Electronic spins provide an additional degree of freedom that can be used in modern spin-based electronic devices. Some benefits of spintronic devices include nonvolatility, energy efficiency, high endurance, and CMOS compatibility, which can be leveraged for data processing and storage applications in today's digital era. To implement such functionalities, controlling and manipulating electron spins is of prime interest. One of the efficient ways of achieving this in spintronics is to use the electric field to control electron spin or magnetism through the voltage-controlled magnetic anisotropy (VCMA) effect. VCMA avoids the movement of charges and significantly reduces the Ohmic loss. This article reviews VCMA-based spintronic devices for magnetic memory applications. First, we briefly discuss the VCMA effect and various mechanisms explaining its physical origin. We then mention various challenges in VCMA that impede it for practical VCMA-based magnetic memory. We review various techniques to address them, such as field-free switching operation, write error rate improvement, widening the operation window, enhancing the VCMA coefficient, and ensuring fast-read operation with low read disturbance. Finally, we draw conclusions outlining the future perspectives.

The development of substantial data storage and processing capabilities of the devices is essential for modern applications, such as the Internet of Things and artificial intelligence. For several decades, semiconductor memories like static and dynamic random access memories (RAMs) have been at the forefront of data storage solutions.1,2 As semiconductor memories are scaled down, charge leakage issues increase, raising standby power dissipation.3,4 This limits their application in areas requiring substantial data storage and processing. However, the standby power dissipation in nonvolatile memories is negligible, making pursuing nonvolatile memory technologies essential. Spintronic devices meet these criteria and hold potential as one of the future alternative technologies.5–7 Magnetic random access memories (MRAMs) based on the magnetic tunnel junction (MTJ), the flagship spintronic device, are very promising in this direction.8–10 The MTJ device consists of a thin insulating spacer (non-magnetic) sandwiched between two ferromagnetic (FM) layers,8 a pinned layer (PL), and a free layer (FL). The spin direction of the PL is immutable due to its high coercivity and cannot be changed by applying the practical magnetic field, while the spin orientation of the FL can be modulated relatively easily.5 The relative spin directions of these FM layers define the electrical conductivity of the MTJ device.11,12 The electrical resistance is low when the magnetization direction of the FM layers is parallel (P) and high when it is antiparallel (AP) to each other.13,14

Controlling and manipulating the magnetization of the FL is crucial for developing MRAMs. Various techniques have been employed to achieve this, including the use of the Oersted field,15–17 spin transfer torque (STT),18–23 thermal gradient,24,25 spin–orbit torque (SOT),26–31 and VCMA.32–37 The Oersted field technique, which powered the initial generation of MRAMs, encounters scalability challenges as device miniaturization necessitates a larger magnetic field.28 The STT mechanism was first theoretically introduced in 1996 by Slonczewski18 and Berger21 and can resolve this shortcoming. STT can manipulate the magnetization of the FL using a spin-polarized current that flows through the MTJ device without needing an external magnetic field.38 In STT-MRAMs, both the read and write currents share the same path, which may cause disturbance in the reading procedure.39,40 Moreover, the writing process requires a considerably higher current magnitude that can break down the insulating barrier, causing reliability issues.41–44 In this geometry, the spin directions of the FL and PL are colinear, leading to a long incubation delay45 and making it unsuitable for high-speed applications. Substantial switching current in STT necessitates oversized access transistors, hampering the component density of the STT-based MRAM and limiting its competitiveness.46–48 Many researchers are actively working on STT to improve the switching efficiency and speed of the device.49–52 Hu and Worledge from IBM introduced a double spin-torque MTJ (DS-MTJ) device using two PLs with one FL.49,50 Due to these two PLs, the FL experienced spin torque from both PL/FL interfaces. Thus, this reduces the switching current by two times. With this technique, they reported less switching error with a sub-nanosecond current pulse. Safranski et al. experimentally demonstrated53 STT switching in the DS-MTJ device with a current 10 times and power consumption 3–10 times less than the conventional three-terminal SOT switching for a similar energy barrier.

As mentioned earlier, the SOT technique can also manipulate the FL spin. Here, write current flows through high spin–orbit coupling (SOC) material, such as heavy metal (HM),27,54 topological insulator,55–57 antiferromagnet (AFM),58 etc. The SOC converts charge current into spin current via the spin Hall effect in the bulk SOC material59 and via the Rashba–Edelstein effect at the material interfaces.60,61 This method separates the read and write current paths29,33,62–64 and alleviates the electrical stress on the oxide spacer when performing write operations.65 In SOT, the initial magnetization direction of the FL and spin polarization direction at the HM/FL interface are perpendicular. Therefore, fast switching can be achieved;29 in other words, it needs a very short current pulse to complete the magnetization switching. The extra third terminal required in the SOT devices penalizes the chip density.66 However, Wu et al. experimentally studied voltage-gated SOT (VGSOT) with four MTJ devices and claimed component density close to the two-terminal device.35 Joule heating is another major concern in the STT and SOT mechanisms due to the current flow via the device.67,68 VCMA is one of the methods used to mitigate this Ohmic loss issue. Instead of the motion of charge carriers and current flow, VCMA employs the electric field to control the electron spin. Typically, it involves an MTJ device characterized by a high resistance-area (RA) product.69–71 As a result, the current that passes through the thin insulating barrier is extremely minimal, enhancing the energy efficiency of VCMA and making it appealing for a wide range of applications, such as MRAMs,72,73 random number (RN) generators,74–76 physically unclonable functions (PUFs),77–79 neuromorphic computing,80,81 and spintronic oscillators.82–87 In this article, our primary focus is the VCMA-MRAM, which is discussed in Secs. II and III.

Reducing the size of the device is essential to achieve a high chip density. The perpendicular magnetic anisotropy (PMA) system, known for its scalability and thermal stability, is more advantageous than the in-plane magnetic anisotropy (IMA) system.88,89 Consequently, devices utilizing PMA nanomagnets are favored over those with IMA. However, the PMA system necessitates an external symmetry-breaking field to facilitate deterministic switching,90,91 which poses challenges for practical applications. Hence, field-free switching operation is very desirable and a hot research topic. VCMA switching is precessional in nature, and, consequently, it also suffers from problems such as high write error rate (WER) and narrow operation window. Moreover, a low VCMA coefficient and a read disturbance rate (RDR) are some additional concerns associated with VCMA-based MRAMs. This article looks into the recent advancements toward addressing these challenges. This article is structured as follows: Sec. II briefly discusses the VCMA effect and its physical origin, Sec. III reviews various challenges in VCMA and techniques to address them, Sec. IV outlines future perspectives and draws concluding remarks.

First, we present the phenomenological description of the VCMA effect for easy understanding to the readers. Later, we briefly discuss different mechanisms proposed in the literature to explain the microscopic origin of VCMA.

The energy barrier separates two energy minimum states of the FL of the MTJ device. A significant energy barrier is necessary to ensure the thermal stability of the FL. First, we discuss the modulation of this energy barrier due to applied voltage, as can be seen in Fig. 1. The voltage-dependent energy barrier [ E b ( V ) ] due to the VCMA effect can be given as92,
E b ( V ) A M T J [ K i ( V ) 2 π M s 2 ( N z N x , y ) t F L ] ,
(1)
where A M T J, K i ( V ), M s, t F L, N x , y, and N z are the cross-sectional circular MTJ area, voltage-dependent interfacial PMA, saturation magnetization, FL thickness, and demagnetization factors in the in-plane and perpendicular directions, respectively. The VCMA effect might exhibit a non-linear relationship with the applied voltage. However, for the typical voltage range used for MRAM devices, it can be assumed that K i ( V ) linearly depends on the voltage (V) at oxide and can be expressed as92,93
K i ( V ) = K i ( V = 0 ) β V / t o x ,
(2)
where β and t o x are the VCMA coefficient and oxide thickness, respectively. As can be seen from Eqs. (1) and (2), as well as in Fig. 1, the energy barrier increases for the negative voltage (light blue color) and decreases for the positive voltage (red color).
FIG. 1.

(a) MTJ device with applied voltage and (b) modulation of the energy barrier by applying voltage on the MTJ device. The ± V on the MTJ reduces (enhances) the energy barrier between two stable states.

FIG. 1.

(a) MTJ device with applied voltage and (b) modulation of the energy barrier by applying voltage on the MTJ device. The ± V on the MTJ reduces (enhances) the energy barrier between two stable states.

