Ferroelectric and magnetic properties of multiferroic BiFeO₃-La_0.7Sr_0.3MnO₃ heterostructures integrated with Si (100)

Ferroelectric (FE) and multiferroic materials^1,2 have been drawing increasing attention owing to their extraordinary multifunctional properties. Of particular note, BiFeO₃ (BFO) shows multifunctional behavior due to coexistence of FE and antiferromagnetic (AFM) ordered parameters above room temperature. The lead-free composition and above room temperature multi functionality make BFO an attractive choice for a wide variety of applications in terms of sensors, memories, and spintronic devices. With the objective of addressing low power, non-volatile memory applications, there has been widespread interest, over the past decade, in utilizing the FE-AFM and FM (external)-AFM couplings that are present in layered films composed of a multiferroic BFO in combination with FM overlayers, such as La_0.7Sr_0.3MnO₃ (LSMO) to achieve electric-bias induced magnetic moment switching. Along that direction, there have been numerous studies reported on the heterostructures deposited on closely lattice matched insulating substrates, such as DyScO₃ (DSO), NdGaO₃ (NGO), and SrTiO₃ (STO). These reports demonstrate the potential that BFO thin films have for ferroelectric random access memory (FeRAM) device applications.

Single phase multi ferroics with large room-temperature polarization and permeability do not exist, thus, researchers have started exploring the two phase multiferroic materials. The combination of BFO and ferromagnetic LSMO is a model system for studying cross-coupling between different ferroic order parameters, and thus is drawing much attention because of its potential for providing E-field control of magnetism in magnetic devices at relatively low levels of power consumption. For instance, the formation of novel magnetic states has been observed³ at the interface between multiferroic BFO and ferromagnetic LSMO when they were deposited on lattice matched STO substrates. In another study reported⁴ by You and co-authors, it was shown that a uniaxial magnetic anisotropy could be artificially induced in LSMO thin films deposited onto BFO by introducing electrically patterned stripe domains. The magneto crystalline anisotropy of the LSMO was reported to be due to the shear strain produced by the BFO lattice. Unfortunately, the oxide substrates used to date are incompatible with CMOS-based technologies, where Si (100) has been the traditional substrate material. In previous work,⁵ we have epitaxially integrated BFO/LSMO heterostructures on Si (100) using pulsed laser deposition (PLD). This was accomplished by invoking a unique buffer layer approach using “domain matching epitaxy” (DME)⁶ to deposit multi-layer oxide-heterostructures on Si (100). Using this same approach, we have epitaxially deposited room temperature ferroelectric BaTiO₃ on Si (100).⁷ 

In this paper, we have measured the electrical, ferroelectric, and anisotropic magnetic properties of BFO/LSMO bilayers deposited on buffered Si (100). Notably, we observed a switchable diode effect and ferroelectric induced resistive switching (RS) at low temperatures (200–300 K) in Pt/BiFeO3/LSMO thin film capacitors prepared on Si (100) substrates. Anisotropic isothermal magnetization data were collected at various temperatures and provide a clear indication of uniaxial magnetic anisotropy being present in the BFO/LSMO bilayer system.

We have prepared BFO/LSMO samples using pulsed laser deposition and characterized them using several techniques, as reported in our earlier work. This work is facilitated by the epitaxial deposition of an TiN (a = 4.24 Å) layer. TiN was chosen because it grows epitaxially on Si(100) and has superior diffusion barrier properties, consistent with the principles of DME. Subsequent layers of MgO (a = 4.22 Å) and STO (a = 3.905 Å) were then sequentially deposited epitaxially onto the previous layers. MgO has an excellent lattice match with TiN and a misfit of about 8% with STO. Finally, the lattice constant of STO (3.905 Å) matches closely with that of LSMO (3.85 Å) and BFO (3.966 Å). This unique selection of buffer layers composed of large (>8%) and small (<2%) lattice mismatch pairings made it possible to integrate epitaxial thin film of BFO/LSMO on Si(100). The BFO (100-nm thick)-based devices were fabricated with LSMO (250 nm thick) acting as the bottom electrode and multiple 200 μm circular top Pt electrodes.

