Novel physical properties of functional oxide thin films
过渡金属氧化物具有比半导体材料更复杂的晶体结构、化学配比关系和价态变化、以及相图等, 其内部还存在晶格、电荷、自旋、轨道等多个自由度之间的竞争耦合关系, 因此具有更丰富的物理性能 (如高温超导、庞磁电阻、铁电/介电、多铁性、金属-绝缘体相变、光电、热电、光学非线性、重费米子等) 和更多可调控其性能的外场激励 (电、磁、光、应力、温度等外场). 尽管现今基于半导体材料的器件在信息工业中占统治地位, 这些具有快速、庞大响应及高电容特性的功能氧化物材料, 实现了操控多个自由度 (包括自旋、电荷、轨道、晶格、氧空位等) 对电、光、磁学性能调控而具有更多功能的氧化物器件, 这可能将会是下一代理想器件. 另外, 区别于块体材料, 薄膜材料具有低维度和异质界面等特点, 可以针对量子约束、量子相干、量子涨落、拓扑电子态、电子-电子相互作用、自旋-轨道耦合以及对称性破缺等物理问题进行人工结构设计和多场调控. 随着薄膜技术的精进和飞速发展, 科学家们已经实现了单原胞层薄膜的精确制备和准确化学计量比的控制, 产生了一系列具有重要影响力的工作. 例如, 美国斯坦福大学的 Hwang 教授等在过渡金属氧化物界面处观测到铁磁相与超导相共存、高迁移率二维电子气以及新型超导—镍基超导等, 中国科学院的马秀良研究员和美国加州大学伯克利大学的 Ramesh 教授在 PbTiO3薄膜中发现拓扑铁电极化畴结构等. 多功能氧化物薄膜体系为力、光、电、磁等外场调控提供广阔平台, 从而衍生出许多新奇量子物性和功能, 进而大大拓展物质科学的研究空间, 同时对新物态的探索和量子临界现象的研究具有重要意义. 同时,多功能氧化物薄膜的发展也将为未来量子信息和量子计算提供可靠的材料基础, 甚至突破目前材料体系的壁垒, 有望在上述领域产生深远的影响.
应《物理学报》编辑部的邀请, 编者邀请了部分活跃在多功能氧化物材料研究的第一线的中青年科学家, 组织了本期的专题. 本期专题文章大致分成如下几个方面: 在氧化物薄膜的新奇物性方面, 张坚地教授报道了氧化物异质界面上的准二维超导现象, 廖昭亮教授报道了氧化物异质结中的反常霍尔效应, 翟晓芳教授和成龙教授报道了超薄膜制备条件和拓扑霍尔效应之间的关联, 汪志明教授报道了过渡金属氧化物中新奇量子态与电荷-自旋互转换研究进展, 郭尔佳研究员报道了钴氧化物中晶格与自旋的关联耦合效应; 在氧化物铁电薄膜的研究方面, 陈祖煌教授报道了锆酸铅基反铁电薄膜研究现状与展望, 金魁研究员、王旭研究员和石兢教授报道了面向宽温域功能器件的连续组分外延铁电薄膜的研究, 刘明教授和董国华教授报道了自支撑单晶氧化物薄膜的应用研究进展; 在外场对氧化物薄膜物性的调控方面, 李千教授报道了应变增强 Nb 掺杂 SrTiO3薄膜的热电性能, 编者报道了外场对拓扑相变类氧化物薄膜物性的调控效应; 在氧化物薄膜的应用方面, 樊贞教授和刘俊明教授报道了钙钛矿相界面插层对 SrFeOx基忆阻器的性能提升, 袁国亮教授和陆旭兵教授报道了 HfO2基铁电薄膜的结构、性能调控及典型器件应用, 吴真平教授和王月晖博士后报道了基于 HfO2插层的 Ga2O3基金属-绝缘体-半导体结构日盲紫外光电探测器.
本专题从不同的角度描述了多功能氧化物薄膜的进展, 反映了此领域的一些现状, 希望对读者了解此前沿课题有所帮助.
2023, 72 (9): 097401.
doi: 10.7498/aps.72.20230044
Abstract +
Oxide interfaces manifest many fascinating phenomena with synergetic correlations among multiple degrees of freedom, including the interplay of broken symmetry, lattice mismatch, charge transfer, spatial confinement. In particular, the interface superconductivity in oxide heterostructure has attracted extensive attention due to the rich underlying physical connotations. The interfaces not only provide alternative research platforms with respect to the bulk material counterpart for exploring new superconductors and investigating superconducting mechanisms, but also create new opportunities for applying superconductors to future electronic devices. In recent years, owing to the rapid development of heteroepitaxial techniques and accurate characterization methods, researchers have found quasi-two-dimensional interface superconductivity in various oxide heterostructures and revealed numerous novel quantum phenomena associated with interface superconductivity, which not only promotes the development of condensed matter physics, but also lays important foundation for the practical application of interface superconductivity. In this brief review, we mainly focus on the quasi-two-dimensional superconductivity at oxide interface. Taking the typical quasi-two-dimensional superconductivity at the LaAlO3/SrTiO3 interface and copper oxides such as La2CuO4/La1.56Sr0.44CuO4 for example, we summarize and examine some novel physical phenomena with interface superconductivity in complex oxide heterostructures. Then we address the related problems that remain to be solved, and finally we prospect the possible future development of the interface superconductivity.
2023, 72 (9): 097301.
doi: 10.7498/aps.72.20221934
Abstract +
SrFeOx (SFO) is a kind of material that can undergo a reversible topotactic phase transformation between an SrFeO2.5 brownmillerite (BM) phase and an SrFeO3 perovskite (PV) phase. This phase transformation can cause drastic changes in physical properties such as electrical conductivity, while maintaining the lattice framework. This makes SFO a stable and reliable resistive switching (RS) material, which has many applications in fields like RS memory, logic operation and neuromorphic computing. Currently, in most of SFO-based memristors, a single BM-SFO layer is used as an RS functional layer, and the working principle is the electric field-induced formation and rupture of PV-SFO conductive filaments (CFs) in the BM-SFO matrix. Such devices typically exhibit abrupt RS behavior, i.e. an abrupt switching between high resistance state and low resistance state. Therefore, the application of these devices is limited to the binary information storage. For the emerging applications like neuromorphic computing, the BM-SFO single-layer memristors still face problems such as a small number of resistance states, large resistance fluctuation, and high nonlinearity under pulse writing. To solve these problems, a BM-SFO/PV-SFO double-layer memristor is designed in this work, in which the PV-SFO layer is an oxygen-rich interfacial intercalated layer, which can provide a large number of oxygen ions during the formation of CFs and withdraw these oxygen ions during the rupture of CFs. This allows the geometric size (e.g., diameter) of the CFs to be adjusted in a wide range, which is beneficial to obtaining continuously tunable, multiple resistance states. The RS behavior of the designed double-layer memristor is studied experimentally. Compared with the single-layer memristor, it exhibits good RS repeatability, small resistance fluctuation, small and narrowly distributed switching voltages. In addition, the double-layer memristor exhibits stable and gradual RS behavior, and hence it is used to emulate synaptic behaviors such as long-term potentiation and depression. A fully connected neural network (ANN) based on the double-layer memristor is simulated, and a recognition accuracy of 86.3% is obtained after online training on the ORHD dataset. Comparing with a single-layer memristor-based ANN, the recognition accuracy of the double-layer memristor-based one is improved by 69.3%. This study provides a new approach to modulating the performance of SFO-based memristors and demonstrates their great potential as artificial synaptic devices to be used in neuromorphic computing.
2023, 72 (9): 097503.
doi: 10.7498/aps.72.20221852
Abstract +
Many emergent and novel phenomena occur in nonmagnetic/ferromagnet heterostructures. In particular, Pt/ferromagnet heterostructures where the Pt has strong spin-orbit coupling and thus can convert spin current into charge current, has attracted a great attention recently. The anomalous Hall effect (AHE) has been found in many Pt/ferromagnet heterostructures. However, the underlying physics remains elusive, so it is necessary to find more heterostructures in order to provide more experimental data. In this work, we investigate anomalous Hall resistances (AHRs) in Pt thin films sputtered on epitaxial La0.67Sr0.33MnO3 (LSMO) ferromagnetic films. High-quality Pt/LSMO heterojunctions are fabricated by pulsed laser deposition and RF-magnetron sputtering. The physical properties of LSMO films are characterized by the measurements of magnetic and transport properties. The AHR mainly contributed by Pt in the Pt/LSMO heterojunction increases sharply with temperature decreasing and changes its sign below 40 K. Furthermore, the AHR decreases sharply with the increase of Pt thickness. Those facts suggest that the ferromagnetism of Pt originates from interface due to magnetic proximity effect. Interestingly, this heterojunction can exhibit possible signal of topological Hall effect under low applied magnetic field. The above results provide an experimental basis for further understanding the interactions between electron spin and charge transport in nonmagnetic/ferromagnetic heterostructures.
2023, 72 (9): 096802.
doi: 10.7498/aps.72.20221854
Abstract +
As one of the magnetic transition metal oxides, SrRuO3 (SRO) has received much attention in recent years, which is mainly due to its unique itinerate ferromagnetism and the unusual electrical transport properties–behaving as Fermi liquid at low temperature and bad metal at high temperature. In the growth of SRO thin films, there are many factors that can affect the quality of thin films. In this work, we study various factors affecting the growth and quality of SRO thin films by using laser molecular beam epitaxy (laser MBE), including laser energy density, substrate temperature and target surface conditions, and explore their influences on the topological Hall effect (THE) in SRO. For thin films grown at high laser energy density and high temperature, we found that there are large trenches at the edge of steps, which deteriorate the transport properties of the thin films. When using low laser energy density, extra SrO may exist in the films, which also suppresses the conductivity. Films grown at low temperature tend to have poor crystallinity while films grown at high temperature exhibit island structures. The ablation degree of the target surface increases the decomposition of SRO to SrO, Ru and volatile RuO4, resulting in Ru defects in the grown thin film. The SRO thin film grown under the optimal conditions (1.75 J·cm–2, 670 ℃, fresh target surface) exhibits the optimal conductivity and the strongest THE. For non-optimal growth conditions that favors thickness inhomogeneity or Ru defects in the film, THE becomes weaker or even disappears. Therefore, we believe that the THE is due to the Dzyaloshinskii-Moriya interaction (DMI) resulting from the interfacial inversion asymmetry and the associated chiral spin structures.
Composition-spread epitaxial ferroelectric thin films for temperature-insensitive functional devices
2023, 72 (9): 097701.
doi: 10.7498/aps.72.20230154
Abstract +
BaxSr1–xTiO3 (BST) ferroelectric thin films are widely used in microwave tunable devices due to their high dielectric constants, strong electric field tunabilities and low microwave losses. However, because of the temperature dependence of dielectric constant in ferroelectric material, the high-tunability for conventional single component ferroelectric thin film can only be achieved in the vicinity of Curie Temperature (TC) which leads the ferroelectric thin films to be difficult to operate in a wide temperature range. To obtain ferroelectric thin films for temperature stable functional devices, single composition Ba0.2Sr0.8TiO3 thin films, Ba0.5Sr0.5TiO3 thin films, and Ba0.2Sr0.8TiO3/Ba0.5Sr0.5TiO3 heterostructure thin films are deposited by pulsed laser deposition (PLD). By comparing their dielectric properties in a wide temperature range, it is found that the temperature sensitivity of BST film can be effectively reduced by introducing a composition gradient along the epitaxial direction. However, the heterostructure engineering may bring extra troubles caused by interfaces, which may limit the quality factor Q. In this paper, we extend our combinatorial film deposition technique to ferroelectric materials, and we successfully fabricate in-plane composition-spread Ba1–xSrxTiO3 thin films, which are expected to broaden the phase transition temperature ranges of BST films while avoiding the problem of interface control.
2023, 72 (9): 097302.
doi: 10.7498/aps.72.20222222
Abstract +
Solar-blind photodetector (PD) converts 200–280 nm ultraviolet (UV) light into electrical signals, thereby expanding application range from security communication to missile or fire alarms detections. As an emerging ultra-wide bandgap semiconductor, gallium oxide (Ga2O3) has sprung to the forefront of solar blind detection activity due to its key attributes, including suitable optical bandgap, convenient growth procedure, highly temperture/chemical/radiation tolerance, and thus becoming a promising candidate to break the current bottleneck of photomultiplier tubes. The Ga2O3-based solar blind PDs based on various architectures have been realized in the past decade, including photoconductive PDs, Schottky barrier PDs, and avalanche PDs. Till now, the metal-semiconductor-metal (MSM) structure has been widely used in developing photoconductive Ga2O3 solar-blind PDs because of its simple preparation method and large light collection area. Unfortunately, despite unremitting efforts, the performance metric of reported MSM-type Ga2O3 solar-blind PDs still lags behind the benchmark of commercial PMTs. Apparently, lack of solution to the problem has greatly hindered further research and practical applications in this field. One effective strategy for further enhancing the device performance such as detectivity, external quantum efficiency (EQE), and light-to-dark ratio heavily relies on blocking the dark current. In this work, high-quality single crystalline β-Ga2O3 with a uniform thickness of 700 nm is grown by using a metal organic chemical vapor deposition (MOCVD) technique. Then atomic layer deposition (ALD) fabricated ultrathin hafnium oxide (HfO2) films ( $ \sim $ 10 nm) are introduced as inserted insulators and passivation layers. The 30 nm/100 nm Ti/Au interdigital electrodes (length: 2800 μm, width: 200 μm, spacing: 200 μm, 4 pairs) are fabricated by sputtering on the top of the film as the Ohmic contacts. Taking advantage of its novel dielectric and insulating properties, the leakage current on Ga2O3 thin film can be effectively inhibited by the inserted ultrathin HfO2 layer, and thus further improving the performance of PDs. Compared with simple MSM structured Ga2O3 PD, the resulting metal-insulator-semiconductor (MIS) device significantly reduces dark current, and thus improving specific detectivity, enhancing light-to-dark current ratio, and increasing response speed. These findings advance a significant step toward the suppressing of dark current in MSM structured photoconductive PDs and provide great opportunities for developing high-performance weak UV signal sensing in the future.
2023, 72 (9): 097703.
doi: 10.7498/aps.72.20222221
Abstract +
The rapid developments of big data, the internet of things, and artificial intelligence have put forward more and more requirements for memory chips, logic chips and other electronic components. This study introduces the ferroelectric origin of HfO2-based ferroelectric film and explains how element doping, defects, stresses, surfaces and interfaces, regulate and enhance the ferroelectric polarization of the film. It is widely accepted that the ferroelectricity of HfO2-based ferroelectric film originates from the metastable tetragonal phase. The ferroelectricity of the HfO2-based film can be enhanced by doping some elements such as Zr, Si, Al, Gd, La, and Ta, thereby affecting the crystal structure symmetry. The introduction of an appropriate number of oxygen vacancy defects can reduce the potential barrier of phase transition between the tetragonal phase and the monoclinic phase, making the monoclinic phase easy to transition to tetragonal ferroelectric phase. The stability of the ferroelectric phase can be improved by some methods, including forming the stress between the substrate and electrode, reducing the film thickness, constructing a nanolayered structure, and reducing the annealing temperature. Compared with perovskite oxide ferroelectric thin films, HfO2-based films have the advantages of good complementary-metal-oxide-semiconductor compatibility and strong ferroelectricity at nanometer thickness, so they are expected to be used in ferroelectric memory. The HfO2-based 1T1C memory has the advantages of fast reading and writing speed, more than reading and writing 1012 times, and high storage density, and it is the fast reading and writing speed that the only commercial ferroelectric memory possesses at present. The 1T ferroelectric field effect transistor memory has the advantages of non-destructive reading and high storage density. Theoretically, these memories can achieve the same storage density as flash memory, more than reading 1010 times, the fast reading/writing speed, low operating voltage, and low power consumption, simultaneously. Besides, ferroelectric negative capacitance transistor can obtain a subthreshold swing lower than 60 mV/dec, which greatly reduces the power consumption of integrated circuits and provides an excellent solution for further reducing the size of transistors. Ferroelectric tunnel junction has the advantages of small size and easy integration since the tunneling current can be largely adjusted through ferroelectric polarization switching. In addition, the HfO2-based field effect transistors can be used to simulate biological synapses for applications in neural morphology calculations. Moreover, the HfO2-based films also have broad application prospects in antiferroelectric energy storage, capacitor dielectric energy storage, memristor, piezoelectric, and pyroelectric devices, etc. Finally, the current challenges and future opportunities of the HfO2-based thin films and devices are analyzed.
2023, 72 (9): 098502.
doi: 10.7498/aps.72.20222382
Abstract +
Flexible electronics have aroused great interest of researchers because of their wide applications in information storage, energy harvesting and wearable device. To realize extraordinary functionalities, freestanding single crystal oxide thin film is utilized due to its super elasticity, easy-to-transfer, and outstanding ferro/electric/magnetic properties. Using the state-of-art synthesis methods, functional oxide films of various materials can be obtained in freestanding phase, which eliminates the restrictions from growth substrate and is transferable to other flexible layers. In this work, we first introduce wet etching and mechanical exfoliation methods to prepare freestanding single crystal oxide thin film, then review their applications in ferroelectric memory, piezoelectric energy harvester, dielectric energy storage, correlated oxide interface, and novel freestanding oxide structure. The recent research progress and future outlooks are finally discussed.
2023, 72 (9): 096803.
doi: 10.7498/aps.72.20222301
Abstract +
The development of high-performance thermoelectric materials can help solve the energy crisis in the future. Thin-film thermoelectric materials can meet the requirement for flexibility of wearable devices while supplying electrical power to them. In this study, high-quality Nb-doped SrTiO3 films (Nb:STO) with different thickness are prepared on SrTiO3 (STO) and La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrates by pulsed laser deposition. The surface morphologies, crystal structures, and thermoelectric performances of the films are characterized. The results show that the thermoelectric performance of the strain-free film increase with thickness increasing. The power factor at room temperature increases by 187%. The Seebeck coefficient of the 144 nm-thick Nb:STO/LSAT sample with strain is greatly improved to $265.95\;{\text{μ}}{\rm{V}}/{\rm{K}}$ at room temperature, which is likely to be due to the strain induced changes in the energy band of the thin film. The improvement of the thermoelectric performances of Nb:STO thin films by strain engineering provides a new approach to improving the thermoelectric properties of oxide thin films.
2023, 72 (9): 096801.
doi: 10.7498/aps.72.20222266
Abstract +
Perovskite transition-metal oxides can undergo significant structural topological phase transition between perovskite structure, brownmillerite structure, and infinite-layer structure under the external field through the gain and loss of the oxygen ions, accompanied with significant changes in physical properties such as transportation, magnetism, and optics. Topotactic phase transformation allows structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for utilizing the redox reaction and oxygen diffusion within transition metal oxides, and establishing great potential applications in solid oxide fuel cells, oxygen sensors, catalysis, intelligent optical windows, and neuromorphic devices. In this work, we review the recent research progress of manipulating the topological phase transition of the perovskite-type oxide films and regulating their physical properties, mainly focusing on tuning the novel physical properties of these typical films through strong interaction between the lattice and electronic degrees of freedom by the action of external fields such as strain, electric field, optical field, and temperature field. For example, a giant photoinduced structure distortion in SrCoO2.5 thin film excited by photons is observed to be higher than any previously reported results in the other transition metal oxide films. The SrFeO2 films undergo an insulator-to-metal transition when the strain state changes from compressive state to tensile state. It is directly observed that perovskite SrFeO3 nanofilament is formed under the action of electric field and extends almost through the brownmillerite SrFeO2.5 matrix in the ON state and is ruptured in the OFF state, unambiguously revealing a filamentary resistance switching mechanism. Utilizing in situ electrical scanning transmission electron microscopy, the transformation from brownmillerite SrFeO2.5 to infinite-layer SrFeO2 under electric field can be directly visualized with atomic resolution. We also clarify the relationship between the microscopic coupling mechanism and the macroscopic quantum properties of charges, lattices, orbits, spin, etc. Relevant research is expected to provide a platform for new materials, new approaches and new ideas for developing high-sensitivity and weak-field response electronic devices based on functional oxides. These findings about the topological phase transition in perovskite oxide films can expand the research scope of material science, and have important significance in exploring new states of matters and studying quantum critical phenomena.
