Magneto-ionic Control of Ferrimagnetic Order by Oxygen Gating.

Ferrimagnets are considered an excellent spintronic material candidate which combines ultrafast magnetic dynamics and straightforward electrical detectability. However, efficient routes toward magneto-ionic control of ferrimagnetic order remain elusive. In this study, a solid-state oxygen gating device was designed to control the magnetic properties of the ferrimagnetic CoTb alloy. Experimental results show that applying a small voltage can irreversibly tune a Tb-dominant device to a stable Co-dominant state and decrease the magnetization compensation temperature by 130 K. In addition, a reversible voltage control of the magnetization axis between out-of-plane and in-plane states is observed, which indicates that the migrated oxygen ions can bond to both Tb and Co sublattices. First-principles calculations indicate that voltage can dynamically control the flow-in and flow-out of oxygen ions that bond to the Co sublattice. Our work provides an effective means to manipulate ferrimagnetic order and contributes to the development of ultra-low-power spintronic devices.

Dynamics of Anisotropic Oxygen-Ion Migration in Strained Cobaltites.

Orientation control of the oxygen vacancy channel (OVC) is highly desirable for tailoring oxygen diffusion as it serves as a fast transport channel in ion conductors, which is widely exploited in solid-state fuel cells, catalysts, and ion-batteries. Direct observation of oxygen-ion hopping toward preferential vacant sites is a key to clarifying migration pathways. Here we report anisotropic oxygen-ion migration mediated by strain in ultrathin cobaltites via in situ thermal activation in atomic-resolved transmission electron microscopy. Oxygen migration pathways are constructed on the basis of the atomic structure during the OVC switching, which is manifested as the vertical-to-horizontal OVC switching under tensile strain but the horizontal-to-diagonal switching under compression. We evaluate the topotactic structural changes to the OVC, determine the crucial role of the tolerance factor for OVC stability, and establish the strain-dependent phase diagram. Our work provides a practical guide for engineering OVC orientation that is applicable to ionic-oxide electronics.

Comment on "Bipolar Resistive Switching in Lanthanum Titanium Oxide and an Increased On/Off Ratio Using an Oxygen-Deficient ZnO Interlayer".

The Mott-based resistive switching random access memory (RRAM) has been considered as a promising candidate for next-generation nonvolatile mass storage. Its performance relies on the oxygen migration process. In ACS Appl. Mater. Interfaces 2022, 14, 17682-17690, Wang et al. demonstrated that inserting a ZnO layer between the top tungsten electrode and the lanthanum titanium oxide (LTO) layer can improve the ON/OFF current ratio of the RRAM. This improvement was attributed to the increased oxygen vacancies in the ZnO layer. We argue their interpretation with the role, significance, and statistics of the oxygen vacancy in the samples. Our experimental evidence contradicts their claim, and we propose that hydroxy groups should be the responsible candidate for their claimed oxygen vacancy peak at around 531-532 eV in the O 1s photoelectron spectroscopy observed in their samples. We thus propose an alternative mechanism for the ON/OFF ratio enhancement with the ZnO interlayer: the ZnO layer prevents the LTO oxygen and hydroxy groups migration and from reacting with the multivalent tungsten electrode.

Memristive Characteristics of the Single-Layer P-Type CuAlO2 and N-Type ZnO Memristors.

Memristive behaviors are demonstrated in the single-layer oxide-based devices. The conduction states can be continually modulated with different pulses or voltage sweeps. Here, the p-CuAlO2- and n-ZnO-based memristors show the opposite bias polarity dependence with the help of tip electrode. It is well known that the conductivity of p-type and n-type semiconductor materials has the opposite oxygen concentration dependence. Thus, the memristive behaviors may attribute to the oxygen ion migration in the dielectric layers for the single-layer oxide based memristors. Further, based on the redox, the model of compressing dielectric layer thickness has been proposed to explain the memristive behavior.

Bipolar Resistive Switching in Lanthanum Titanium Oxide and an Increased On/Off Ratio Using an Oxygen-Deficient ZnO Interlayer.

The present study pioneered an oxygen migration-driven metal to insulator transition Mott memory, a new type of nonvolatile memory using lanthanum titanium oxide (LTO). We first show the reset first bipolar property without an initial electroforming process in LTO. We used oxygen-deficient ZnO as an interlayer between LTO and a W electrode to clarify whether oxygen migration activates LTO as the Mott transition. ZnO oxygen deficiency provides oxygen ion migration paths as well as a reservoir, facilitating oxygen migration from LTO to the W electrode. Thus, including the ZnO interlayer improved oxygen migration between LTO and the W electrode, achieving a 10-fold increased on/off current ratio. The current research contributes to a better understanding of valence change Mott memory by exploring the LTO resistive switching mechanism and ZnO interlayer influences on the oxygen migration process.

A 1D Vanadium Dioxide Nanochannel Constructed via Electric-Field-Induced Ion Transport and its Superior Metal-Insulator Transition.

Nanoscale manipulation of materials' physicochemical properties offers distinguished possibility to the development of novel electronic devices with ultrasmall dimension, fast operation speed, and low energy consumption characteristics. This is especially important as the present semiconductor manufacturing technique is approaching the end of miniaturization campaign in the near future. Here, a superior metal-insulator transition (MIT) of a 1D VO2 nanochannel constructed through an electric-field-induced oxygen ion migration process in V2 O5 thin film is reported for the first time. A sharp and reliable MIT transition with a steep turn-on voltage slope of <0.5 mV dec-1 , fast switching speed of 17 ns, low energy consumption of 8 pJ, and low variability of <4.3% is demonstrated in the VO2 nanochannel device. High-resolution transmission electron microscopy observation and theoretical computation verify that the superior electrical properties of the present device can be ascribed to the electroformation of nanoscale VO2 nanochannel in V2 O5 thin films. More importantly, the incorporation of the present device into a Pt/HfO2 /Pt/VO2 /Pt 1S1R unit can ensure the correct reading of the HfO2 memory continuously for 107 cycles, therefore demonstrating its great possibility as a reliable selector in high-density crossbar memory arrays.

Controlling Oxygen Mobility in Ruddlesden-Popper Oxides.

Discovering new energy materials is a key step toward satisfying the needs for next-generation energy conversion and storage devices. Among the various types of oxides, Ruddlesden-Popper (RP) oxides (A2BO4) are promising candidates for electrochemical energy devices, such as solid oxide fuel cells, owing to their attractive physicochemical properties, including the anisotropic nature of oxygen migration and controllable stoichiometry from oxygen excess to oxygen deficiency. Thus, understanding and controlling the kinetics of oxygen transport are essential for designing optimized materials to use in electrochemical energy devices. In this review, we first discuss the basic mechanisms of oxygen migration in RP oxides depending on oxygen nonstoichiometry. We then focus on the effect of changes in the defect concentration, crystallographic orientation, and strain on the oxygen migration in RP oxides. We also briefly review their thermal and chemical stability. Finally, we conclude with a perspective on potential research directions for future investigation to facilitate controlling oxygen ion migration in RP oxides.

Effect of Electronegativity on Bipolar Resistive Switching in a WO3-Based Asymmetric Capacitor Structure.

This study investigates the transport and switching time of nonvolatile tungsten oxide based resistive-switching (RS) memory devices. These devices consist of a highly resistive tungsten oxide film sandwiched between metal electrodes, and their RS characteristics are bipolar in the counterclockwise direction. The switching voltage, retention, endurance, and switching time are strongly dependent on the type of electrodes used, and we also find quantitative and qualitative evidence that the electronegativity (χ) of the electrodes plays a key role in determining the RS properties and switching time. We also propose an RS model based on the role of the electronegativity at the interface.

Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaO x interface.

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >10(5) cycles, and good data retention of >10(4) s with a good resistance ratio of >10(2) at 85°C are obtained using a Ti nanolayer to form a W/TiO x /TaO x /W structure under a low current operation of 80 µA, while few switching cycles are observed for W/TaO x /W structure under a higher current compliance >300 µA. The low resistance state decreases with increasing current compliances from 10 to 100 µA, and the device could be operated at a low RESET current of 23 µA. A small device size of 150 × 150 nm(2) is observed by transmission electron microscopy. The presence of oxygen-deficient TaO x nanofilament in a W/TiO x /TaO x /W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaO x interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.(2).

Co nanoparticles induced resistive switching and magnetism for the electrochemically deposited polypyrrole composite films.

The resistive switching behavior of Co-nanoparticle-dispersed polypyrrole (PPy) composite films is studied. A novel design method for resistive random access memory (ReRAM) is proposed. The conducting polymer films with metal nanocrystal (NC)-dispersed carbon chains induce the spontaneous oxidization of the conducting polymer at the surface. The resistive switching behavior is achieved by an electric field controlling the oxygen ion mobility between the metal electrode and the conducting polymer film to realize the mutual transition between intrinsic conduction (low resistive state) and oxidized layer conduction (high resistive state). Furthermore, the formation process of intrinsic conductive paths can be effectively controlled in the conducting polymer ReRAM using metal NCs in films because the inner metal NCs induce electric field lines converging around them and the intensity of the electric field at the tip of NCs can greatly exceed that of the other region. Metal NCs can also bring new characteristics for ReRAM, such as magnetism by dispersing magnetic metal NCs in polymer, to obtain multifunctional electronic devices or meet some special purpose in future applications. Our works will enrich the application fields of the electromagnetic PPy composite films and present a novel material for ReRAM devices.