Magnetic tunnel junctions using epitaxially grown FeAlSi electrode with soft magnetic property

Magnetic tunnel junction (MTJ)-based magnetic sensors using the tunnel magnetoresistance (TMR) effect are highly promising next-generation devices for detecting weak magnetic signals such as bio-magnetic fields. In recent years, the detectivity of MTJ sensors has improved dramatically¹ and is very close to that of superconducting quantum interference devices (SQUIDs),² which are currently the most sensitive magnetic sensors. In addition, compared with SQUIDs, MTJ sensors have advantages such as small size, low cost, and wearability, enabling easy measurement of bio-magnetic fields and the widespread use of bio-magnetic medical diagnostics. Furthermore, since weak cerebral magnetic fields are measured with high spatial resolution, it contributes to the further understanding of the brain function and to the development of BMI technology, with the sensitivity sufficient to detect cerebral fields in real time (∼100× compared to the current value).^1,3 Thus, MTJ sensors have a wide range of potential applications not only in the medical field but also in the fields of neuroscience and IT, so further improvement of their sensitivity is strongly required.

Since the sensitivity of a TMR sensor is defined as TMR/2H_k,⁴ where H_k is the magnetic anisotropy field of the free layer, ferromagnetic materials with both a high TMR ratio and low H_k are required. Previous studies have investigated soft magnetic materials such as NiFe alloys,^5–8 amorphous CoFeSiB,^1,9,10 and perpendicular CoFeB,^11–13 and, for example, there is a report of successful observation of MCG using an MTJ sensor based on the synthetic ferrimagnetic free layer.^1,14 However, the signal-to-noise ratio for MEG measurements is still low, and the sensitivity of MTJ sensors using the above-mentioned conventional materials is gradually reaching a ceiling. Therefore, the sensitivity of MTJ sensors needs to be dramatically improved to measure very small bio-magnetic fields, including cerebral fields. For this purpose, the approach of investigating new soft magnetic materials, which are completely different from conventional materials, and their potential for MTJ sensor applications is quite interesting.

In this study, we focused on the Sendust alloy (Fe₈₅Al_5.4Si_9.6 wt. %; hereinafter “FeAlSi”) as a new free-layer material. The soft magnetic properties of Sendust alloys have been utilized for the read/write head of magnetic recording¹⁵ and are very sensitive to their composition.^16,17 The permeability significantly increases near the Sendust center composition of Fe₈₅Al_5.4Si_9.6,¹⁸ which is close to that of pure Fe. In addition, since FeAlSi has the D0₃ structure, which is the ordered structure of bcc-A2,^15,19 it is similar in composition and structure to bcc-Fe and is expected to have a high TMR ratio due to $Δ$₁ coherent tunneling, as in previous studies.^20–22 Therefore, there is a possibility to achieve both high TMR and low H_k by using FeAlSi as a free layer, so this is very much worth investigating for achieving highly sensitive MTJ sensors.

Ferromagnetic materials used in MTJs need to be thin films of nm order. Although there have been several reports on FeAlSi films of μm order,^23–29 our previous study is the first report on FeAlSi films of nm order. We developed 30 nm-thick epitaxial FeAlSi films with excellent soft magnetic properties superior to other typical soft magnetic materials such as NiFe or CoFeSiB in our previous study by fine-tuning the composition and annealing temperature, as shown in Fig. S1.³⁰ In addition, we studied MTJs with 30 nm-thick polycrystalline FeAlSi films on MgO buffer layers and first observed the TMR effect (TMR ratio = 35.9%) of MTJs with the FeAlSi electrode with relatively low magnetic anisotropy.³¹ However, the measured TMR ratio and H_k were inferior to those of other MTJs using typical free layer materials such as NiFe or CoFeSiB because of low crystallization for FeAlSi polycrystalline films. In the current study, we used FeAlSi epitaxial films as a free layer of MTJs for two objectives. The first objective is to clarify the potential of MTJs with an FeAlSi free layer as TMR sensors by investigation of the TMR effect in epitaxially grown MTJs. By revealing its high potential with this study, the development of polycrystalline TMR sensors with the FeAlSi electrode prepared on conventional SiO₂ substrates will be accelerated. Another objective is to realize ultimate TMR sensors using epitaxially grown MTJs, which are expected to have a high TMR ratio and low noise due to their high crystallinity and will be used in special applications for detecting ultra-weak magnetic fields. From the above-mentioned research motivation, in this study, FeAlSi epitaxial films with soft magnetic properties and a flat surface were prepared on MgO single-crystal substrates, and the annealing temperature dependence of TMR properties was investigated.

