Multi-Ferroic Properties on BiFeO₃/BaTiO₃ Multi-Layer Thin-Film Structures with the Strong Magneto-Electric Effect for the Application of Magneto-Electric Devices

Abstract

The characteristics of X-ray Diffraction (XRD), Current (I)-Voltage (V), and the Polarization-Electric field (P-E) on the BiFeO₃ (BFO), BaTiO₃ (BTO), and our proposed Multi-Layer (ML) BiFeO₃/BaTiO₃ (BFO/BTO) samples are studied systemically. The influence of these films with and without different external Magnetic (M_ex.) inputs is also investigated. Our proposed multi-ferroic ML BFO/BTO sample has a lower tunneling current (J_g) (~5 × 10³ times) and a stronger magneto-electric effect (an ~25 times higher value of the change of saturated polarization P_s with the M_ex. inputs than the pure multi-ferroic BFO sample. The material properties for ML BFO/BTO samples can be optimized with the combination of the advantages of BFO and BTO thin films. The stronger magneto-electric effect in our proposed ML samples can have many applications such as magnetic sensors, magnetic electric devices, and logic Magneto-Electric Spin–Orbit (MESO) devices.

1. Introduction

With the growth and the recent development of magnetic sensors [1,2], magnetic electric devices [3,4], and logic Magneto-Electric Spin–Orbit (MESO) devices [5,6], a multi-ferroic material with a good ferroelectricity, low tunneling current (J_g), and strong magneto-electric effect is needed urgently. In the past, we investigated the material properties of pure ferroelectric BaTiO₃ (BTO) and pure multi-ferroic BiFeO₃ (BFO) thin films separately [7,8]. The ferroelectric BTO thin film has a higher permittivity (ε) and a smaller saturated polarization (P_s), while the multi-ferroic BFO thin film has a higher P_s, poor ferroelectricity, and a higher J_g. The single thin film not only for pure ferroelectric BTO, but also for pure multi-ferroic BFO has its own shortcomings and application bottlenecks. In order to meet the different requirements for different kinds of applications in magneto-electric spin devices, we propose to use the Multi-Layer (ML) BFO/BTO thin-film structures to optimize the multi-ferroic material properties [7,8]. The atomic scale combination of dissimilar materials is expected to produce not only striking control, but also an ample scope of enhancement of new combinations of functional properties. Novel MLs comprising different ferroelectric/multi-ferroic materials have been intensively studied to find better systems with superior electrical and ferroelectric properties having strong coupling and interaction among the layers. Sharma [9] discussed the stress effect to enhance the polarization in the ML BFO/BTO structure for application in energy storage devices. However, the characteristics of the Polarization-Electric field (P-E) curves with and without different external Magnetic (M_ex.) inputs and the magneto-electric effect for the pure BFO, the pure BTO, and ML BFO/BTO devices were not discussed. A better J_g, an acceptable P_s, the coercive field (E_c), and the superior magneto-electric effect (ΔP_s/Δexternal magnetic (M_ex.) inputs) have been demonstrated in the proposed ML BFO/BTO thin film structures. The superior material properties for multi-ferroic materials in this sample can be used for different kinds of applications including magnetic sensors and logic spin electrics.

2. Materials and Methods

In this work, 4” silicon wafers with a (100) orientation were used as the substrate. After an initial cleaning and passivation, the thin film of platinum Pt (t = 50 nm) was deposited on the passivated Si (SiO₂/Si) as the bottom electrode via DC sputtering. A buffer layer of TiO₂ was deposited in situ over the passivated substrate prior to the bottom electrode Pt deposition for enhancement of the adhesion. Then, the wafers were cut into several small pieces to investigate different conditions of ML stack processing. The BFO and BTO dielectric was deposited on Pt/TiO₂/SiO₂/Si (100) via RF magnetron sputtering [10,11,12] at a substrate temperature of 27 and 750 °C, respectively. The detailed process parameters are summarized in

Table 1. The process parameters for our deposition of BaTiO₃ and BiFeO₃ films.

