Magnetic-field and white-light controlled resistive switching behaviors in Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device

Abstract

Resistive switching memory devices, in which the resistance can be modulated between two nonvolatile states by applying an electrical pulse, have been proposed as the fascinating candidates for next generation logic and nonvolatile memory devices. Herein we report on the observation of magnetic-field controlled resistive switching behaviors in the Ag/[BiFeO₃/γ-Fe₂O₃]/FTO structure. Moreover, this resistive switching behavior can be modulated by white light. Therefore, such a resistive switching memory can be controlled simultaneously by voltage pulses, magnetic field and white light. This study is helpful for exploring the nonvolatile multistate memory devices manipulated by various means.

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Introduction

It is undeniable that non-volatile memory technology is highly important in information science and electronic memory device.^1–4 The resistive switching random access memory (RRAM) is currently under intense investigation due to its fascinating application that offers high reading/writing speed, high storage density and low power consumption, which applies the reversal resistive switching between the high resistance state (HRS) and the low resistance state (LRS) as the logic “0” and “1”,^5,6 in which the two states can be controlled by traditional electrical pulse. Recently, the resistive switching behaviors in a large amount of materials, such as binary metal oxides and the perovskite materials, were observed including NiO, ZnO, TiO₂, ZrO₂, Al₂O₃, Nb₂O₅, Ta₂O₅, Fe₃O₄, Zn₂SnO₄, SrZrO₃, SrTiO₃, and Pr_0.7Ca_0.3MnO₃.^7–27 In addition, the application of multiferroic materials in the resistive switching has also gained great attention recently.^28–36

In the past few years, light as new control parameter in the resistive switching is reported.^7–10 The extra control parameter in the resistive switching can greatly broaden its applications. More importantly, the magnetic field has been also involved in the ZnO based-resistive switching memory device in recent many reports.^37–39

In this paper, we report the magnetic-field controlled resistive switching behavior in Ag/[BiFeO₃/γ-Fe₂O₃]/FTO structure. To the best of our knowledge, the magnetic-field controlled resistive switching behavior in multiferroic materials, such as BiFeO₃, has not been reported so far. Moreover, the white-light controlled resistive switching behavior in Ag/[BiFeO₃/γ-Fe₂O₃]/FTO structure have been not also observed yet. Therefore, we report on the observation, for the first time, of a resistive switching memory that can be controlled simultaneously by voltage pulses, magnetic field and white light. The magnetic-field and white-light controlled resistive switching reveals the potential for next-generation nonvolatile memory applications based on multifunctional materials.

Experimental

Materials preparation

In our works, BiFeO₃ powder was synthesized by a wet chemical method, which is similar to the method reported in Toshiyuki Mashino' works,⁴⁰ and γ-Fe₂O₃ powder was prepared by common precipitation method, which is similar to the method reported in previous literature.⁴¹

Detailed preparation procedures of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device are shown as follows: we mixed as-synthesized BiFeO₃ powder and γ-Fe₂O₃ powder together with a certain proportion, and then grinded the mixed powder for 2 hours. Then we dissolved the mixed powder in toluene solution to prepare precursor gel. As-prepared precursor gel was spin-coated on the FTO substrate. The spin-coating process at 5000 rpm for 10 s was used for BiFeO₃/γ-Fe₂O₃ composite film preparation. Then the sample was dried at 120 °C in vacuum for 12 hours to remove toluene. The thickness of the film was detected by the XRF (PW2404 R, Philips Japan Co. Ltd.), which shows the thickness of BiFeO₃/γ-Fe₂O₃ composite film is about 5 μm. Finally, the Ag electrodes with area of 1 mm² were prepared by vacuum deposition. The BiFeO₃/γ-Fe₂O₃ samples with masks on them were put into the vacuum sputtering system to grow Ag electrodes.

Materials characterization

Crystal structure of BiFeO₃/γ-Fe₂O₃ composite film was characterized by X-ray diffraction (XRD, Shimadzu XRD-7000 X-ray diffractometer) with Cu Kα radiation. The microstructure characterization, selected area electron diffraction (SAED) and the energy-dispersive X-ray (EDX) spectra of the BiFeO₃/γ-Fe₂O₃ composite film were tested by transmission electron microscopy (JEM-2100) at an acceleration voltage of 200 kV. Magnetic properties of BiFeO₃/γ-Fe₂O₃ composite film were characterized by a vibrating sample magnetometer (VSM) from Microsense Corporation in USA. The measurements of ferroelectric hysteresis loops (P–E) were performed using a Precision Premier Workstation ferroelectric test system (Radiant Technology, USA).

Performance test

For the test of electrical transport characterizations, Ag is top electrode and FTO is bottom electrode. Current–voltage (I–V) curves and resistance–cycles curves were tested using the electrochemical workstation CHI-660D at room temperature. The positive direction is defined as current or electronic pulse direction from the top Ag electrode to the bottom FTO. We used an ordinary filament lamp with various power densities as white light source, and the wavelength range of white light is 400–760 nm.

Results and discussion

Fig. 1(a) shows the schematic representation of the device. The Ag electrodes with the area of less than 1 mm² were deposited onto the surface of BiFeO₃/γ-Fe₂O₃ composite film. Fig. 1(b) exhibits the XRD diffraction patterns of [BiFeO₃/γ-Fe₂O₃]/FTO structure and FTO substrate. In order to make diffraction peaks of BiFeO₃/γ-Fe₂O₃ composite film more clear, we also present the XRD pattern of the pure FTO substrate without BiFeO₃/γ-Fe₂O₃ composite film in Fig. 1(b)A. The peaks of FTO substrate appear in the patterns of [BiFeO₃/γ-Fe₂O₃]/FTO structure (Fig. 1(b)B). We can see there are only the peaks of BiFeO₃ and γ-Fe₂O₃ besides peaks of FTO substrate. The diffraction peaks of BiFeO₃ can be assigned to the pure phase of BiFeO₃ (JCPDS no. 20-0169), indicating a rhombohedrally distorted perovskite structure in a space group of R3c, which is consistent with the literature.⁴⁰ All diffraction peaks of γ-Fe₂O₃ can be attributed to the pure γ-Fe₂O₃ phase without trace of other phase. Therefore, the film contains only pure BiFeO₃ and γ-Fe₂O₃ phase.

Fig. 1 (a) The schematic representation of the device. (b) The X-ray diffraction (XRD) of BiFeO₃/γ-Fe₂O₃ composite film and FTO substrate. (c) The high resolution transmission electron microscope (HRTEM) of BiFeO₃/γ-Fe₂O₃ composite film, the inset is corresponding selected area electron diffraction (SAED) pattern of BiFeO₃. (d) The energy-dispersive X-ray (EDX) spectrum of BiFeO₃/γ-Fe₂O₃ composite film, the inset shows the BiFeO₃/γ-Fe₂O₃ mole ratio in the composite film is about 2.85.

Fig. 1(c) presents the high resolution transmission electron microscope (HRTEM) image of BiFeO₃/γ-Fe₂O₃ composite film. The fringes with a spacing of 0.28 nm correspond to (110) planes of BiFeO₃, and the fringes with a spacing of 0.48 nm correspond to (311) planes of γ-Fe₂O₃. The inset to Fig. 1(c) shows the corresponding selected area electron diffraction (SAED) pattern of BiFeO₃, which indicates the BiFeO₃ of BiFeO₃/γ-Fe₂O₃ composite film is single-crystalline structure.

The composition of BiFeO₃/γ-Fe₂O₃ composite film is further confirmed by elemental analysis using energy-dispersive X-ray spectra (EDX). The EDX data in Fig. 1(d) confirms that the element compositions of composite film are Bi, Fe and O without any other impurities. In addition, the BiFeO₃ : γ-Fe₂O₃ mole ratio in the composite film is about 2.85 from the inset of Fig. 1(d).

The magnetic hysteresis loop (M–H) of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device with magnetic field from −2.5 T to 2.5 T is shown in Fig. 2. The Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device presents obvious ferromagnetism with coercivity about 150 Oe at room temperature from the inset of Fig. 2, and the magnetization is about 85 emu cm⁻³ at 2.5 T.

Fig. 2 The hysteresis loop (M–H) of BiFeO₃/γ-Fe₂O₃ composite film.

Fig. 3(a) displays typical voltage-dependent current density (J–V) curves in linear scale of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device in various magnetic fields, which exhibit asymmetric behaviour with significant hysteresis. The arrows in the figure denote the sweeping direction of voltage. The inset of Fig. 2(a) shows the experimental test circuit. In order to exclude the effect of the variability of the high resistance state on the characteristics that the applied magnetic field can regulate the resistance and the set/reset voltage, we first measured the I–V curve without magnetic field after measurement of 200 I–V curves. Then we increased magnetic field gradually and measured the I–V curves in various magnetic field. From these curves, it is concluded that the applied magnetic field can regulate the resistance and the set/reset voltage. And then we decreased magnetic field gradually and measured the I–V curves in various magnetic field, including zero magnetic field. From these curves, it is also concluded that the applied magnetic field can modulate the resistance and the set/reset voltage. Therefore, the effect of the variabilities of the high resistance state and the set/reset voltage on the characteristics that the applied magnetic field can regulate the resistance and the set/reset voltage can be excluded. Furthermore, the resistance–cycles curve, the set/reset voltage–cycle curve and the set/reset current–cycle in magnetic field are obviously separated from those without magnetic field (Fig. 3(c) and (d) and 4), especially the set/reset current–cycle, which undoubtedly indicate the applied magnetic field can modulate the resistance and the set/reset voltage. In addition, the resistance–cycles curve, the set/reset voltage–cycle curve and the set/reset current–cycle without magnetic field were firstly measured, and then those in magnetic field were measured. Furthermore, the R–H was measured and shows R increase with the applied magnetic field. A similar result takes place for the situation of white light.

Fig. 3 (a) The typical J–V characteristics curves in linear scale of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO structure in various magnetic fields, the inset is the real test circuit in magnetic field. (b) The corresponding J–V characteristics curves in logarithmic scale, (c) The evolution of switching voltages including V_Set and V_Reset during the 100 resistive switching cycles. (d) The evolution of switching currents including I_Set and I_Reset during the 100 resistive switching cycles.

Fig. 4 The resistance–cycles curve with a positive bias voltage of 0.1 V.

Fig. 3(b) shows corresponding J–V curves in logarithmic scale. The arrows in the Fig. 3(b) denote the sweeping direction of voltage. With the increasing of voltage from 0 V, the current densities suddenly increase at voltage about 0.82 V without magnetic field and about 0.98 V with magnetic field of 2.5 T, indicating a resistive switching from the high resistance state (HRS or ‘OFF’) to the low resistance state (LRS or ‘ON’), which was called the “Set” process, and the switching voltage is called set voltage (V_Set). The device can keep at LRS sweeping from 1.5 V → 0 V. When the applied voltage sweeps from zero to a certain negative reset voltage (V_Reset) of about −1.23 V without magnetic field and about −1.38 V with magnetic field of 2.5 T, the device returns from the LRS to the HRS, which was called the “Reset” process. No electroforming process is required for the Ag/[BiFeO₃/γ-Fe₂O₃]/FTO devices. Therefore, the resistive switching should be attributed to abundant oxygen vacancies (V_O) pre-existing in the as-grown BiFeO₃/γ-Fe₂O₃ composite films. It is worth noting that the magnetic field can control the resistive switching. Both the LRS resistance and the HRS resistance increase with the increasing of the magnetic fields. Especially, the magnetic field can control the switching voltage V_Reset and V_Set. The absolute values of V_Reset and V_Set increase with the increasing of the magnetic fields. Our results reveal that the magnetic field can be a control parameter of the resistive switching in this device, which provides the potential for magnetism-controlled nonvolatile memory applications.

During the subsequent “Set” and “Reset” cycles, the device shows the same J–V curves as the first cycle. The V_Reset and V_Set are almost unchanged in the different cycles. Fig. 3(c) displays the evolutions of V_Set and V_Reset over 100 successive resistive switching cycles. There is no obvious decay for V_Set and V_Reset. The V_Set and V_Reset are 0.82 ± 0.1 V and −1.23 ± 0.1 V without magnetic field, respectively, and the V_Set and V_Reset are 0.98 ± 0.1 V and −1.38 ± 0.1 V with magnetic field of 2.5 T, respectively.

Moreover, the evolution of set and reset currents with cycle number is also shown in Fig. 3(d). We defined the set and reset current according to previous reports.^42–46 We can see the set current is decrease and reset current is increase with magnetic field.

In addition, the evolutions of the resistance in the HRS and LRS with a positive bias of 0.1 V over 100 successive resistive switching cycles are tested and shown in Fig. 4. The resistances at the LRS (ON state) and at the HRS (OFF state) keep stable to a certain extent. The above results reflect excellent repeatability and reliability of the magnetic-field controlled resistive switching in Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device.

Furthermore, we tested the resistive switching characteristics of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO devices under white light illumination with various power densities. The inset of Fig. 5(a) shows the experimental test circuit. Fig. 5(a) presents J–V characteristics curves in linear scale of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO under white light illumination with various power densities. Fig. 5(b) exhibits corresponding J–V curves in logarithmic scale, which indicates the white-light controlled resistance switching. The absolute values of reset voltage (V_Reset¹) and set voltage (V_Set¹) increase with the increasing of illumination power densities. Fig. 5(c) displays the evolutions of V_Reset¹ and V_Set¹ over 100 successive resistive switching cycles. Fig. 5(d) displays the evolutions of I_Reset¹ and I_Set¹ over 100 successive resistive switching cycles.

Fig. 5 (a) The J–V characteristics curves in linear scale of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO structure in the dark and under white-light illumination with various power densities, the inset is real test circuit. (b) The corresponding J–V characteristics curve in logarithmic scale. (c) The evolution of switching voltages including V_Set¹ and V_Reset¹ during the 100 resistive switching cycles in the dark and under light illumination with power density of 20 mW cm⁻². (d) The evolution of switching currents including I_Set¹ and I_Reset¹ during the 100 resistive switching cycles in the dark and under light illumination with power density of 20 mW cm⁻².

Fig. 6 show the evolutions of the resistances in the HRS and LRS with a positive bias of 0.1 V over 100 successive resistive switching cycles. The resistances, V_Reset¹, V_Set¹, I_Reset¹ and I_Set¹ keep stable to a certain extent, which shows the white-light controlled resistive switching in Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device is highly repeatable and reliable. In addition, the HRS/LRS ratio increases with the increasing of illumination power densities. Our results reveal that the white light can be a control parameter of the resistive switching in this device.

Fig. 6 The resistance–cycles curve with a positive bias voltage of 0.1 V in the dark and under light illumination with power density of 20 mW cm⁻².

BiFeO₃ is multiferroic materials,^47,48 which shows ferroelectric and antiferromagnetic at room temperature. BiFeO₃ exhibits strong magnetoelectric effect, where ferroelectricity is coupled with magnetism. The mechanisms for resistive switching have been extensively investigated, and it is generally believed that the electrically driven migration of oxygen ions/vacancies plays a critical role.^49–54 In our works, both the magnetic field and white light can control the resistive switching. Especially, both the magnetic field and white light can modulate the absolute values of reset voltage and set voltage. The controlling of magnetic field and white light over the resistive switching of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device should originate from the couple among magnetism, optical property and ferroelectricity of BiFeO₃. Fig. 7(c) shows the ferroelectric hysteresis loops (P–E) of BiFeO₃ film without magnetic field and with magnetic field of 2.5 T. The magnetic field can improve the ferroelectric polarization of BiFeO₃, which is due to the strong magnetoelectric effect.^55,56 Fig. 7(d) exhibits the ferroelectric hysteresis loops (P–E) of BiFeO₃ film in the dark and under white light illumination. The white light can improve the ferroelectric polarization of BiFeO₃ due to the strong photoferroelectric effect, which is explained by trapping of photogenerated charge at domain boundaries to minimize internal depolarizing fields.^57,58 Therefore, both magnetic field and white light can improve the ferroelectric polarization of BiFeO₃. The mechanism for the controlling of magnetic field and white light over the resistive switching of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device is displayed in Fig. 7(a) and (b). Ferroelectric polarization of BiFeO₃ appears in applied electrical field E_e. Ferroelectric polarization of BiFeO₃ results in an extra opposite-direction electrical field E_i in BiFeO₃/γ-Fe₂O₃ composite film (Fig. 7(a)). When applying magnetic field or white light, ferroelectric polarization of BiFeO₃ increases, which leads to increasing of the extra opposite-direction electrical field E_i in BiFeO₃/γ-Fe₂O₃ composite film (Fig. 7(b)). Therefore, larger applied voltages are necessary to finish the ‘Set’ process and the ‘Reset’ process. Therefore, both the magnetic field and white light can modulate the absolute values of reset voltage and set voltage. In addition, the larger resistances of LRS and HRS induced by magnetic field in Fig. 3(d) should be attributed to increasing of the extra opposite-direction electrical field E_i in magnetic field. However, the effects of light on resistances of LRS and HRS are more complex because of photogenerated charges. While the model claimed here remains phenomenological, it is needed for further study to find a more reasonable mechanism for elucidating the origin of the phenomenon of magnetism and light controlled resistive switching.

Fig. 7 (a) Ferroelectric polarization of BiFeO₃ results in an extra opposite-direction electrical field E_i in BiFeO₃/γ-Fe₂O₃ composite film. (b) When applying magnetic field or white light, ferroelectric polarization of BiFeO₃ increases, which leads to increasing of the extra opposite-direction electrical field E_i in BiFeO₃/γ-Fe₂O₃ composite film. (c) The ferroelectric hysteresis loops (P–E) of BiFeO₃ film without magnetic field and with magnetic field of 2.5 T. (d) The ferroelectric hysteresis loops (P–E) of BiFeO₃ film in the dark and under white light illumination. (e) Our device added the magnetic field and white-light as extra control parameter besides the electrical pulse in resistive switching memory.

Conclusions

In summary, the magnetic-field and white-light controlled resistive switching of Ag/[BiFeO₃/γ-Fe₂O₃]/FTO device is observed. The device adds magnetic field and white light as extra control parameters besides the traditional electrical pulse to manipulate resistive memories. Therefore, such a resistive memory can be controlled simultaneously by voltage pulses, magnetic field and white light (Fig. 7(e)), which reveals the potential for next-generation nonvolatile memory applications based on multifunctional materials.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (Grant no. 51372209).

Notes and references

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