All films were deposited on MgO (001) substrates using DC/RF magnetron sputtering (P_base< 2 × 10⁻⁶ Pa). The Ar gas pressure was 1.0 Pa for preparing the MgO barrier layer and 0.1 Pa for the other layers. The composition of FeAlSi films and their annealing condition were optimized.³⁰ The composition of the sputtering target was Fe_81.6Al_10.7Si_7.7 (wt. %), and the composition of prepared FeAlSi thin films measured by inductively coupled plasma (ICP) spectrometry was Fe_85.7Al_8.1Si_6.2. The FeAlSi layer was annealed in the sputtering chamber after the deposition without applying a magnetic field. The annealing temperature (T_FeAlSi) and duration were 400 °C and 1 h, respectively. The stacking structure to investigate the crystal structure, magnetic properties, and surface properties of the FeAlSi free layer is as follows: MgO-sub./MgO(20)/FeAlSi(30)/Ta(5) (thickness in nm). The MgO(20) layer is a buffer layer to epitaxially grow FeAlSi films with high (001) orientation and a flat surface. The measurements for the structural, surface, and magnetic properties were performed by x-ray diffraction (XRD) and atomic force microscopy (AFM) and using a vibrating sample magnetometer (VSM), respectively.

The stacking structure for investigating the TMR properties of MTJs with the FeAlSi free layer is as follows: MgO-sub. /MgO(20)/FeAlSi(50)/MgO(2.0)/CoFeB(3)/Ru(0.85)/CoFe(5)/IrMn(10)/Ta(5)/Ru(10). FeAlSi free layers were annealed at 400 °C in the sputtering chamber for 1 h after the FeAlSi deposition. Thick FeAlSi films of 50 nm were also used as bottom electrodes of MTJs. The deposited MTJ films were microfabricated using photolithography and Ar ion milling. The bottom electrode was patterned into rectangles with 1200 × 300 µm² (the end point of the ion milling was the surface of the MgO substrate). The pillar of MTJs was patterned into rectangles of 80 × 40, 40 × 20, and 20 × 10 µm² (the end point of the ion milling was the middle of the MgO barrier). Microfabricated devices were annealed (T_a) in a vacuum annealing furnace for 1 h with a magnetic field of 10 kOe to fix the magnetization of the pinned layer and improve crystallinity of the MgO barrier.³² 

First, the results of the characterization of the FeAlSi free layer are summarized. Figure 1(a) shows XRD patterns (2θ-θ scan) for the FeAlSi free layer annealed at T_FeAlSi = 400 °C. The FeAlSi (004) A2 peak was observed, and we confirmed crystallization of the (001)-oriented epitaxial FeAlSi films on the MgO substrates. The result indicates that the (001)-oriented MgO barrier for $Δ$₁ coherent tunneling can be formed on the (001)-FeAlSi bottom electrode. Moreover, the presence of the (002) B2 peak showed the films with a B2-ordered structure. Figure 1(b) shows the AFM image for the FeAlSi free layer annealed at T_FeAlSi = 400 °C. The bottom electrode with a flat surface is needed to suppress the electric current leak and orange-peel coupling between free and pinned layers through a very thin MgO tunneling barrier of 2.0 nm in the MTJ structure.³³ The AFM image shows a relatively smooth surface, and the average roughness is 0.17 nm, comparable to another bottom free layer in previous work.³⁴ Figure 1(c) shows the magnetization curves for the FeAlSi free layer applying the magnetic field along the [100] and [110] directions of the FeAlSi free layer, where [100] is a magnetic easy axis while [110] is a hard axis. The coercivity for the easy axis shows a low value of 1.7 Oe, reflecting the soft magnetic property of FeAlSi. The results indicate that the fabricated (001)-oriented FeAlSi free layer with a flat surface and soft magnetic property can be applied to MTJs for TMR sensors.

Figure 2(a) shows the annealing temperature dependence of magnetoresistance curves for MTJs using FeAlSi free layers with a bias voltage of 10 mV. The applied fields were along the exchange bias of the pinned layer (FeAlSi [110] direction). We first observed the TMR effect in spin-valve type MTJs using an FeAlSi epitaxial electrode at room temperature (RT). The TMR ratio depended on the annealing temperatures, and the highest TMR ratio was 121% with an annealing temperature of 325 °C. The observed high TMR ratio over 100% infers that $Δ$₁ coherent tunneling through the crystallized MgO barrier is realized and the observed TMR ratio is comparable to those of other MTJs for TMR sensors.^1,6,8,11,35 Since the FeAlSi films prepared by optimum target composition and annealing temperature showed a very low H_k of 0.43 Oe, as shown in supplementary material Fig. S1,³⁰ the potential sensitivity is ∼140%/Oe, which is larger than those for MTJs with an amorphous CoFeSiB free layer.^9,11