Material	Type	Based Pressure (Torr)	Working Pressure (Torr)	Power (W)	Gas Flow (sccm)	Rotating (rpm)
BaTiO3	RF	4.9 × 10−6	6 × 10−3	50	40	10
BiFeO3	RF	4.9 × 10−6	1.5 × 10−2	50	99.6	10

. The commercial sputtering targets of BFO and BTO (4N purity, 2” φ, t = 3 mm w/Cu bonding) were used as the deposition sources. The base pressure of the chamber and the rotation speed of holder were kept as 5 × 10^–6 Torr and 10 rpm. As we known and as shown in Figure 1, ferroelectric BTO thin films have a higher ε and a smaller P_s while multi-ferroic BFO thin films have a higher P_s, poor ferroelectricity, and a higher J_g. The device applications of BFO are mainly hindered by the high J_g, resulting in poor ferroelectric properties [13]. We propose and study ML BFO/BTO thin-film structures to optimize the multi-ferroic material properties with the largest magneto-electric coefficient (α_ME) to meet the requirements of applications on logic MESO devices. The interfacial strain between ML BFO/BTO thin-film structures due to the lattice mismatch can lead to the modulation of the ferromagnetic anisotropy and improvement of the electrical properties. Furthermore, the ferroelectric switching affects the interface magnetization due to the change in the collection of spin-polarized electrons at the ferromagnetic/ferroelectric interface.

Different sequence and thickness splits for BFO and BTO were composed for the study from ML-A to ML-C. The detailed final film scheme for ML-A, ML-B, and ML-C was BFO 100 nm/BTO 100 nm/BFO 100 nm/BTO 100 nm, BTO 100 nm/BFO 100 nm/BTO 100 nm/BFO 100 nm, and BFO 200 nm/BTO 200 nm, respectively. It should be noted that due to the limitation of the grain size to form the ferroelectric domain, the thickness of BFO and BTO film cannot be too thin (>100 nm). Upon the formation of ML BFO/BTO samples, part of the samples was put in an O₂ atmosphere for Rapid Thermal Annealing (RTA) with an annealing temperature of 550 °C for 60 s.

For electrical measurements, square top electrodes of Pt (t = 50 nm) with an area of 1 mm² were deposited on the ML BFO/BTO stack via DC sputtering with the shadow mask process. The phase structures and crystallographic orientation of the BFO film, BTO film, and ML BFO/BTO structures were characterized by X-ray Diffraction (XRD) and Raman spectroscopy. The small-angle X-ray scattering is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified. The room temperature capacitance-voltage (C-V) and current-voltage (I-V) characteristics were measured by the semiconductor device electricity system (Agilent E4980A, Santa Clara, CA, USA). The P-E measurement and analysis were done with a ferroelectric and capacitor test station, provided by polyK Technologies, Philipsburg, PA, USA. The tested electric field range was from −1200 to 1200 kV/cm. The external magnetic field can be applied by the homemade system. We also note that all the electrical and magnetic analysis for the single layer BTO, single layer BFO, and ML BFO/BTO structures (ML-A to ML-C) with different thicknesses was normalized in this work in order to have a fair comparison.

3. Results and Discussion

Figure 2a shows the XRD patterns of the single BTO thin film with different annealing conditions from 600 to 800 °C. From the diffraction patterns, only weak peaks of (110) around 2θ = 32° and (200) around 2θ = 45° were observed. However, the growth of the (110) and (111) peaks and the splitting of the (200) peak indicated that the cubic-to-tetragonal phase transition of BTO occurred after the 60 s and 800 °C RTA process. On the other hand, Figure 2b shows the XRD patterns of the single BFO thin film with different annealing conditions from 550 to 650 °C. A better material quality of the BFO sample with post annealing at 550 °C with an O₂ atmosphere was observed. In the BFO/BTO ML system, the complex BFO and BTO diffraction patterns in the split conditions from Condition A (ML-A) to Condition C (ML-C) are shown in Figure 2c. It can be observed that all prepared ML multi-ferroic BFO/BTO samples were polycrystalline in nature with a rhombohedral distortion for BFO and a tetragonal distortion for BTO [4], respectively.