To probe the ferroelectric characteristics at room temperature, we have used switching spectroscopy piezo-response force microscopy (SSPFM). For this purpose, the commercial scanning probe microscope (Cypher, Asylum Research) equipped with a Pt-coated conducting tip (AC240TM, Olympus) was operated at a resonance frequency of about 260 kHz and the ac bias amplitude of 2 V. The same setup was augmented to carry out advanced SSPFM measurements. We used a 25 × 25 grid on 2 μm × 2 μm scan area to map the local polarization switching using a variable tip bias between +6 and −6 V. I-V characteristics of BFO/LSMO capacitors were measured at different temperatures using a Keithley 4200 semiconductor characterization system. The sample was mounted on a liquid N₂ cooled chuck in the absense of ambient light. Temperature- and magnetic-field dependent magnetization measurements were carried out using a Quantum design MPMS SQUID VSM dc magnetometer with a sensitivity ≤10⁻⁸emu at 0 T. It should be noted that TiN, MgO, and STO buffer layers are all non-magnetic, and hence, are not expected to contribute to the magnetic properties of BFO/LSMO heterostructures that are presented in this work. For magnetization measurements, a 4 mm × 4 mm piece was cut from each sample. Measurements were performed at various temperatures with the magnetic field applied either parallel or perpendicular to the film plane. Great care was taken to avoid magnetic contamination of the samples.

The out-of-plane (OOP) XRD pattern⁵ shows that all the layers have a preferential (00l) orientation, suggesting the epitaxial growth of the multilayered structure. The phi-scan (in-plane) pattern shows four peaks separated by ∼90° indicating its pseudo cubic/rhombohedral symmetry and establishing the cube-on-cube relationship of the BFO with the underlying substrate Si (100). The bright-field cross-section TEM image⁵ of the sample shows the thicknesses of BFO and LSMO to be ∼100 and 250 nm, respectively. HREM images⁵ taken at the BFO/LSMO and LSMO/STO interfaces show that they are sharp and clean, with no evidence of inter-diffusion or secondary phase formation. Our high resolution XPS data show finger print signatures for the presence of Fe³⁺ with no indication of Fe²⁺. The magnetization data⁵ show that the Curie temperatures (T_C) are of ∼350 K, consistent with the reported value for bulk LSMO.

Figure 1 shows the typical leakage current (I-V curves) measured at various temperatures ranging from 200 to 350 K. The bias voltage was swept from 0 V → 5 V → 0 V → −5 V → 0 V. The I-V curves shift upward with increasing temperature (particularly, the curve recorded at 350 K) and exhibit non-leaky behavior up to at least 50 cycles (not shown). The curves are asymmetric as a consequence of the different work functions of Pt and LSMO electrodes. The literature⁸ has identified three current limiting mechanisms that can be active in BFO and other similar ferroelectric perovskite oxides deposited on lattice matching substrates, such as SrTiO₃. The first mechanism is interface-limited Schottky emission, which arises from a difference in Fermi levels between a metal electrode and an insulator or semiconductor film. The energy difference creates a potential barrier between the metal and insulator that charges must overcome. The second mechanism frequently cited is bulk-limited space-charge-limited conduction (SCLC). Current limitation arises from the formation of a current impeding space charge that develops as charges are injected into the film from the electrode at a rate faster than they can travel through the film. The third mechanism is bulk limited Poole-Frenkel emission. This conduction mechanism involves the consecutive hopping of charges between defect trap centers. The ionization of the trap charges can be both thermally and field activated.

To test the applicability of these models for our heterostructures, we have plotted (not shown) our data (at 200 K, from 1 to 5 V, positive bias) in the form of current density versus square of voltage (J vs V²); J/T² (T is temperature) vs V^1/2; conductivity (σ) vs V^1/2 coordinates to test the validity of bulk-limited SCLC, interface-limited Schottky emission, bulk limited Poole-Frenkel emission mechanisms, respectively. From our analysis, we noticed that the trends did not follow the expected straight line behavior that is characteristic of SCLC, but, did generally fit the linear behaviors predicted for the remaining two cases. Thus far, our data show Schottky and Poole-Frenkel leakage mechanisms are operational and we are planning to test these devices by varying the top electrode diameter and other growth conditions.