Close modal
The energy barrier can be eliminated entirely for a voltage called critical voltage ( V C ) and this critical voltage is given by94 
V C = Δ ( V = 0 ) t o x k B T / ( A M T J β ) ,
(3)
where Δ ( V = 0 ) is the thermal stability at zero voltage and is defined as [ K i ( V = 0 ) 2 π M s 2 ( N z N x , y ) t F L ] / k B T, where k B is the Boltzmann constant and T is the operating temperature of the device. The thermal stability of the FL of the MTJ device is also affected by the voltage at the oxide and can be estimated by solving Eqs. (1) and (2) as
Δ ( V ) = Δ ( V = 0 ) β V A M T J / k B T t o x .
(4)

Once the energy barrier is eliminated, an in-plane magnetic field is required to break the symmetry and induce magnetization switching deterministically. This in-plane magnetic field can be achieved through various sources detailed in Sec. III.

The VCMA coefficient is the modulation in areal density of anisotropy energy per unit electric field.95 It can be obtained from the slope of PMA energy density vs the electric field curve [see Eq. (2)]. This curve can be obtained by different methods, such as hysteresis measurements,96 microwave ferromagnetic resonance,97,98 and magnetoresistance curve.98 

A few parameters, such as saturation magnetization and PMA constant, are significantly affected by the operating temperature of the device. These variations reasonably impact the switching performance of the device. Therefore, these must be considered during device operations. The temperature-dependent saturation magnetization ( M s ( T ) ) and PMA constant ( K i ( T ) ) can be given as99–101 
M s ( T ) = M s ( 0 ) [ 1 ( T T C ) ϑ ] ,
(5a)
K i ( T ) = K i ( 0 ) [ 1 δ T ] ,
(5b)
where M s ( 0 ) and K i ( 0 ) are the saturation magnetization and PMA constant at 0 K, respectively, and T C is the Curie temperature. ϑ and δ are the fitting parameters and their values can be obtained from experimental measurements if required.

The theoretical prediction of the VCMA effect in the metal was given by Nie and Blügel in 2000.102 In 2007, Weisheit et al.103 first demonstrated the VCMA effect in ultrathin FePt and FePd layers. They utilized propylene carbonate electrolytes to prevent hydrogen formation and diffusion into the film under negative voltage. They observed reversible changes in the coercivity of the FePt or FePd films when immersed in the electrolyte, depending on the applied voltage. Later, in 2009, Maruyama et al.104 studied the effect of the electric field on the magnetic anisotropy in a thin-film structure of Fe/MgO. They reported a large change in magnetic anisotropy (40%) by a small voltage (<100 mV/nm). Hereafter, Leistner et al.105 reported a significant change in magnetic anisotropy by 25% using FePt-based thin film due to charge-induced differences between surface compositions. They demonstrated the manipulation of magnetism in metal oxides using an electric field via reversible oxidation/reduction processes.105 There are various possibilities for VCMA emanation, such as the voltage-induced redox reaction,106,107 electromigration,108 charge trapping,109 Rashba effect,110,111 piezoelectric effect,112 and purely electronically induced VCMA,113,114 etc. The first three mechanisms can cause an adequate VCMA coefficient (in order of thousands of fJ/V m).106,108,109 Nonetheless, their compatibility with high-speed MRAM applications is compromised by the sluggish pace of chemical reactions.95,113 The prevailing explanation for the VCMA effect in thin films attributes it to the accumulation or depletion of charges at the HM/oxide interface.113 This can change the band structure115 along with the electronic occupancy of 3d orbitals.116 Such changes can induce significant PMA.

Purely electronically induced VCMA effects, such as anisotropy in the orbital angular momentum resulting from charge doping114 and induction of an electric quadrupole,114 are discussed below. The variation in the orbital magnetic moment is elucidated by selectively introducing electron/hole doping in the electronic orbitals of atoms at the interface, which explains the VCMA effect.114,117 Kawabe et al.117 demonstrated the modulation of the orbital magnetic moment in the ultrathin Co induced by an electric field. The orbital magnetic moment mechanism is shown in Fig. 2(a). The PMA energy can be changed due to this effect through the SOC from the spin-conserved excitation processes.114,117,118 The PMA energy in ferromagnetic metals is defined as the disparity between the SOC energies associated with the perpendicular and in-plane magnetizations, which can be given by the first term of the following equation:119 
λ 4 [ Δ L ξ , ↓↓ Δ L ξ , ↑↑ ] + 7 λ 2 [ Δ T ζ , ↓↑ Δ T ζ , ↑↓ ] ,
(6)
where λ and are the SOC coefficient and reduced Planck's constant, respectively. The orbital angular momentum is Δ L ξ = L z L x with L z and L x being the z- and x-components of the spin angular momentum, respectively. The magnetic dipole moment is Δ T ζ = T z T x with T z and T x being the corresponding z- and x-components of a magnetic dipole, respectively. The symbols ↑(↓) represent the majority (minority) spin bands' contributions, respectively.
FIG. 2.

Emergence of the VCMA effect. (a) Orbital magnetic moment and (b) electric quadrupole. Reproduced under the terms of the CC BY 4.0 license from Nozaki et al., Micromachines 10(5), 327 (2019). Copyright 2019 MDPI.

FIG. 2.

Emergence of the VCMA effect. (a) Orbital magnetic moment and (b) electric quadrupole. Reproduced under the terms of the CC BY 4.0 license from Nozaki et al., Micromachines 10(5), 327 (2019). Copyright 2019 MDPI.

Close modal

Another mechanism responsible for the VCMA effect is the electric field at the metal and dielectric interface.120 This field is typically non-uniform along the direction perpendicular to the interface. It interacts with the electric dipole and the quadrupole moments of the electron shells within the metal layer. The mechanism is illustrated in Fig. 2(b). An electric quadruple is associated with the magnetic dipole moments within an electron shell and affects the magnetic anisotropy through the SOC.120,121 In 5d materials like Pt that exhibit proximity-induced spin polarization, the magnetic dipole T z term plays a crucial role in the VCMA effect. This term corresponds to an electric quadruple in the atoms.122 This can be seen from the second term of Eq. (6). Interested readers can explore several insightful review articles by Nozaki et al. in 2019,114 Pardede et al. in 2020,123 and Shao and Khalili Amiri in 2023.124 They primarily focused on the mechanisms and applications of the VCMA. Rather than discussing these aspects again, we primarily focus on reviewing various challenges associated with VCMA switching for VCMA-MRAM applications and possible methods to address them. We now discuss these issues sequentially in Sec. III.

With various advantages, the VCMA-based MRAM has some issues, such as external field requirement, high WER, narrow operation window, low VCMA coefficient, and RDR and read delay, as shown in Fig. 3. In Secs. III AIII E, we review the recent advancements toward addressing these issues.

FIG. 3.

Various challenges associated with VCMA switching. It includes the requirement of the external field, high write error rate, narrow operation window, low VCMA coefficient, and read disturbance and delay.

FIG. 3.

Various challenges associated with VCMA switching. It includes the requirement of the external field, high write error rate, narrow operation window, low VCMA coefficient, and read disturbance and delay.

Close modal

Field-free switching is crucial for implementing high-density MRAMs. Here, we focus on field-free VCMA switching assisted by various mechanisms, such as VGSOT, built-in in-plane magnetic field, STT, conically magnetized FL (CFL), strain, self-regulated precessional switching, Rashba field, and skyrmion techniques, to accomplish this. We reviewed all these techniques reported until the writing of the paper to the best of our knowledge.

1. VGSOT technique

When used independently, VCMA or SOT switching in the PMA nanomagnet requires an external symmetry-breaking magnetic field.92 However, this field requirement can be eliminated when it (VCMA or SOT) is used in a suitable combination with an exchange bias (EB) or STT. Many researchers have reported field-free switching operations in such devices theoretically and experimentally.33,35,125–129 The VGSOT device comprises an MTJ on top of a high SOC material. The voltage pulse on the MTJ device induces the VCMA as well as STT effects. The minimum current density required for deterministic SOT switching, called critical current density, can be reduced using this voltage pulse.33 However, its mechanism is not clear and needs to be clarified. One possibility is due to modifications of Rashba SOC (RSOC) by the electric field.111,130 Another possible mechanism is the modulation of interfacial PMA by the voltage pulse.116,126

In 2016, Yoda et al. studied131 the magnetization dynamics of the IMA MTJ of the VGSOT device. They fabricated a prototype structure (eight MTJs). The MTJ voltage was employed as a selection criterion, while the SOT effect served as a writing mechanism. Their simulation results showed the possibility of a low WER ( 10 12 ) with a reasonable VCMA coefficient and thermal stability in a wide range of write current densities.131 In 2017, Inokuchi et al. experimentally demonstrated a voltage-controlled spintronic memory (VoCSM) device125 by modulating the strength of the energy barrier. The Ta/CoFeB/MgO/CoFeB/Ru/CoFe/IrMn multilayer stack was fabricated on a thermally oxidized Si substrate. The HM (Ta) was grown over a thermally oxidized Si substrate and treated as a write line. The strings of the IMA MTJs were fabricated on the common write line on Ta. They found improvement in the read disturbance as well as WER [no switching error for the 5 × 10 8 Landau–Lifshitz–Gilbert (LLG) simulations] by adequately controlling the read and write sequences. The variation in the SOT current density is attributed to alterations in the surface anisotropy of the FL induced by control voltage.125 These works mentioned above were for the IMA MTJ-based devices, and the discussion of VCMA switching in the perpendicular MTJ (pMTJ) device assisted by EB field or a combination of STT and SOT is given below.

a. VGSOT with antiferromagnet

An AFM is vital to realize field-free switching operations in today's spintronic devices. Experimental evidence supports the capability of an AFM to exhibit SOT and induce an EB field.132,133 This EB field can mitigate the external field requirement and has already been used to realize field-free SOT switching. Figure 4(a) depicts a VGSOT device with AFM as the high SOC material. In 2019, Peng et al. experimentally demonstrated134 field-free switching in a pMTJ device using a combination of SOT, exchange bias, and VCMA. They reported a reduction in the current density by 49% using 0.6 V on the MTJ with a small write energy of 6.2 fJ/bit. In 2020, Zhang et al.33 realized field-free switching in a three-terminal VGSOT device. They used a pMTJ device (CoFeB/MgO/CoFeB) on an AFM (IrMn) for the simulation. The EB field from the AFM can tilt the FL magnetization.135 They started the simulation with SOT only. This resulted in an incomplete field-free switching with a significant plateau time.33 However, they found a substantially reduced plateau time by including the VCMA effect. An MTJ voltage of 1.3848 V could completely eliminate the effective PMA field and reduce the critical current by 30 times compared to the SOT case without the VCMA effect.33 

FIG. 4.

VGSOT device structure with a pMTJ device on (a) an AFM and (b) an HM. The light orange color arrows in the AFM adjacent to the FL represent the EB field (x-direction). The white color arrows in the MTJ show the spin directions of the FM layers (z-direction). The spin polarization direction at the HM and FL interface is in the y-direction.

FIG. 4.

VGSOT device structure with a pMTJ device on (a) an AFM and (b) an HM. The light orange color arrows in the AFM adjacent to the FL represent the EB field (x-direction). The white color arrows in the MTJ show the spin directions of the FM layers (z-direction). The spin polarization direction at the HM and FL interface is in the y-direction.

Close modal

Later, in 2021, Kang et al. experimentally reported127 field-free SOT switching using an electric field. They used two side gates to create lateral electric fields that could break the symmetry. This resulted in deterministic switching without requiring an external field. In 2022, Alla et al. reported128 a simulation-based field-free VCMA switching in the VGSOT device. They studied the effect of various writing mechanisms (VCMA, SOT, and EB field). In 2023, Lu et al. recently realized field-free switching operation in the VGSOT device.129 They reported improvement in the annealing temperature and PMA by introducing a thin W layer at the AFM/FM interface. A voltage of 0.76 V to the MTJ can reduce the required current by 73% and power consumption by 84%. They also reported a reduction in the coercive field due to the VCMA effect.

b. Combination of SOT, STT, and VCMA

The voltage pulse at the MTJ device induces two different effects: STT and VCMA. SOT and STT, in combination with the VCMA effect, promise faster and more energy-efficient switching than STT or SOT switching alone. In 2020, Grimaldi et al.136 combined these three effects in a three-terminal pMTJ device and experimentally demonstrated magnetization switching. Later, in 2021, Krizakova et al.137 experimentally realized field-free SOT switching operation assisted by a dual-bias voltage pulse in a three-terminal device. The group also performed micromagnetic simulations to verify the experimental outcomes. Figure 4(b) depicts the operation of the device. With this combination, they showed an improvement in switching efficiency or critical switching energy below the STT technique. This voltage does not compromise the ability of SOT switching with sub-ns current pulse. A year later, Yoshida et al.138 reported micromagnetic simulation-based field-free switching driven by SOT and STT, including the VCMA effect in a pMTJ device. By properly optimizing the SOT and STT pulses, they achieved 100% switching for 100 trials. This combination reduces power consumption tenfold and the switching time by ∼50% compared to STT switching alone.

Due to the various benefits of the VGSOT device discussed, it avails in many applications, such as multilevel switching using voltage-controlled SOT,139 computing-in-memory,140 logic locking for hardware security using VGSOT device,141 and logic locking for high-speed digital circuits.142 However, the three-terminal VGSOT device faces a low component density issue compared to the two-terminal device. It penalizes the component density of the chip. Moreover, the requirement for two different pulses is another concern with such devices.

2. Built-in in-plane magnetic field technique

Another possible way to get field-free switching involves using a magnetic field generated by integrating an additional magnetic layer in the device structure. The strength of the magnetic field can be manipulated by adjusting the distance between the embedded magnet and the FL. This embedded magnet allows its use in the STT, SOT, and VCMA in different forms.32,90,143–145 In 2017, Deng et al. theoretically proposed32 an elliptical bias layer (BL) in an elliptical pMTJ device for VCMA switching. The BL is engineered inside the MTJ stack to provide an EB field, resulting in field-free precessional VCMA switching.32 The schematic shown in Fig. 5 comprises an elliptical pMTJ on an extra BL (10 nm) separated by a metal (3 nm). Micromagnetic/macro-spin simulations showed that an EB field of up to 50 mT can be obtained in such structures.

FIG. 5.

Diagram of an elliptical pMTJ device with the major axis along the x-direction. The extra bias layer (BL) provides an in-plane magnetic field through exchange coupling or EB field in the x-direction. The metal separates the BL and FL. Reproduced under the terms of the CC BY 4.0 license from Deng et al., Sci. Rep. 7(1), 16562 (2017). Copyright 2017 Nature Portfolio.

FIG. 5.

Diagram of an elliptical pMTJ device with the major axis along the x-direction. The extra bias layer (BL) provides an in-plane magnetic field through exchange coupling or EB field in the x-direction. The metal separates the BL and FL. Reproduced under the terms of the CC BY 4.0 license from Deng et al., Sci. Rep. 7(1), 16562 (2017). Copyright 2017 Nature Portfolio.

Close modal

The experimental realization of such devices by adding additional magnetic layers or nanomagnets is available in the literature by many researchers for SOT143–145 and VCMA.146 The fabrication of the MTJ-based devices requires a metallic hard mask (MHM). The Co metal can be fabricated directly on the MTJ without an additional mask. Leveraging this advantage, Garello et al. used magnetic Co metal as an MHM in 2019,145 demonstrating a field-free SOT switching. Later, Tsou et al.99 performed a write margin analysis for this structure through numerical simulation. By employing this method, they showed a 60% reduction in the required switching current. Another major advantage of this structure was the elimination of the plateau time, which significantly improves the operation speed of the device. Later, in 2020, Kong et al.144 used an additional in-plane magnetized CoFeB layer to couple with a perpendicularly magnetized CoFeB layer via the Ta spacer in the pMTJ device and experimentally performed field-free switching. After two years of the Kong group, Cai et al.143 demonstrated it in a similar structure with CMOS-compatible 300 mm integrated with the pMTJ device for MRAMs. They reported an ultra-fast (current pulse duration of 0.3 ns) and energy-efficient switching for the MRAM device.

In 2020, Wu et al. reported146 field-free VCMA switching using an embedded nanomagnet (MHM). VCMA needs the same polarity of the write pulse for AP-P or P-AP transitions because it is unipolar. Therefore, a pre-read process is required before writing VCMA-based memories to ensure the magnetization state of the FL.114,146,147 Due to the presence of MHM, the threshold voltages of these transitions (AP-P and P-AP) can be different.146 This can be helpful for the pre-read operation. They showed a WER value of the order of 10 5 for this device.

3. STT technique

As mentioned, STT switching can be achieved without relying on external field assistance but requires a large current when used alone. Usually, STT switching requires a lengthy current pulse of several nanoseconds; shortening this pulse width necessitates a significantly higher switching current due to the inverse relationship between pulse duration and required current amplitude.28 The switching current required for magnetization is directly proportional to the energy difference between two minima, necessitating a substantial current to overcome the large energy barrier to ensure thermal stability.93 This large current may damage the thin oxide layer of the MTJ. However, combining SOT with STT can lower the necessary switching current, preventing the breakdown of the tunnel barrier. A few researchers reported magnetization switching by combining the SOT and STT techniques.46,148,149 This may help the switching dynamics in two ways: STT-assisted SOT switching and SOT-assisted STT switching. In the former case, STT can provide the necessary bias to break the symmetry and allow magnetization switching without an external field.148 In the latter scenario, SOT causes an initial deviation in the magnetization alignment within the ferromagnet. This would eliminate the incubation delay and, thus, allow fast switching.148 This combination ensures field-free switching operation.148 VCMA helps the STT switching as a suitable combination of STT and VCMA ensures switching at a lower current.92,93,150,151 This can be achieved through voltage pulse engineering. A voltage pulse can reduce the energy barrier due to the VCMA effect. After this, STT current due to a small voltage pulse can complete the magnetization switching.93,103 Figure 6 illustrates the STT-assisted VCMA switching mechanism.

FIG. 6.

(a) Device structure of a pMTJ device with applied voltage, (b) consecutive voltage pulses for the VCMA and STT effects, and (c) energy barrier height modulation by voltage. The first voltage pulse ( V V C M A ) reduces the energy barrier. Consequently, a small STT current by small voltage ( V S T T ) ensures the magnetization flipping without external field assistance.

FIG. 6.

(a) Device structure of a pMTJ device with applied voltage, (b) consecutive voltage pulses for the VCMA and STT effects, and (c) energy barrier height modulation by voltage. The first voltage pulse ( V V C M A ) reduces the energy barrier. Consequently, a small STT current by small voltage ( V S T T ) ensures the magnetization flipping without external field assistance.

Close modal

In 2012, Alzate et al. experimentally demonstrated152 VCMA switching without external field assistance. They used a small non-zero leakage current insufficient for purely STT switching through the pMTJ device, achieving a tenfold reduction in switching energy. In 2014, Kanai et al. experimentally demonstrated150 that combining consecutive voltage pulses for the VCMA and STT effects enables faster and more reliable magnetization switching without external magnetic fields and explored how the width of these pulses influences switching probability. A switching probability of 1 can be achieved using optimized voltage pulses. Kang et al.92 used this technique for VCMA-MRAM cell design and showed that this could be useful in avoiding source degeneration problems.

For getting deterministic switching using a combination of the STT and VCMA mechanisms, there are two concerns: for a small voltage (very small STT), a non-zero stray magnetic field from the PL is required, and second, for a high voltage, large STT can overcome this stray field effect and switch the magnetization, but switching speed is reduced (few μs to few ms). This makes the above technique unsuitable for high-speed MRAMs.151 The proper optimization of these pulses is required to get field-free VCMA switching with STT assistance.

4. CFL method

An elliptical MTJ can provide the required in-plane magnetic field owing to its shape anisotropy. This field can help in getting field-free switching operations for a few cases.124,153 This field, however, depends upon the in-plane component of the magnetization and cannot be helpful for fully perpendicular FL cases.153 A deviation of the magnetization from the perpendicular direction is required to execute precessional switching. To deal with this concern, numerous researchers suggested the idea of a CFL (see Fig. 7).153–156 The stabilization of the conically magnetized state arises from the interplay between the first- and second-order magnetic anisotropy.157–160 The cone state is experimentally observed in thin films of Co deposited on Pt or Pd,161 Co/Pt multilayers,162 MgO/CoFeB/Ta heterostructure,163 and FM/AFM interfaces.164–166 

FIG. 7.

(a) An elliptical MTJ device with the Cartesian coordinate system. The major axis of the FL lies along the x-direction. (b) Defining different initial magnetic states as a function of effective first-order ( K 1 , e f f ( 0 ) ) and second-order ( K 2 ( 0 ) ) anisotropies at zero voltage. The shaded region in the second quadrant represents the stable cone state with polar angle θ. The hatched region is a bistable state. Redrawn from Casimir et al., J. Phys. Radium 20, 360 (1959). Copyright 1959 EDP Sciences.

FIG. 7.

(a) An elliptical MTJ device with the Cartesian coordinate system. The major axis of the FL lies along the x-direction. (b) Defining different initial magnetic states as a function of effective first-order ( K 1 , e f f ( 0 ) ) and second-order ( K 2 ( 0 ) ) anisotropies at zero voltage. The shaded region in the second quadrant represents the stable cone state with polar angle θ. The hatched region is a bistable state. Redrawn from Casimir et al., J. Phys. Radium 20, 360 (1959). Copyright 1959 EDP Sciences.

Close modal

The initial magnetization state at minimum energy for zero applied voltage can be given by153,156 m z ( 0 ) = ± 1 + K 1 , e f f ( 0 ) / 2 K 2 ( 0 ), m x ( 0 ) = ± 1 ( m z ( 0 ) ) 2, and m y ( 0 ) = 0, where K 1 , e f f ( 0 ) ( = K 1 ( 0 ) 0.5 π M s 2 ( N z N x ) ) and K 2 ( 0 ) are the effective first-order anisotropy constant and second-order anisotropy constant at zero voltage, respectively. The equilibrium polar angle ( θ ( 0 ) = sin 1 K 1 , e f f ( 0 ) / 2 K 2 ( 0 ) ) at 0 K can be found by competition between K 1 , e f f ( 0 ) and K 2 ( 0 ).167 The cone state region, which is shown in Fig. 7 by the shadow, having K 1 , e f f ( 0 ) < 0 and K 2 ( 0 ) > 0.5 K 1 , e f f ( 0 ). These anisotropy constants depend linearly on the voltage [see Eq. (2)]. In the CFL, the magnetization tilts from the out-of-plane (OOP) [see Fig. 7(b)], reducing the energy required to write the MTJ device, but this necessitates symmetry breaking through the application of an in-plane magnetic field.

In 2015, Matsumoto et al. reported154 STT switching with the CFL MTJ device. They observed a reduction in the current density by 22% and switching time by 56% than the perpendicular FL for the same thermal stability. In 2016, Park et al. experimentally showed159 the manipulation of the easy cone state in the Ta/Pt/CoFeB/MgO structure using an electric field. They reported the canting of the easy axis from the perpendicular direction to form a cone state. They reported the control of the cone angle ( θ ) from 22° to 32° by an electric field of 4 MV/cm. In 2018, Matsumoto et al. demonstrated153 field-free VCMA switching in the CFL pMTJ device with elliptical nanomagnets that use shape anisotropy to induce an in-plane magnetic field, offering a practical design guide for magnetic memory applications. They also derived an analytical formula for precessional switching and conducted a comprehensive study of WER in CFL-based VCMA switching devices.156 They reported a WER less than 10 5. They reported voltage pulse width ( t P W ) effect on the WER for K 1 , e f f = 80 kJ / m 3 and K u 2 = 40 kJ / m 3 with optimum WER ( 8 × 10 7 ) at t P W = 0.55 ns and θ ( 0 ) = 45 0. They also reported the dependence of the optimum WER on θ ( 0 ) for the optimum values of voltage pulse width (0.55 ns) and ( K 1 , e f f , K u 2 ). Such CFL-based field-free VCMA switching is yet to be demonstrated experimentally. Mass production of the elliptical MTJ devices may suffer fabrication problems due to a lack of precise control of the cross section of the MTJ device. This may lead to different shape anisotropy fields for different MTJs. Our group recently studied168 WER in the CFL MTJ device with circular nanomagnets using the SOT method without using the external field. Our device structure consists of a CoFeB/MgO/CoFeB/PtMn multi-structure stack. We exploited the EB field from AFM (i.e., PtMn) to realize field-free switching and also used field-like torque (FLT) to improve the switching performance of the device. We reported a WER of the order of 10 7. A similar WER study in a circular MTJ can be achieved using the VCMA mechanism.

5. Strain control

Strain is of great interest in controlling the magnetization dynamics. Magnetostriction is a well-known phenomenon where an applied magnetic field can change the shape or dimensions of the magnetic materials. The inverse of magnetostriction, the Villari effect, allows change in magnetization in response to the mechanical strain.169,170 This happens because of the presence of magnetocrystalline anisotropy. Much research has been done on strain-mediated magnetization switching using techniques such as STT,86,171–175 SOT,26,176,177 and VCMA.178,179 In 2013, Biswas et al. reported energy-efficient STT switching assisted by surface acoustic waves.172 In 2014, Khan et al. studied strain-assisted STT switching and reported a reduction in the required energy by 50 times that of conventional STT-MRAMs.174 In 2018, Wang et al. theoretically reported field-free SOT switching assisted by strain.177 In 2020, Chen et al. achieved a switching probability of 0.98 by implementing field-free strain-assisted SOT switching, notably reducing the required current.176 Later, in 2023, Mishra et al.26 reported this in the pMTJ device with the best-case switching probability of 0.99.

A few researchers theoretically reported field-free VCMA switching assisted by strain.178,179 Magnetization can be rotated up to 90 ° using the VCMA effect alone.180,181 As a result, achieving a predictable 180 ° rotation necessitates an external field assistance. In 2017, Drobitch et al. investigated VCMA switching assisted by strain and STT179 in a two-terminal device. The device used perpendicularly magnetized elliptical FM layers. The FL was made of Terfenol-D, a highly magnetostrictive material. This device is very crucial for a high-density MRAM. The schematic of such a strain-assisted VCMA device can be seen in Fig. 8. The pMTJ is on a thin piezoelectric layer attached to a conducting substrate. The applied voltage across the structure partly drops across the MTJ and partly across the piezoelectric layer, assuming that the highly conducting substrate drops negligible voltage. The voltage across the MTJ induces the VCMA effect, and the voltage across the piezoelectric layer generates the strain. This strain influences the FL anisotropy via magnetoelastic coupling between the piezoelectric layer and FL. This magnetic field can break the symmetry and result in deterministic switching. This magnetoelastic anisotropy field depends on the magnetization components of FM layers along the stress axis.179 Thus, it may lead to a high WER. They reported a WER of the order of 10 4.

FIG. 8.

The device structure of the strain-assisted VCMA switching. The pMTJ is made on the piezoelectric layer. The voltage ( V P P ) is applied across the structure. It partly drops across the MTJ ( V M T J ) and partly drops across the piezoelectric layer ( V P I E Z O ). Magnetization of the FL is rotated by 90 ° (black color arrow) from OOP to in-plane due to the VCMA effect. Another 90 ° rotation can be achieved by symmetry-breaking field due to strain.

FIG. 8.

The device structure of the strain-assisted VCMA switching. The pMTJ is made on the piezoelectric layer. The voltage ( V P P ) is applied across the structure. It partly drops across the MTJ ( V M T J ) and partly drops across the piezoelectric layer ( V P I E Z O ). Magnetization of the FL is rotated by 90 ° (black color arrow) from OOP to in-plane due to the VCMA effect. Another 90 ° rotation can be achieved by symmetry-breaking field due to strain.

Close modal

As mentioned earlier, Terfenol-D can have good magnetostriction properties, but the VCMA coefficient may be low. Moreover, it is a very fragile and costly material.182 Our group has, therefore, recently studied strain-assisted VCMA switching in mainstream materials like CoFeB.178 We used a similar device structure to that of Drobitch et al.179 We performed macro-spin simulations to investigate the effect of strain and EB field on the WER of the device.178 We reported the WER value of the order of 10 5 with a short voltage pulse of the duration of 0.6 ns. These strain-mediated devices are yet to be realized experimentally. Furthermore, strain can affect parameters like exchange bias,183 PMA,184,185 and VCMA.184 Therefore, optimizing these parameters while demonstrating the device experimentally is crucial.

6. Self-regulated precessional switching method

Most methods to realize field-free switching discussed here are based on the MTJ device with a single PL and FL. However, the advantages of using two PLs on the device performance for STT are already discussed in Sec. I.51 Numerous researchers studied the double-barrier MTJ device structure.186–188 Recently, Sin and Oh have proposed189 a novel method to achieve field-free switching operation without external field assistance. They exploited the fact that the resistance of the device in the antiparallel state is larger than that in the parallel state to break the energy symmetry. The device structure is shown in Fig. 9 and comprises two PLs with one FL. The “up” state and read operations are executed by applying a constant current through MTJ1 via terminals T1 and T3, as seen in Fig. 9(b). The purpose of current is not to create STT but to induce charge in the oxide layer for the VCMA effect. The VCMA effect reduces the PMA, thus reducing the resistance and the voltage drop across the device. The energy symmetry is broken if the current is sufficient, and the parallel “up” state becomes more favorable. MTJ2 similarly enables deterministic writing of the “down” state through the injection of current through terminals T2 and T3 [see Fig. 9(c)]. They reported improved switching performance of the device compared to other VCMA techniques with a small mean energy consumption of 38.22 fJ and a mean switching delay of 3.77 ns.

FIG. 9.

(a) Double-barrier MTJ device structure with two PLs and one FL, (b) writing “up” state and read operations by current flows through terminals T1 and T3, and (c) writing “down” state by current flows through terminals T2 and T3. Reproduced under the terms of the CC BY 4.0 license from Sin et al., Sci. Rep. 13(1), 16084 (2023). Copyright 2023 Nature Portfolio.

FIG. 9.

(a) Double-barrier MTJ device structure with two PLs and one FL, (b) writing “up” state and read operations by current flows through terminals T1 and T3, and (c) writing “down” state by current flows through terminals T2 and T3. Reproduced under the terms of the CC BY 4.0 license from Sin et al., Sci. Rep. 13(1), 16084 (2023). Copyright 2023 Nature Portfolio.

Close modal

7. Rashba field method

Another way to realize a field-free switching operation is to use the Rashba field. Some researchers have studied magnetization dynamics induced by the Rashba field.110,190,191 The Rashba effect occurs due to the breaking of inversion symmetry in heterojunctions. Deng et al. utilized this technique and performed field-free VCMA switching.192 Brief mechanisms can be understood from Fig. 10. The device comprises an elliptical pMTJ on an HM. Voltage pulse V 2 reduces the interfacial PMA due to the VCMA effect, and simultaneously applied voltage V 3 generates the Rashba field (−y-direction) at the FM/HM interface, assisting the magnetization switching. The Rashba field can be given as
B R = α R 2 μ B M s ( z ^ × J F L ) ,
(7)
where α R, J F L, and μ B are the Rashba coefficient, the current density through the HM, and Bohr magnetron, respectively. They reported fast (0.5 ns) and energy-efficient (6 fJ) switching in the pMTJ device with a large thermal stability of 61 and a large operation window of 0.8 ns. The three-terminal device shown in Fig. 10 requires two pulses (voltage and/or current). The additional terminal may penalize the chip density. Moreover, this method is yet to be reported experimentally for field-free VCMA switching.
FIG. 10.

Schematic of a three-terminal device structure with an elliptical MTJ on the HM. The direction of the effective Rashba field is shown by a blue color arrow (−y-direction), and the current flows in the −x-direction. Reproduced from Deng et al., Appl. Phys. Lett. 112(25), 252405 (2018) with the permission of AIP Publishing.

FIG. 10.

Schematic of a three-terminal device structure with an elliptical MTJ on the HM. The direction of the effective Rashba field is shown by a blue color arrow (−y-direction), and the current flows in the −x-direction. Reproduced from Deng et al., Appl. Phys. Lett. 112(25), 252405 (2018) with the permission of AIP Publishing.

Close modal

8. Skyrmion-assisted method

A skyrmion is a topologically stable spin texture characterized by swirling magnetic moments and emerges in certain thin-film FM materials.193,194 The skyrmion can be manipulated using an electric field,195 spin-polarized current,196 etc. In the switching process, the skyrmion acts as an intermediate state and assists in switching the PMA nanomagnet.197,198 The applied voltage pulse reduces the interfacial PMA and creates a skyrmion. The voltage pulse is removed coincidently with the skyrmion's inbreathing (shrinking) motion.199 Therefore, once the PMA is restored to its original value, the intermediate skyrmion annihilates to other ferromagnetic magnetic states. The illustrative switching process is shown in Fig. 11. In 2018, Bhattacharya and Atulasimha197 proposed a two-terminal skyrmion-mediated VCMA switching mechanism for magnetic memory. They studied VCMA switching without an external field using Dzyaloshinskii–Moriya interaction (DMI).200,201 The equivalent magnetic field due to DMI can be formulated as H D M I = ( 2 D / μ 0 M s ) [ ( m ) z ^ m z ],202 where D is the effective DMI constant and m z is the z-component of the reduced magnetization ( m ). The DMI is in significant quantities at the FM/HM interface with high SOC and promotes the magnetization canting between the neighboring spins.197 This mitigates the requirement of the external magnetic field. They performed 10 4 LLG simulations and found no switching error. Later, in 2020, Rajib et al. studied field-free VCMA switching assisted by skyrmion.199 

FIG. 11.

(a) Schematic of an MTJ device on the HM and (b) illustration of skyrmion-mediated switching operation. The ferromagnetic state can be reversed by simultaneously removing the voltage while inducing the inhalation of the skyrmion. Reproduced with permission from Rajib et al., IEEE Trans. Electron devices 67, 3883–3888 (2020). Copyright 2020 IEEE.199 

FIG. 11.

(a) Schematic of an MTJ device on the HM and (b) illustration of skyrmion-mediated switching operation. The ferromagnetic state can be reversed by simultaneously removing the voltage while inducing the inhalation of the skyrmion. Reproduced with permission from Rajib et al., IEEE Trans. Electron devices 67, 3883–3888 (2020). Copyright 2020 IEEE.199 

Close modal

An MTJ device with smaller dimensions needs higher VCMA, PMA, and DMI values for the switching. Nozaki et al. experimentally reported an immense value of the PMA.95 Large values of the VCMA and DMI are theoretically predicted.203–205 However, these are yet to be reported experimentally. Moreover, skyrmion-mediated VCMA switching is yet to be done experimentally.

Various field-free VCMA switching techniques reported experimentally and theoretically until the writing of the article are summarized in Table I (with corresponding WER values) and Fig. 12.

FIG. 12.

The summarization of all the field-free VCMA switching operation techniques theoretically or experimentally reported in the literature.

FIG. 12.

The summarization of all the field-free VCMA switching operation techniques theoretically or experimentally reported in the literature.

Close modal
TABLE I.

List of various field-free techniques and WER values.

Field-free VCMA techniquesMinimum WERsComments
VGSOT technique IMA system: 10−12 Ref. 131 and <5 × 10−9 Ref. 125  There is no switching error for 5 × 108 simulations.125  
PMA system: 10−5 Ref. 35  105 events with tPW of 0.4 ns. 
Embedded extra magnet 9 × 10−5 Ref. 146  This WER is measured at tPW = 1 ns. 
Strain-assisted VCMA 10−4 Ref. 179 and 5 × 10−5 Ref. 178  Simulated for 104 times with tPW = 0.7 ns179  and 105 times with tPW = 0.6 ns.178  
Rashba field-assisted VCMA 0 Ref. 192  Data extraction from the graph with 100 LLG simulations.192  
STT-assisted VCMA ≈0.01 Ref. 152 and 0 Ref. 150  Data extraction from the graph with 100 LLG simulations.150,152 
Skyrmion-mediated VCMA <10−4 Ref. 197  No switching error for 104 LLG simulations. 
CFL method <10−5 Ref. 156  Error correction is feasible at 300 K, excluding the external field. 
Field-free VCMA techniquesMinimum WERsComments
VGSOT technique IMA system: 10−12 Ref. 131 and <5 × 10−9 Ref. 125  There is no switching error for 5 × 108 simulations.125  
PMA system: 10−5 Ref. 35  105 events with tPW of 0.4 ns. 
Embedded extra magnet 9 × 10−5 Ref. 146  This WER is measured at tPW = 1 ns. 
Strain-assisted VCMA 10−4 Ref. 179 and 5 × 10−5 Ref. 178  Simulated for 104 times with tPW = 0.7 ns179  and 105 times with tPW = 0.6 ns.178  
Rashba field-assisted VCMA 0 Ref. 192  Data extraction from the graph with 100 LLG simulations.192  
STT-assisted VCMA ≈0.01 Ref. 152 and 0 Ref. 150  Data extraction from the graph with 100 LLG simulations.150,152 
Skyrmion-mediated VCMA <10−4 Ref. 197  No switching error for 104 LLG simulations. 
CFL method <10−5 Ref. 156  Error correction is feasible at 300 K, excluding the external field. 

A large WER is another concern in the VCMA. VCMA switching is precessional in nature, and it leads to a large WER. The WER has a non-monotonic dependence on the voltage pulse (amplitude and width) with a characteristic minimum, as has been observed by many researchers.114,151,178,179,206 This could occur due to the transition between precession orbits induced by thermal effects.207 This can indeed limit the development of VCMA-based MRAMs. A very low WER is required for memory applications to avoid error correction and write verification processes. For practical memories to work accurately, a WER less than 10 9 for chips with error correction code (ECC) and less than 10 18 for chips without the ECC is desired.47,208 The error correction and write verification processes slow down the device operation speed. Here, we discuss various approaches to improve the WER.209,210

1. Improving the thermal stability

An enormous thermal stability of the MTJ device is necessary to make the device immune to thermal effects, resulting in a lower WER. In 2016, Shiota et al. studied209 the impact of thermal stability on the WER. They reported improvement in the WER by increasing the thermal stability of the device and decreasing the damping constant ( α ). It has been shown that small α leads to a low switching error.211 A WER less than 10 15 can be obtained with a small α of 0.01 and thermal stability ( Δ ) value greater than 48.209 

2. Pulse engineering

The switching performance of the device is sensitive to the voltage pulse parameters, such as amplitude, width, and rise/fall time. Utilizing a triangular pulse suggests a gradual change in the effective field from the OOP to the in-plane direction. Therefore, it makes the FL precession unstable. A rectangular pulse (zero rise and fall times) abruptly eliminates the effective PMA, causing the effective magnetic field to align in the in-plane direction. The magnetization can precess around a stable axis and switch efficiently. The illustrative 3D magnetization trajectories for triangular (black color) and rectangular (green color) voltage pulses are shown in Fig. 13. These are simulated from the parameters given in Ref. 178 and are mentioned for the sake of illustration only. Compared to the rectangular pulse, a significant precession can be seen for the triangular pulse (large rise and fall times). Large precession leads to a large WER once thermal fluctuations are considered.

FIG. 13.

Effect of pulse shaping on the switching performance of the device. (a) Triangular (black color) and rectangular (green color) voltage pulses and (b) corresponding magnetization trajectories ( m z ) with a large precession in the x–y plane for the triangular pulse (black color) than small precession for the rectangular pulse (green color).

FIG. 13.

Effect of pulse shaping on the switching performance of the device. (a) Triangular (black color) and rectangular (green color) voltage pulses and (b) corresponding magnetization trajectories ( m z ) with a large precession in the x–y plane for the triangular pulse (black color) than small precession for the rectangular pulse (green color).

Close modal

In 2019, Yamamoto et al. experimentally reported212 the importance of properly optimizing rise and fall times in getting a lower WER. The magnetization may be subjected to additional fields during the rise or fall time because of the uncompensated PMA. Because of this field, torque can pull the magnetization along the in-plane direction during the pulse rise time. This can increase the WER as the pulse rise time increases. With respect to the fall time, they found that a short and properly optimized pulse fall time (to compensate for the damping torque) can reduce the WER.

3. Inverse bias method

A large PMA is necessary to ensure adequate thermal stability of the device. Therefore, a large voltage pulse and/or VCMA coefficient is required to eliminate the PMA to achieve precessional switching. Applying a voltage pulse with an opposite polarity (inverse bias) can resolve the abovementioned issues. The inverse bias pulse applied before and after the write pulse enhances PMA. This, therefore, improves the thermal stability of the device at the initial and relaxation states and can result in a lower WER. This method was experimentally confirmed by Noguchi et al. in 2016.213 The technique has been illustrated in Fig. 14. Figure 14(a) shows the conventional voltage pulse and its associated PMA. Because of thermal noise, there can be m z distribution at the initial or the final states. Figure 14(b) shows the inverse bias method with the voltage pulse of opposite polarity to the write pulse being used before and after the write pulse. The opposite polarity voltage enhances the PMA and reduces the distribution of the m z states, as shown schematically in Fig. 14(b). The inverse bias method has another advantage in terms of pre-read and read-verify steps. As mentioned earlier, VCMA switching is unipolar, and the polarity of the applied voltage cannot define the magnetization direction. Therefore, pre-read is necessary to know the magnetization direction before the writing process. Before the actual write pulse, the inverse bias pulse can be used to pre-read the state. In the same way, after the writing is done, it is essential to verify that the intended bit has been written correctly. This read-verify action can be done using the inverse bias pulse after the actual write pulse.

FIG. 14.

Modulating PMA using (a) the conventional and (b) inverse bias voltage pulse. The inverse bias pulse enhances PMA before and after the write pulse, thus improving the thermal stability of the device. Magnetization distributions for both methods can be seen as confined in a smaller region for the inverse bias method than the conventional method. Reproduced with permission from Ikeura et al., Jpn. J. Appl. Phys. 57(4), 040311 (2018). Copyright 2018 IOP.

FIG. 14.