Figure 3 shows the I-V curve for the samples of pure BTO thin film, pure BFO thin film, and ML BFO/BTO structures after 550 °C post annealing with an O₂ atmosphere. The ferroelectric BTO films had a lower J_g, while the pure multi-ferroic BFO films had the largest one. With the optimization of ML BFO/BTO structures, we can find that the J_g of our split ML-A and ML-B can be close to that of the pure ferroelectric BTO sample, which had the lowest J_g level.

Besides the characteristics of the I-V measurements of these samples, the P-E curve was also measured and is shown in Figure 4. The multi-ferroic BFO thin film had the highest value of P_s (~75 µC/cm²), while the ferroelectric BTO thin film had a small value of ~15 µC/cm². The E_c of both samples was close to 280 kV/cm. The enhanced polarization with the increase in the number of interfaces could be due to the presence of some interfacial polarization and trapped charges at the ML interfaces. Our developed ML BFO/BTO structure (ML-A) had values of P_s and E_c close to the pure multi-ferroic BFO thin film sample (Figure 4), and it also had the lowest J_g, the same as in the pure ferroelectric BTO sample (Figure 3). This indicates that the developed ML BFO/BTO structure (ML-A) can have the combined advantages of the pure BTO thin film (low J_g) and the pure BFO thin film (high P_s) at the same time.

In order to extract the value of α_ME to study the magneto-electric effect, the plot of the P-E curves for the pure 100 nm BFO thin film and our designed ML BFO/BTO structures for Condition A (ML-A) with different M_ex. inputs (0, 600, 1200 Oe) was further measured and is shown in the Figure 5. The improvement in the magnetic property of ML-A could be due to the interfacial effects and the multi-ferroic coupling in the BFO/BTO multi-layered system.

Figure 6 shows the response and sensitivities of P_s with different M_ex. inputs (0, 600, 1200 Oe) on both samples of the pure multi-ferroic BFO thin film and our proposed ML BFO/BTO structure (ML-A). The stronger magneto-electric effect (ΔP_s/ΔM_ex. inputs) and a higher value of α_ME can be observed in our proposed ML BFO/BTO structure (ML-A) compared to those in the pure multi-ferroic BFO thin film sample.

Figure 7 summarizes the key performance indices such as (a) J_g, (b) P_s, (c) E_c, and (d) ΔP_s/ΔM_ex. inputs for the extraction of α_ME from the pure multi-ferroic BFO thin film and our developed ML BFO/BTO structures. Besides the similar values of P_s and E_c, our proposed ML BFO/BTO structure had a lower J_g (~5 × 10³ times) and a stronger magneto-electric effect (~25 times higher value upon the change of P_s with the M_ex. inputs) than the pure multi-ferroic BFO thin film [7,8]. The superior material properties and the stronger magneto-electric effect of our proposed ML BFO/BTO structure (ML-A) provide for wider applications in industry, including magnetic sensors, logic spin electrics, and embedded magnetic memory.

4. Conclusions

The lower J_g and the higher response of P_s with different M_ex. inputs and the value of α_ME were found in our proposed ML BFO/BTO structures compared to the pure multi-ferroic BFO thin-film sample. The ferro-electric BTO thin film had a higher ε and a smaller P_s, while the multi-ferroic BFO thin film had a higher P_s, poor ferroelectricity, and a higher J_g. The proposed ML BFO/BTO structures can combine the advantages of pure ferroelectric BTO and pure multi-ferroic BFO thin films. The superior material properties and the stronger magneto-electric effect in our proposed ML BFO/BTO structures provide for wider applications in industry, including magnetic sensors, logic spin electrics, and embedded magnetic memory.