The I-V curves exhibit rectifying behavior, which is attributed to the asymmetric barrier heights at the two electrode/film interfaces. In addition, bipolar RS of BFO is revealed by the strong I-V hysteresis under positive and negative bias, which is prominent at low temperatures (200 K) and disappears at 350 K. Such strong hysteresis in the I-V curves has been observed in epitaxial BFO (Ref. 9) and Ca doped BFO (Ref. 10) films, where the authors attributed the hysteresis to the redistribution of oxygen vacancies and interplay between ionic and electronic conduction. We attribute this resistive switching to the ferroelectric switching^9,11 of BFO that occurs at around 5 V (see below our PFM data). However, we cannot rule out the possible additional influence of oxygen vacancies when electric field was applied.

To demonstrate polarization switching of the films, a dc bias was applied between the conducting PFM probe and LSMO bottom electrode while scanning over the desired areas. The ferroelectric local switching behavior of our films is shown in Fig. 2. The applied dc sweep voltage is a triangular pattern increasing from 0 to +6 V then down to −6 V and finally back to 0 V; the measurements are repeated five times to improve the signal to noise ratio. As illustrated in Fig. 2, the characteristic butterfly loops (of several cycles) were observed in out of plane PFM amplitude (black) signals of all the BFO films. In addition, the phase signal (blue) indicated a clear switching behavior at a voltage of 4–5 V, providing unambiguous evidence¹² for the occurrence of ferroelectricity in the BFO films. The ferroelectricity remained mostly unaffected throughout the measurement grid and over multiple cycles of voltage applications.

Figures 3(a) and 3(b) display isothermal in-plane (red) and out of plane (black) isothermal M-H data collected at temperatures 5 and 300 K with the magnetic field varied from +1000 to −1000 Oe, respectively. As it can be seen, in the direction parallel to the film plane, the sample shows saturated M-H loops, with a coercive field H_c as high as 300 Oe measured at 5 K. As expected for ferromagnetic material, it can be noticed that the coercive field and magnetic moment decreases with increasing temperature. A small moment and no magnetic hysteresis behavior are seen in the direction perpendicular to the film. Magnetic exchange anisotropy has been carefully ruled out by the insertion of a thin nonmagnetic STO interlayer, which has previously been shown to interrupt exchange coupling between BFO and LSMO films.⁵ As presented in Fig. 3(c), the out of plane magnetic behavior retains its as-deposited character even when the sample was cooled under different magnetic fields (varying from 0.2 to 1 T) and polarity. This behavior is typical of that expected¹³ for a film with its easy direction lying in the plane of the film, common for systems showing uniaxial magneto anisotropy. Such uniaxial magnetocrystalline anisotropy has been reported earlier in epitaxial multiferroic materials, such as BaTiO₃-CoFe₂O₄ (Ref. 14) nanostructures, LSMO/BFO,^4,15,16 LSMO/PZT,¹⁶ LSMO/BTO (Ref. 17) bilayers deposited on STO substrate. In these studies, using ferromagnetic resonance (FMR) and angle dependent bulk magnetization measurements coupled with temperature dependent magnetic force microscopy (MFM) measurements, the authors have shown that the domains in the ferroelectric layer induce uniaxial magnetic anisotropy. We believe that such mechanism is operational in the present sample as well.

In summary, we have reported on the electrical, ferroelectric, and magnetic properties of epitaxial BFO/LSMO bilayers deposited on CMOS compatible substrate Si (100). Most importantly, bipolar resistive switching and rectifying behavior, complete local ferroelectric switching, and in-plane magneto crystalline anisotropy were observed from the capacitor structures made from BFO/LSMO bilayers. The observed resistive switching is mediated by ferroelectric switching. The pronounced hysteric I–V shape can potentially be used for a resistive memory device for CMOS applications. The present work represents a significant step forward in realizing the BFO/LSMO based two phase multiferroic material for CMOS compatible memory and non-volatile applications.

S.R.S acknowledges National Academy of Science (NAS), U.S. for awarding the NRC postdoctoral research associate fellowship. The authors are pleased to acknowledge the support of the Army Research Office under Grant No. W911NF-04-D-0003. Also, the authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation.