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Voltage-Driven Room-Temperature Resistance and Magnetization Switching in Ceramic TiO2/PAA Nanoporous Composite Films Xidi Sun,†,‡ Xiaoming Mo,† Lihu Liu,§ Huiyuan Sun,*,§ and Caofeng Pan*,‡
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Center on Nanoenergy Research, College of Physical Science and Technology, Guangxi University, Nanning 530004, Guangxi, PR China ‡ CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro−Nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, PR China § College of Physics Science & Information Engineering and Hebei Advanced Thin Film Laboratory, Hebei Normal University, Shijiazhuang, Hebei 050024, China ABSTRACT: Voltage control of room-temperature ferromagnetism has remained a big challenge which will greatly influence the multifunctional memory devices. In this paper, porous TiO2 thin films were deposited by dc-reactive magnetron sputtering onto ordered porous anodic alumina (PAA) substrates. Voltage-driving room-temperature resistance and magnetization switching without external magnetic field are simultaneously found in an Ag/TiO2/ PAA/Al (Ag/TP/Al) device. Further analysis indicates that the formation/rupture of oxygen vacancy defect-based conductive filaments would be responsible for the changes of resistivity and magnetization. Our present results suggest that the TP nanoporous composite film material may therefore be used to achieve voltage control of magnetism and resistance switching in the future multifunctional memory devices. The Ag/TP/Al devices can also be used for new spintronic devices, neuromorphic operations, and alternative logic circuits and computing. KEYWORDS: storage materials, ferromagnetism, oxygen vacancy, nanoporous composite films, voltage control of magnetism
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INTRODUCTION With the development of science and technology, ultra-highdensity and powerful memory devices are of significant importance. Therefore, multifunctional materials for achieving voltage control of room-temperature ferromagnetism (RTFM) and resistance behavior are desirable in the development of multifunctional data storage and spintronic devices.1−7 Since Ohno et al. discovered the electric manipulation of ferromagnetic properties in ferromagnetic semiconductor materials,8,9 same behavior was also found in ABO3 perovskite structures10,11 and metallic oxide semiconductor materials.12−14 The problem is that many magnetoelectric composites prepared on some special substrates have weak RTFM, which cannot satisfy the practical needs of data storage and spintronic devices at all. Hence, the search for multifunctional materials with obvious resistance switching and large RTFM is one of the key problems. Recently, redox-based resistive switching (RS) memories have attracted great attention in both scientific and industrial communities for new data storage because of their advantages of high-density, nonvolatile properties; fast operation speed; low power consumption; and simple synthesis procedure.15−17 In addition, most of the binary transition-metal oxides have an obvious RS behavior, which are compatible with the complementary metal-oxide-semiconductor (CMOS) technol© 2019 American Chemical Society
ogy. Therefore, a binary transition-metal oxide system had been brought to the forefront by researchers. In most literature studies, researchers believe that the physical origins of the RS phenomenon in these oxides is that conducting channels would be selectively formed under the applied external electric field at crystalline defects such as vacancies and grain boundaries. 18−22 Ahmad et al. 21 and Huang et al. 22 demonstrated that the total oxygen vacancy density had obvious influence on the RS features of zinc oxide film separately, which indicated that the oxygen vacancies were in the corresponding positive-charged state. Among the various binary oxides, TiO2 nanostructures have been studied for decades because of their broad applications.23−26 It is noteworthy that the presence of oxygen vacancies in TiO2 nanotubes has a strong impact on its ferromagnetic properties.27 Then, we analysed whether it was possible to achieve the magnetization transform through the electric field control of RS in TiO2-based oxide materials to meet the burgeoning development of low-power-consumption multifunctional memory devices. With these motivations, the electric field control of Received: February 14, 2019 Accepted: May 28, 2019 Published: May 28, 2019 21661
DOI: 10.1021/acsami.9b02593 ACS Appl. Mater. Interfaces 2019, 11, 21661−21667