Research progress of novel quantum states and charge-spin interconversion in transition metal oxides
2023, 72 (9): 097702.
doi: 10.7498/aps.72.20222219
Abstract +
For efficient storage and processing of massive data in the information technology era, spintronic device attracts tremendous attention due to its low power consumption and non-volatile feature. Spin source material, which can efficiently generates spin current, is an important constituent of novel spin-orbit torque device. The efficiency of spin current generation in spin source material directly determines the performances of various spintronic devices. In the past two decades, great progress has been made in exploring high-efficient spin source material systems and understanding the relevant physical mechanisms. A wide variety of materials are explored, ranging from traditional heavy metals and semiconductors to topological insulators and two-dimensional (2D) materials. Recently, the material family of transition metal oxides attracts tremendous attention due to its efficient and highly tunable charge-spin conversion intimately related to its emerging novel quantum states and electronic structure. The mechanism of charge-spin conversion generally has two contributions: the bulk spin Hall effect and the spin-momentum locked interface with inversion symmetry breaking. Novel electronic structures such as topological band structures and spin-momentum locked surface states can realize efficient charge-spin conversion. For example, the Weyl points in SrRuO3 and the topological Dirac nodal line in SrIrO3 are predicted to give rise to a large Berry curvature and corresponding spin Hall conductance; the topological surface states can generate spin accumulation due to spin-momentum locking; the Rashba states at the oxide interface such as the 2D electron gas in SrTiO3 and KTaO3 can generate spin current by Rashba-Edelstein effect. Furthermore, the entanglement of various degrees of freedom, including spin, charge, lattice and orbit in transition metal oxides lead to the electronic structure being highly tunable by various methods including gate voltage, substrate constraint, thickness, interface engineering, etc. Therefore, charge-spin conversion in transition metal oxides is of great significance for both modulating of novel electronic structure in fundamental research and exploring its promising potential in future spintronic devices. In this review, we focus on introducing aspects of exotic electronic structures, spin transport mechanism, charge-spin interconversion characterization, efficiency and manipulation in transition metal oxides, and giving a prospect on the future development trend.
2023, 72 (9): 097704.
doi: 10.7498/aps.72.20230389
Abstract +
It has been more than 70 years since the first anti-ferroelectric was discovered. Its unique electric-field-induced phase transition behavior shows great potential applications in the fields of energy storage, electrocaloric, negative capacitance, thermal switching, etc. With the development of advanced synthesis technology and the trend of miniaturization and integration of devices, high-quality functional oxide films have received more and more attention. A large number of studies have shown that anti-ferroelectric thin film exhibits more novel properties than bulk, but it also faces more challenges, such as the disappearance of antiferroelectricity under a critical thickness induced by size effect. In this paper, we review the development history of lead zirconate-based anti-ferroelectric thin films, and discuss their structures, phase transitions and applications. We hope that this paper can attract more researchers to pay attention to the development of anti-ferroelectric thin films, so as to develop more new materials and explore new applications.
2023, 72 (9): 097502.
doi: 10.7498/aps.72.20230206
Abstract +
Strongly correlated electronic system contains strong coupling among multi-order parameters and is easy to efficiently tune by external field. Cobaltite (LaCoO3) is a typical multiferroic (ferroelastic and ferromagnetic) material, which has been extensively investigated over decades. Conventional research on cobaltites has focused on the ferroelastic phase transition and structure modulation under stress. Recently, researchers have discovered that cobaltite thin films undergo a paramagnetic-to-ferromagnetic phase transition under tensile strain, however, its origin has been controversial over decades. Some experimental evidence shows that stress leads the valence state of cobalt ions to decrease, and thus producing spin state transition. Other researchers believe that the stress-induced nano-domain structure will present a long-range ordered arrangement of high spin states, which is the main reason for producing the ferromagnetism of cobalt oxide films. In this paper, we review a series of recent researches of the strong correlation between spin and lattice degrees of freedom in cobalt oxide thin films and heterojunctions. The reversible spin state transition in cobalt oxide film is induced by structural factors such as thin-film thickness, lattice mismatch stress, crystal symmetry, surface morphology, interfacial oxygen ion coordination, and oxygen octahedral tilting while the valence state of cobalt ions is kept unchanged, and thus forming highly adjustable macroscopic magnetism. Furthermore, the atomic-level precision controllable film growth technology is utilized to construct single cell layer cobaltite superlattices, thereby achieving ultra-thin two-dimensional magnetic oxide materials through efficient structure regulation. These advances not only clarified the strong coupling between lattice and spin order parameters in the strongly correlated electronic system, but also provided excellent candidate for the realization of ultra-thin room temperature ferromagnets that are required by oxide spintronic devices.