We discuss the annealing temperature dependence of the TMR effect in more detail. Figure 2(b) summarizes the annealing temperature dependence of the TMR ratio and resistance area product (RA). We evaluated RA values by fitting the R vs A⁻¹ plot, where we use the value of R_min in a parallel magnetic configuration as R. An example for T_a = 325 °C is shown in supplementary material Fig. S2 as a typical example of a plot of R vs A⁻¹. The TMR improvement with increasing annealing temperature is due to the promotion of $Δ$₁ coherent tunneling by crystallization of the MgO barriers, and this leads to RA reduction from 250 to 325 °C. TMR degradation above 325 °C is due to deterioration of the synthetic pinned layer with over-annealing. As shown in Fig. 2(a), the shift field for the pinned layer decreased above 325 °C. To clarify this reason, we performed VSM measurements for MTJ films annealed at T_a = 325 and 400 °C, as shown in supplementary material Fig. S3. We confirmed that the exchange bias due to the CoFe/IrMn interface was mostly maintained even after the annealing at 400 °C whereas the RKKY synthetic anti-ferromagnetic coupling through Ru was degraded. Therefore, the dominant reason for the decrease in the shift field due to over-annealing was Ru diffusion, not Mn diffusion.^36–38 However, further investigations will be needed to clarify the mechanism of relatively low durability against the annealing process in detail using transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) measurements in the future. The degradation of pinned layers causes the reduction in the TMR ratio above 325 °C. In the magnetoresistance curve for MTJs with the highest TMR ratio (T_a = 325 °C), we found a relatively low switching field of 4 Oe, reflecting the soft magnetic property for the FeAlSi free layer, but this value is slightly larger than the coercivity observed in the FeAlSi single layer shown in Fig. 1(c). The increase in coercivity for MTJs can be caused by the difference in FeAlSi thickness and shape magnetic anisotropy because the FeAlSi layer was micro-fabricated into a rectangular shape of 1200 × 300 µm² for the bottom electrodes. Since the direction of applying the magnetic field for MR curve measurements is along the magnetic easy axis of shape anisotropy (longitudinal direction of the FeAlSi electrode), the switching field increases. Lower magnetic anisotropy can be realized by optimizing the thickness and shape of the FeAlSi bottom electrode in the future works.

Finally, we discuss in detail the interface between the ferromagnetic and insulating layer and tunneling processes. In Fig. 2(b), the observed RA with the order of 10⁶ Ω μm² was relatively higher than that in previous work of MTJs with a high TMR ratio.^1,35 This is possibly caused by oxidation of the interface between the FeAlSi bottom layer and the MgO barrier. Figure 3(a) shows the cross-sectional TEM image of the MTJ using an FeAlSi free layer annealed at T_a = 325 °C. We observed the very thin bright contrast layer at the interface between the FeAlSi free layer and the MgO barrier. This contrast possibly implies oxidization of the FeAlSi layer surface, and it can cause a significant reduction in the TMR ratio.³⁹ However, detailed chemical analyses, such as EELS and x-ray photoelectron spectroscopy (XPS), will be needed to confirm the surface oxidation of FeAlSi layers. Figure 3(b) shows the bias voltage dependence of the normalized TMR ratio and typical tunnel conductance for the prepared MTJ annealed at T_a = 325 °C using the FeAlSi free layer. The direction in which the current flows from the top electrode to the bottom electrode is defined as the positive direction of the bias voltage. V_half, where the TMR ratio becomes a half of the zero-biased value, was 0.36 V, which is less than the value of typical MgO-based MTJs (∼0.7 V) in a previous work.⁴⁰ Low bias voltage dependence of G_p shows a large gap between the bottom edge of the conduction band and the top edge of the valence band for both ferromagnetic layers of FeAlSi and CoFeB. Therefore, the reduction in the TMR ratio with increasing bias voltage is caused by voltage dependence of G_AP originating from an inelastic tunneling process by magnetic impurity scatterings.^41,42 There are several possible causes of inelastic tunneling, such as oxidation of the FeAlSi interface and defects in the MgO barriers. If the dominant reason for bias voltage dependence was oxidation of the FeAlSi bottom electrode, then the TMR ratio with lower RA can be improved by suppressing the oxidation of FeAlSi by inserting an anti-oxidation layer such as Mg at the interface between FeAlSi/MgO. We believe that the MTJs with an FeAlSi electrode with soft magnetic property will be a promising device for high-sensitivity TMR sensors.

In summary, we fabricated (001)-oriented FeAlSi epitaxial films on MgO(001) single-crystal substrates and found a flat surface and soft magnetic property in prepared films. The prepared FeAlSi films were applied to the free layer of MTJs, and we investigated their magnetoresistance properties. We first observed the TMR effect of MTJs using epitaxial FeAlSi films at room temperature, and the TMR ratio was a high value of 121% comparable to those of other typical MTJs for TMR sensors. The switching field of the free layer was relatively low, reflecting good soft magnetic properties of FeAlSi films. The FeAlSi film is a promising free layer material for highly sensitive TMR sensors. On the other hand, according to the results for TEM and bias voltage dependence of the TMR ratio, the interface between FeAlSi and MgO is supposedly able to be degraded by FeAlSi surface oxidation.

See the supplementary material for (1) VSM results for FeAlSi films prepared by optimum target composition and annealing temperature, (2) a typical example of a plot of R vs A⁻¹, and (3) VSM results for MTJ films annealed at T_a = 325 and 400 °C.

This work was supported by the Japan Society for the Promotion of Science (JSPS), the Japan Science and Technology Agency (JST) S-Innovation Program, the Center for Science and Innovation in Spintronics (CSIS, Tohoku University), the Center for Spintronics Research Network (CSRN, Tohoku University), the Instrumental Analysis Group (Technical Division, School of Engineering, Tohoku University), GP-Spin at Tohoku University, the ANRI Fellowship, and Tohoku University.