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Surface scanning probe Microscopy image and XRD pattern (inset) of the PAA substrate. (b) Surface and (c) cross-sectional SEM image of the nanoporous TP film. (d) EDX spectrum of the TP film. (e) XRD pattern for the porous TiO2 thin film. work. Finally, the films were annealed in air and vacuum atmosphere at 400 °C for 3 h. To study the RS properties of TP films, dot structural Ag top electrodes with a diameter of 200 μm were deposited with a metal shadow mask. In the subsequent measurement, an external electric field can be applied on the Ag top electrodes, the Al substrate serving as the bottom electrode. The morphology and constituent of the materials have been examined by using field-emission scanning electron microscopy (FESEM) with energy-dispersive X-ray (EDX) spectroscopy. The phase structure of the TiO2 film was investigated by X-ray diffraction (XRD) with Cu Kα radiation. The surface feature of the samples was obtained by using an atomic force microscope. The photoluminescence (PL) spectrum was achieved on a fluorescence spectrophotometer with an excitation wavelength of 340 nm. The magnetic properties of the samples had been tested on a superconducting quantum interference device (MPMSXL-5). In the measurements, all the samples were cut into a 2 mm × 2 mm square because of the limitation of the sample holder and the magnetic field was applied perpendicular to the film plane. It should be noted that all the memory cells in this square area can be trigged to a high-resistance state (HRS) or a low-resistance state (LRS) at the same time before we measure their magnetism. Resistance switching experiments were performed in a two-point, vertical geometry using a Keithley 2612A current source at room temperature, where the current is limited to 100 mA to guard the device from complete dielectric breakdown. It should be noted that all the tools used for handling the samples were nonferromagnetic during sample preparation and magnetic measurement in order to eliminate any influence of alien magnetic impurities.
nonvolatile magnetization accompanied with RS characteristic in a CMOS-compatible structure is greatly desirable. On the basis of the above discussion, TiO2/porous anodic alumina (PAA) (TP) nanoporous laminate films were prepared by a magnetron sputtering method. The porous nanostructured TiO2 possesses higher specific surface area than the bulk ones, which can provide a large number of oxygen vacancies in it and further performance improvements may be possible by controlling such defects. In our work, we have investigated the properties of the formed nanoscale oxide layers and analyzed the role of oxygen vacancies in room temperature resistance switching and magnetization switching characteristics without external magnetic field.
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EXPERIMENTAL SECTION
To fabricate nanoporous PAA substrates, the commercial high-purity Al foils (99.999% purity, 0.5 mm thickness) were first annealed at 400 °C for 2 h in argon atmosphere and then polished in a homemade polishing solution of ethanol and perchloric acid (4:1 in volume) for 5 min. After that, the organics on the polished foils were removed by acetone and deionized water and finally dried in air. In the anodization process, the Al foils and a platinum plate were used as the anode and cathode, respectively. The first anodization was carried out in 0.3 M oxalic acid electrolytes under a constant dc voltage of 45 V at a temperature of 5 °C for 6 h. Then, the alumina film produced was removed by a chemical method in a mixture solution of phosphoric acid (6 wt %) and chromic acid (4 wt %) at 30 °C for 8 h. Then, a second anodization was carried out under the same anodization conditions as in the first step. To get the desired pore channel length of the PAA thin films, we can change the oxidation time appropriately. After anodization, the highly ordered PAA substrates were obtained. Porous TiO2 thin films are deposited on the PAA substrates at 300 °C by using dc-reactive magnetron sputtering. To ensure the purity of the samples, a high-purity titanium disk (99.99%) was selected as the sputtering target. On the other hand, the Ti target was presputtered for 5 min, which is needed in order to eliminate surface impurities before depositing. In the sputtering process, high-purity Ar and O2 (99.99%) were used as working and reactive gases, respectively. The working pressure was kept constant at 1.1 Pa by adjusting the flux of argon at 30 sccm and oxygen at 3 sccm during sputtering. The experiment result shows that the TiO2 films deposited for 60 min with a sputtering power of 60 W is appropriate in our
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RESULTS AND DISCUSSION Figure 1a shows the surface morphology and XRD pattern of the PAA thin film. It is obvious that the two-step anodization process yields well-defined hexagonal nanoporous arrays and that the PAA is in an amorphous state (inset of Figure 1a). We can see that the pores have diameters of about 45 nm, and the pore-to-pore length is approximately 125 nm. In our work, the nanoporous TP films are prepared by using a dc magnetron sputtering instrument and PAA as the template. Figure 1b,c displays the FE-SEM images of the surface and the cross section of the as-sputtered TiO2 films on PAA substrates. We can see that the film is a highly ordered 21662
DOI: 10.1021/acsami.9b02593 ACS Appl. Mater. Interfaces 2019, 11, 21661−21667
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Hysteresis loops before and after annealing in air and vacuum condition at 400 °C for 3 h. The bottom inset shows the magnified M− H curve. (b) Comparative analysis PL spectra for a TP film annealed in air and vacuum at 400 °C for 3 h.