Modulating PMA using (a) the conventional and (b) inverse bias voltage pulse. The inverse bias pulse enhances PMA before and after the write pulse, thus improving the thermal stability of the device. Magnetization distributions for both methods can be seen as confined in a smaller region for the inverse bias method than the conventional method. Reproduced with permission from Ikeura et al., Jpn. J. Appl. Phys. 57(4), 040311 (2018). Copyright 2018 IOP.

Close modal

In 2018, Ikeura et al.214 quantitatively studied the efficacy of this inverse bias method using macro-spin simulation. They reported a WER in the order of 10 6 using this technique. As mentioned earlier, adequate thermal stability of the MTJ device can reduce the WER. The inverse bias voltage enhances PMA and, therefore, the thermal stability of the device at the initial and relaxation states. Hence, the WER reduces significantly. In 2020, Yamamoto et al. experimentally demonstrated215 the inverse bias effect on the WER. They reported that inverse biases after the write pulse can significantly improve the WER and operation window. However, the inverse bias voltage pulse before the write pulse does not substantially affect the WER or the operation window.

The WER can be reduced further by the write verification schemes. Grezes et al.216 used a write verification process to achieve WER below 10−9 with a total program time of less than 10 ns. Table II and Fig. 15 summarize improvement in the WER reported theoretically and/or experimentally for the abovementioned techniques.

FIG. 15.

Summary of various WER improvement methods in VCMA switching.

FIG. 15.

Summary of various WER improvement methods in VCMA switching.

Close modal
TABLE II.

List of various WER improvement methods in VCMA switching.

WER techniquesMinimum WERsComments
Thermal stability 4 × 10−3 (experiment) and 10−15 (simulation) Ref. 209  Very less WER of 10−15 at α = 0.01 and high thermal stability (Δ > 48).209  
Pulse engineering Order of 10−4 Ref. 212  Precise voltage pulse fall time significantly improves the WER. 
Inverse bias Order of 10−6 Ref. 214  It enhances thermal stability before and after the write pulse and, thus, improves the WER. 
WER techniquesMinimum WERsComments
Thermal stability 4 × 10−3 (experiment) and 10−15 (simulation) Ref. 209  Very less WER of 10−15 at α = 0.01 and high thermal stability (Δ > 48).209  
Pulse engineering Order of 10−4 Ref. 212  Precise voltage pulse fall time significantly improves the WER. 
Inverse bias Order of 10−6 Ref. 214  It enhances thermal stability before and after the write pulse and, thus, improves the WER. 
Another issue with the VCMA switching is a narrow operation window. To achieve deterministic switching, the voltage pulse should be turned off after the magnetization completes half precession cycle.209 The voltage pulse width ( t P W ) can be given by 209,214,217 
t P W = π ( 1 + α 2 ) μ 0 γ H i p ,
(8)
where μ 0 is the vacuum permeability, γ is the gyromagnetic ratio, and H i p is the component of the effective magnetic field in the in-plane direction. Suppose a case when the pulse duration is less or more than the precession period. In that case, the WER value increases because the magnetization trajectories ( m z ) either do not reach the other easy axis state completely or reach and return toward the initial magnetic state. Proper optimization of the pulse duration is, therefore, necessary. It also affects the operation window. There are a few techniques that address the operation window.

1. Device and pulse engineering

VCMA switching produces WER minima for a short duration of the voltage pulse. The thermal fluctuation influences the WER and operation window.207 Unfortunately, only a few researchers addressed the narrow operation window issue.32,192 In 2017, Deng et al.32 studied ultrafast low-energy VCMA switching through macro-spin simulation. They found that the operation window depends on the applied voltage and oxide thickness. The operation window can be widened by adequately optimizing the oxide thickness and properly selecting the applied voltage. Reducing the amplitude of the voltage pulse enhances the effective PMA field. This increases m z and reduces m x, where m x is the x-component of m . This reduces the effective anisotropy field along the x axis. The precession period depends inversely on this field component [Eq. (8)],207 so the precession period increases, resulting in a more extensive operation window. Therefore, the device can tolerate more variations in the voltage pulse width. They found an operation window of 0.15 ns.32 

Later, in 2018, Deng et al. reported192 an improvement in the operation window up to 0.8 ns. As discussed earlier, the internal Rashba field can be used instead of an external field. This Rashba field can be controlled with the help of the current [Eq. (7)] that is being passed through the HM. By optimizing this current, the Rashba field and operation window can be enhanced. In 2020, the operation window and WER improvement were addressed by Yamamoto et al.215 They reported improvement in the operation window using properly optimized inverse biases before and after the write pulse. Matsumoto et al. theoretically studied the switching performance of the pMTJ device using the VCMA technique at room temperature.218 They reported VCMA switching with low WER for a range of voltage pulses. They assumed pulse width as tolerance pulse or operation window for which the WER lies between 2 × 10 5 and 10 3.218 They reported a long tolerance of few ns where WER values are in the range of 2 × 10 5 10 3.

In 2021, One et al.217 studied VCMA switching using detailed macro-spin simulation. As per their analysis, important switching behavior parameters are the damping constant and the anisotropy field modulation ( p ). p is given as a percentage proportion of the anisotropy field ( H K ), and it can be defined by the relation p = 1 H K / H K 0, where H K 0 is the amplitude of the anisotropy field in the absence of the electric field. They showed that for a certain value of the critical modulation ( p c = 1 2 H i p / H K 0 ), the magnetization will always lose energy and switch to the energy minima, irrespective of the voltage pulse width. One can design and operate the device in the critical modulation bands to improve the operation window significantly.

2. Circuit technique

The narrow operation window in the VCMA technique can be widened not only by the parameters and pulse optimization but also by the external circuit design. In 2019, Cai et al. reported219 VCMA-based magnetic memory design in fully depleted silicon on insulator (FD SOI) assisted by body bias. They explored the 1MTJ-1T (one MTJ and one transistor) structure, varying the transistor size and the VCMA voltage pulse (amplitude and width). They reported that high switching probability could be achieved for an extensive range of voltage pulse width by adequately selecting the supply voltage (more than 1.2 V) and the access transistor dimension (large dimension, i.e., W = 400 nm).219 

The switching performance of each MTJ device in a memory array can be different due to process variations during the fabrication. Therefore, the WER may vary from cell to cell. A write pulse termination (WPT) circuit scheme can address this issue.220 The WER value can be lowered for a range of voltage pulse widths using the WPT technique. This technique applies a voltage pulse to the selected cell where switching failure happens and, thus, improves the switching performance of the device.220 In 2016, Grezes et al. experimentally reported70 VCMA-induced switching in the pMTJ device of different dimensions. They observed that switching voltage and time are insensitive to the junction diameter in high-resistance MTJs. They obtained a relatively high switching probability for an operation window of 0.2 ns.70  Figure 16 illustrates the summary of different techniques to widen the operation window.

FIG. 16.

Summary of various techniques to widen the operation window in the VCMA switching.

FIG. 16.

Summary of various techniques to widen the operation window in the VCMA switching.

Close modal

A lower switching voltage is desirable for the practical implementation of VCMA-based memories. This can be accomplished by fine-tuning the oxide thickness, thermal stability of the device at zero voltage, MTJ cross-sectional area, and VCMA coefficient, as seen in Eq. (3). Equation (3) suggests that reducing the oxide thickness can decrease switching voltage, but the tunneling current may increase. It causes more Ohmic loss as the STT effect may dominate. Furthermore, for smaller devices, a large value of PMA is needed for the same thermal stability. This causes a large VCMA coefficient or write voltage. Since a lower switching voltage is pivotal for technological application, we require an immense VCMA coefficient. There are quite a few mechanisms to enhance the VCMA coefficient, as mentioned below.

1. HM insertions at oxide and FL interface

It has been proved theoretically and experimentally that inserting HM (with high SOC material like Ir or Os) at the FM/dielectric interface can increase the PMA and VCMA coefficient.95,184,203,221–223 Ab initio computations provide a straightforward theoretical approach to studying the HM insertion at the Fe/MgO interface. However, experimental challenges arise due to the interdiffusion of atoms across different layers during the annealing process.124 This causes deviation between the theory and experiment. Nozaki et al. investigated222 how a CoFe termination layer affects the VCMA coefficient and tunneling magnetoresistance (TMR) in the Ir-doped Fe/MgO structures. They reported a large β of −350 fJ/V m. Furthermore, the lattice mismatch between the layers (Ir and CoFe) can induce a strain. Kato et al. experimentally demonstrated224 a giant β (thousands of fJ/V m) in a structurally strained CoFe layer in a MgO/CoFe/Ir system. Using the ab initio computations, it is found that strain can affect the VCMA coefficient. An extremely large value of β (>10000 fJ/V m) was predicted in an atomically thin Ir dusting layer (monolayer) in the Fe/MgO system.184,203 However, such a high VCMA coefficient is yet to be demonstrated experimentally. The metal insertion may negatively impact the TMR in some cases. Therefore, thorough research is needed to optimize these parameters precisely.