Figure 3. (a) I−V curves of the as-prepared Ag/TP/Al device by applying a sweeping voltage as 0 V → 3 V → 0 V → −3 V → 0 V. The arrows indicate the voltage sweeping directions. The inset in (a) shows the schematic structure of the Ag/TP/Al device. (b) Endurance test of the bistable resistance in a Ag/TP/Al device over a course of 10 switching cycles.
As shown in Figure 2b, the room-temperature PL spectra were obtained in the range of 310−590 nm with the excitation wavelength of 340 nm. In the PL spectra, a peak near 360 nm can be observed, which can be attributed to the intrinsic emission of TP. Furthermore, a broad emission band can be seen in the range of 400−520 nm, which may be attributed to the existence of oxygen vacancies.28,30,31 On the other hand, for some semiconductors, an increase in defects can shrink the band gap and then broaden the PL peak. On the contrary, the decrease of oxygen vacancy concentration will lead to band tail decaying. In addition, these results stop the band gap shrinkage and narrow the excitonic level distribution.31 Therefore, as the oxygen vacancy concentration decreases, the width of the spectrum becomes narrower. From Figure 2b, it can be seen that the intensity of the PL band weakens and the half-width also decreases (from 98 to 70 nm) in the TP film annealed in air compared to the TP film annealed in vacuum. Obviously, annealing in vacuum can increase oxygen vacancy concentration, whereas annealing in air can prevent the escape of oxygen and decrease the oxygen defect concentration existed on the surfaces of the TP film. Therefore, the change of PL spectrum intensity can be comprehended by the oxygen vacancy concentration change in different heat treatment procedures.31,32 As discussed above, it is clear that the variation of the PL spectrum (Figure 2b) is synchronous changes of the magnetic moment (Figure 2a). Such a result suggests that the observed RTFM in the TP thin films is related to the vacancy defects in the material. As a focal point, the possible magnetic exchange mechanism will be discussed in detail in the following parts of this paper. The resistive properties of the Ag/TP/Al structures were measured using a semiconductor characterization system (Keithley 2612A), and all the measurements were carried
nanoporous array and the pore diameter of it is about 40 nm. It is clearly shown that the formation of TiO2-ordered pore arrays may attribute to the induction of the PAA substrate during the deposition process. However, because of its growth pattern, the pore diameter of it decreased with the increase of deposition time. In addition, it will form a continuous film as the deposition time runs through the critical time. The thickness of the TiO2 film layer is estimated to be about 140 nm, Figure 1c. Figure 1d shows the EDX spectrum of the nanoporous TiO2 film. Only Al, Ti, and O elements were detected, and any form of transition metals was not found in the films. Obviously, the observed peak of Al and some of O should be ascribed to the PAA substrates. Figure 1e shows the XRD pattern of the asprepared TiO2 films deposited on PAA substrates, which show the prepared sample TiO2 presenting the phase of tetragonal anatase (I41/amd), and no impurity phases can be found within the detection limits. Previous works have proved that the physical properties of metal oxides are influenced by the oxygen vacancies.19,27,28 To examine the relationship between oxygen vacancy concentration and magnetism, the room-temperature magnetic moment versus magnetic field (M−H) curves with the magnetic field perpendicular to the TP film plane and PL spectra under different conditions were measured, as shown in Figure 2. As shown in Figure 2a, we draw the hysteresis loops using raw data in order to eliminate the correction error.7,29 The well-defined hysteresis loops of the samples exhibit the RTFM nature of the TP films. In general, heat treating the oxide in reducing atmosphere (or vacuum) will increase the oxygen vacancy concentration. Contrarily, annealing in O2 gas condition decreases the oxygen vacancy concentration. Evidently, after annealing in vacuum, the magnetic properties of the TP film could be improved by comparing to the one annealed in air condition. 21663
DOI: 10.1021/acsami.9b02593 ACS Appl. Mater. Interfaces 2019, 11, 21661−21667
Research Article
ACS Applied Materials & Interfaces out on an aseismic platform. Figure 3a displays the typical measured current−voltage (I−V) characteristics of the Ag/ TP/Al structure with the application of a sweeping voltage from Ag to Al as 0 V → 3 V → 0 V → −3 V → 0 V, and the inset exhibits the schematic setup for I−V measurements. The current compliance was fixed at 10 mA to prevent permanent breakdown of the device. From Figure 3a, no forming process can be found in the RS, which may attribute to the abundant oxygen vacancies existed in the pristine Ag/TP/Al nanoporous films.32−34 Under forward and reverse bias, the switching hysteresis is asymmetric obviously. We can attribute this phenomenon to the ultrahigh resistance of ∼109 Ω of the sample, as described by Zoolfakar.33 The sudden current increase when the voltage was swept from 0 to +3 V may be due to the forming process of conducting filaments, indicating that the cell switches from HRS to LRS. However, it transformed to HRS again when the voltage swept from 0 to −3 V. The filamentary model has been widely accepted to explain the switching behavior of TiO2-based resistance switching devices. The great number of oxygen vacancies in TP nanoporous composite films can form conducting filaments on the inner walls of the holes naturally. However, the formation/disruption of localized conducting filaments is preferred as the driving mechanism of resistance switching. For clarity, a simplified switching mechanism model is given below to explain the RS effect. Once a bias is applied, O2− ions will move toward the Ag top electrode more effectively. This movement of O2− ions is associated with the formation of oxygen vacancies, and conductive filament channels will be set up, leaving the metal ions in the TP film between the two electrodes. Consequently, with the set process (HRS to LRS) occurring, the incidence of oxygen vacancy capturing electron decreases. When applying a negative voltage, the O2− ions will move back from the Ag top electrode to the TP layer, and the consequent rupture of the conducting filament will occur. In addition, the chance of oxygen vacancies capturing electrons improved greatly in the reset process (LRS to HRS) subsequently. On the other hand, once the conductive filament is formed (or disconnected), RS can still be maintained even if the voltage is dropped. Therefore, this resistance transformation is nonvolatile. Figure 3b shows the I−V curve of the untreated Ag/TP/Al device over the course of a series of 10 repeated measurements, and a stable RS characteristic over the course of 10 switching cycles can be found, where a higher voltage was applied to investigate the stability of the device and to ensure that it can be used in a wide voltage range. Considering about the formation of oxygen vacancies, it is reasonable for us to hypothesize that an oxygen vacancy (V•• O) is formed and existed as a positive center in the TiO2 layer by removing an O2− ion. In other words, it is easy for oxygen vacancies to trap electrons. However, the trapped electrons will have a net magnetic moment because of their localization, so we can ensure that the V•O (an oxygen vacancy occupied by one electron) will have a magnetic moment. The mobilities of electrons are different when they are in different resistive states for the sample, which leads to the different number of V•O. Different resistive states, however, have different electron mobilities; therefore, the number of V•O in HRS and LRS is different. On the basis of the above analysis, we can conclude that the magnetism is different in HRS and LRS.7 Now, we turn to the influence of RS change on the magnetic properties of Ag/TP/Al devices, which is a particularly important question in this work. Figure 4 shows that the
Figure 4. M−H loops in the initial state, HRS and LRS. The bottom inset shows the switching of MS accompanying the RS effects.
M−H curves with the external magnetic field are perpendicular to the TP film plane after triggering the memory devices to the HRS or LRS, where we applied the magnetic field perpendicular to the films because the magnetism of the porous film material along the nanoholes, that is, perpendicular to the films, is bigger than that of parallel to the films.27,28 Remarkably, the device at both LRS and HRS all shows clear M−H curves, which shows that RTFM still retains even the application of bias voltages. This result indicates that the effect of RS has significant influence on the magnetism of the Ag/ TP/Al device. As suggested by the SQUID results, the MS for the device at HRS (about 200 emu/cm3) is larger than that at LRS (about 110 emu/cm3). At the same time, the change of MS of the sample keep same step with the one of RS, as shown in the inset of Figure 4. More interestingly, magnetization switching was also found in the transformation process of HRS and LRS without obvious degradation. This is very crucial for the application of information storage. On the basis of the experimental results described above, we believe that the oxygen vacancies play an essential role in the process of electric-field-induced resistance switching and magnetization change. In fact, until now, the origin of ferromagnetism is still very confusing. However, there have been several different theoretical models on the origin of RTFM in dilute ferromagnetic semiconductor oxides. In 2000, Dietl proposed the first model to explain the origin of ferromagnetism in dilute magnetic semiconductor oxides,35 that is, the Zener/Ruderman−Kittel−Kasuya−Yosida (RKKY) model, which believed that the dopant (transition-metal cation) replaces the positions on the lattice and generate polarized holes. The theoretical model considers that the magnetization of these samples is related to the doping concentration. However, the measured magnetization has been proved to be irrelevant on the consistence of dopants,36,37 which indicates that the RKKY mechanism may not be the principal reason of ferromagnetism in dilute magnetic semiconductor oxides. Later, Coey developed several models to explain the magnetism of thin magnetic semiconductor films. For example, the bound magnetic polaron model38 and the charge-transfer ferromagnetic theoretical model39 emphasize the effect of defects in the origin of RTFM in dilute magnetic semiconductor oxides. These models, however, cannot explain how ferromagnetism occurs in dopant-free dilute magnetic semiconductor oxides because there is no additional carrier source for the absence of dopants. Yang et al. reported that the ferromagnetism in Al2O3 nanoparticles is mediated by the exchange coupling between single charged oxygen vacancies.40 This exchange mechanism assumes that a single charged oxygen vacancy center will overlap with each 21664
DOI: 10.1021/acsami.9b02593 ACS Appl. Mater. Interfaces 2019, 11, 21661−21667
Research Article
ACS Applied Materials & Interfaces
ferromagnetism and thus the MS of the nanoporous TP film in the LRS lower than that in the HRS is evident. Because the resistance switching is accompanied by the formation and breaking of conducting filaments under the modulation of external electric field, in the process of oxygen vacancies arranging orderly, the probability of trapping electrons by the filaments will increase greatly, which results in the magnetism of HRS higher than the pristine state. On the basis of the knowledge of solid-state physics, it is easy to know that oxygen vacancies can be easily formed on the sample surface and thus the conductive filaments formed by oxygen vacancy are most likely to be formed on the inner surface of the pore. Therefore, compared with the pristine state, the probability of oxygen vacancy capturing electrons is higher, no matter in the HRS or in the LRS. Therefore, the magnetization is higher in HRS and LRS than that in the as-prepared state, which is consistent with the expected result (see in Figure 4).2,7,19
other once the density of the nanoparticles reaches the critical value for magnetic percolation and which will bring about a long-range ferromagnetic ordering. However, when the distance between oxygen vacancies is relatively far away, how the ferromagnetic order is generated is not explained by the research group. Very recently, a new defect-induced bound polaron model has been proposed in the undoped but with much defects dilute ferromagnetic semiconductor oxides.41 This extended model assumes that a narrow impurity band is formed just below the conduction band minimum because of the overlap and hybridization of the defect states, that is, the oxygen vacancies associated with different charges.41 Therefore, one view is that the ferromagnetic exchange interactions may arise between the V•O and local 4d1 electron because their net spins possess the same direction.41 However, this model is insufficient to explain why some dilute magnetic semiconductor oxides without any unpaired d electrons also show ferromagnetism. As mentioned above, because the oxygen vacancies are in a state of charge imbalance, which exists as an electron trap to capture electrons,28 the neutral oxygen vacancy center can therefore be ascribed as an electron pair trapped in the hole left by the missing oxygen; thus, the cancellation of paired electron spins occur because of the antiparallel electron spins. Moreover, the formation of neutral oxygen vacancy displays too high formation energies and may occur in negligible amounts. V•O center containing only one single electron is similar to the localized electron, which has a net magnetic moment.7,19 Moreover, V•• O center is strongly electron-deficient with the absence of both electrons, which made no contribution to the total magnetic moment at all. On the basis of direct exchange mechanism, the V•O distributed at the nearest neighbor sites will couple together directly for their spins arranged in parallel.7,19,27,28 In the case of the oxygen vacancies distributing far from each other, the conduction electrons will play the role of itinerant electrons in the material. The free electrons can easily be polarized by V•O, and then the magnetic moment orientation of the polarized electrons will keep in line with that of VO• . Thus, the longer-range ferromagnetic exchange can occur by the intervention of polarized electrons for the V•O, which is regarded as an indirect exchange interaction process (V•O−e−−V•O). This insight may be used to interpret the RTFM of TP nanoporous films and also be applied to comprehend the observed distinct phenomena of d0 ferromagnetism in low-dimensional nanostructures or thin films with high specific surface areas. Furthermore, the model we proposed indicates that magnetic ordering might be obtained theoretically by introducing various anion vacancy defects in semiconductor oxides. Besides, the phenomenon that the saturated magnetization for the HRS is always larger than those for the LRS can be due to the fact that many V•• O can be generated when the oxygen ions are driven by the electric field.7,19 When applying a positive voltage to the Ag top electrode (see in Figure 3a), the device can be switched into the LRS, for which there is a greater electron mobility corresponding to a decreased probability that the V•• O vacancies capture electrons, thereby reducing the number of V•O vacancies. On the contrary, in the case of the application of negative bias, the device switches to HRS again. Compared with LRS, the probability of electrons captured by V•• O vacancies increases because of the lower electron mobility, which means more V•O present in the film. Now, a large number of V•O vacancies can heighten the
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CONCLUSIONS In summary, we focus on the resistance switching and electricfield-induced magnetization modulation based on the Ag/TP/ Al structure. The TiO2 film was synthesized by magnetron sputtering on PAA substrates. Large magnetization changes of the TP films emerge in response to the resistance switching effect of the Ag/TP/Al heterostructure controlled by applied electric field. Our results provide convincing evidence for the influence of oxygen vacancies on resistive behavior and magnetization modulation in undoped TP nanoporous films. Furthermore, the results suggest that the magnetic ordering might be obtained by regulating the kinds of anion vacancy defects in semiconductor oxides. The combination of the resistance switching and magnetism modulation caused by electric field at room temperature makes it possible to integrate both ferromagnetism and electrical properties into a simple Ag/TP/Al structure, which can be extended to produce other novel dilute magnetic semiconductors and is expected to be used in new memory and new spintronic devices.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.S.). *E-mail:
[email protected] (C.P.). ORCID
Xidi Sun: 0000-0002-0211-6555 Lihu Liu: 0000-0001-8362-7754 Huiyuan Sun: 0000-0001-8462-7609 Caofeng Pan: 0000-0001-6327-9692 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the support of the Natural Science Foundation of Hebei Province (no. A2016205237).
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REFERENCES
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.9b02593 ACS Appl. Mater. Interfaces 2019, 11, 21661−21667