2. Interface oxidation state controlling

As previously mentioned, an electric field at the FM/oxide interface, such as the CoFeB/MgO case, can alter the occupancy of hybridized orbitals by causing either charge accumulation or depletion. This has been confirmed by ab initio computations.116,225 Moreover, theoretical investigations suggest that the VCMA coefficient can be significantly influenced by the oxidation level at the Fe/MgO interface.226,227 The VCMA coefficient can be enhanced by properly optimizing the oxidation level at the CoFe/MgO interface. The mechanism of over-oxidation and oxygen migration has been illustrated in Fig. 17. In the former case, additional oxygen atoms are introduced into the Fe monolayer, while later, the oxygen atoms are migrated from the MgO interface toward Fe. In 2017, Li et al. studied228 the effect of the insertion layer X (where X can be Ta, Pt, and Mg) on the VCMA coefficient. They found an increment in the VCMA coefficient (100 fJ/V m) by three times that of the conventional Ta/CoFeB/MgO-based structure (∼30 fJ/V m).229,230 Precise control of the thickness of the Mg atom insertions results in either an under-oxidation or over-oxidation state at the CoFe/MgO interface. The maximum VCMA coefficient (100 fJ/V m) is achieved when the CoFe layer undergoes intense over-oxidation.228 In a recent study, Nozaki et al. investigated231 the effect of post-oxidized MgAl layer insertion on the PMA, TMR, and VCMA of the epitaxial MTJ device. A large β (∼300 fJ/V m) with reasonable PMA was observed by precise interface engineering. Therefore, the VCMA coefficient and PMA can be improved by precise control of MgAl insertion.

FIG. 17.

Various mechanisms to enhance the VCMA coefficient. The blue color, red color, green color, and orange color atoms are Fe, Ir, O, and Mg, respectively. Ir insertion figure: Reproduced with permission from Kwon et al., Phys. Rev. Appl. 12(4), 044075 (2019). Copyright 2019 American Physical Society. Controlling oxidation state figure: Reproduced with permission from Ibrahim et al., Phys. Rev. B, 98(1), 214441 (2018). Copyright 2018 American Physical Society.

FIG. 17.

Various mechanisms to enhance the VCMA coefficient. The blue color, red color, green color, and orange color atoms are Fe, Ir, O, and Mg, respectively. Ir insertion figure: Reproduced with permission from Kwon et al., Phys. Rev. Appl. 12(4), 044075 (2019). Copyright 2019 American Physical Society. Controlling oxidation state figure: Reproduced with permission from Ibrahim et al., Phys. Rev. B, 98(1), 214441 (2018). Copyright 2018 American Physical Society.

Close modal

3. Dielectric constant of the oxide layer

As mentioned earlier, VCMA originates from the accumulation or depletion of charges at the FM/oxide interface. Significant charge accumulation corresponds to a large VCMA coefficient. Quantitatively, it can be expressed as proportional to the relative dielectric constant ( Δ Q / Δ E A s = ε r ε 0 ),232,233 where Δ Q, Δ E, A s, ε r, and ε 0 are the charge accumulation, applied electric field, surface area, relative permittivity of the oxide layer, and vacuum permittivity, respectively. Therefore, a significant value of the VCMA coefficient can be achieved using an insulating barrier with high-κ materials, such as SrTiO3233,234  or HfO2235–237 However, the MTJ device with these materials exhibits poor TMR.238 Chien et al. experimentally studied the VCMA coefficient and TMR in a MgO/PZT/MgO energy barrier-based system.232 They found enrichment in β by 40% for the MgO/PZT/MgO barrier (19.8 ± 1.3 fJ/V m) than the normal MgO barrier-based (14.3 ± 2.7 fJ/V m) MTJ device. They also found a reasonable TMR value of more than 50%.

The thermal fluctuation during the read operation led to a large RDR. This issue was addressed by Lee et al. in 2016.239 They proposed a source line sensing (SLS) scheme other than the conventional bit line sensing (BLS) scheme. This SLS scheme, shown in Fig. 18, utilizes the VCMA effect and imposes a voltage with opposite polarity to that of the MTJ voltage during the sensing process. The concept of the reverse polarity voltage pulse compared to the write voltage pulse was introduced by Wang et al. in 2015.73 The RDR reduced significantly with this scheme.239 They also found that an enhancement in coercivity during the read operation resulted in an improved sensing margin. In 2016, Grezes et al. experimentally reported216 that device performance (WER and RDR) depends on the read/write voltage pulse amplitude and duration. They reported a WER less than 10 9 ( 10 17 ) with 10 ns (20 ns) write time, and an RDR less than 10−16 with 2 ns read time in a 256 kbit MeRAM.

FIG. 18.

Circuit design to mitigate read disturbance and minimize the read delay during sensing. During the read operation mode, sense voltage, write voltage, bit line, and source line are at high, ground, ground, and high potential, respectively. Reproduced with permission from Lee et al., IEEE Magn. Lett. 7, 1–5 (2016). Copyright 2016 IEEE.

FIG. 18.

Circuit design to mitigate read disturbance and minimize the read delay during sensing. During the read operation mode, sense voltage, write voltage, bit line, and source line are at high, ground, ground, and high potential, respectively. Reproduced with permission from Lee et al., IEEE Magn. Lett. 7, 1–5 (2016). Copyright 2016 IEEE.

Close modal

In the MTJ device, the RA product indicates whether the STT or VCMA effect dominates. A thicker oxide layer increases the RA product, enhancing the VCMA domination but also lengthening the read time.240 However, the total read delay can be balanced with the signal transmission latency on the memory chip due to VCMA exhibiting higher chip density than STT- and SOT-based MRAMs.

In summary, the VCMA-based MRAM is recognized as an emerging and energy-efficient technology, notable for its negligible Ohmic loss and rapid switching capabilities. This article covers several challenges in VCMA switching and recent advancements toward addressing these issues, such as field-free switching operation, WER improvement, widening the operation window, improving the VCMA coefficient, RDR, and read delay. Some field-free switching techniques, such as VGSOT and Rashba field methods, can provide a low WER, but these need an extra terminal that can penalize the chip density. Additionally, they require extra current or voltage pulse. VCMA switching using the self-regulated precessional switching using double-barrier device structure, skyrmion, strain, and CFL techniques is interesting but yet to be demonstrated experimentally. Strain can influence the exchange bias, PMA, and VCMA coefficient. Therefore, optimizing this effect could enable us to enhance device performance. Recently, our group has demonstrated simulation-based SOT switching in the CFL MTJ device with circular nanomagnets without needing an external field. Similar investigations could also be extended to VCMA switching.

A large WER is another important issue that impedes the use of these techniques for practical magnetic memory applications. Although a few WER improvement methods, such as increasing barrier height and reducing damping constant,209 engineering of the pulse shape,212 and inverse bias,213,214 have been proposed, they are still not enough to fulfill the requirements for practical MRAM realizations. Other methods for improving WER, such as error correction through multiple writings and write verification, can also reduce the WER. However, these methods may decrease the operational speed of the device. Consequently, thorough research is necessary to tackle this challenge and further enhance the switching performance of the device. A narrow operation window is a significant challenge with VCMA switching, and while some researchers have addressed it, more work is needed to enhance device robustness against pulse width variations. A high VCMA coefficient value can reduce the required voltage, but its immense value and adequate TMR are yet to be demonstrated experimentally. The RDR is another issue with VCMA switching. Although a few researchers have tried to address this, substantial work must be done to make it suitable for practical memory applications.

We acknowledge the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India, under Grant Nos. CRG/2022/007360 and EEQ/2020/000164 for support.

The authors have no conflicts to disclose.

Pinkesh Kumar Mishra: Conceptualization (lead); Data curation (lead); Formal analysis (equal); Investigation (lead); Methodology (equal); Resources (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (equal). Meenakshi Sravani: Conceptualization (supporting); Validation (supporting); Writing – review & editing (supporting). Arnab Bose: Investigation (supporting); Supervision (supporting); Validation (supporting); Visualization (equal); Writing – review & editing (equal). Swapnil Bhuktare: Conceptualization (equal); Funding acquisition (lead); Investigation (lead); Methodology (equal); Supervision (lead); Validation (supporting); Visualization (supporting); Writing – original draft (supporting); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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