Exceptionally High, Strongly Temperature Dependent, Spin Hall Conductivity of SrRuO3

Voltage control of multiferroic magnon torque for reconfigurable logic-in-memory

Magnons, bosonic quasiparticles carrying angular momentum, can flow through insulators for information transmission with minimal power dissipation. However, it remains challenging to develop a magnon-based logic due to the lack of efficient electrical manipulation of magnon transport. Here we show the electric excitation and control of multiferroic magnon modes in a spin-source/multiferroic/ferromagnet structure. We demonstrate that the ferroelectric polarization can electrically modulate the magnon-mediated spin-orbit torque by controlling the non-collinear antiferromagnetic structure in multiferroic bismuth ferrite thin films with coupled antiferromagnetic and ferroelectric orders. In this multiferroic magnon torque device, magnon information is encoded to ferromagnetic bits by the magnon-mediated spin torque. By manipulating the two coupled non-volatile state variables-ferroelectric polarization and magnetization-we further present reconfigurable logic operations in a single device. Our findings highlight the potential of multiferroics for controlling magnon information transport and offer a pathway towards room-temperature voltage-controlled, low-power, scalable magnonics for in-memory computing.

Field-Free Spin-Orbit Torque Magnetization Switching in a Single-Phase Ferromagnetic and Spin Hall Oxide

Current-induced spin-orbit torque (SOT) offers substantial promise for the development of low-power, nonvolatile magnetic memory. Recently, a single-phase material concurrently exhibiting magnetism and the spin Hall effect has emerged as a scientifically and technologically interesting platform for realizing efficient and compact SOT systems. Here, we demonstrate external-magnetic-field-free switching of perpendicular magnetization in a single-phase ferromagnetic and spin Hall oxide SrRuO3. We delicately altered the local lattices of the top and bottom surface layers of SrRuO3, while retaining a quasi-homogeneous, single-crystalline nature of the SrRuO3 bulk. This leads to unbalanced spin Hall effects between the top and bottom layers, enabling net SOT performance within single-layer ferromagnetic SrRuO3. Notably, our SrRuO3 exhibits the highest SOT efficiency and lowest power consumption among all known single-layer systems under field-free conditions. Our method of artificially manipulating the local atomic structures will pave the way for advances in spin-orbitronics and the exploration of new SOT materials.

Oxide Spintronics as a Knot of Physics and Chemistry: Recent Progress and Opportunities

Transition-metal oxides (TMOs) constitute a key material family in spintronics because of mutually coupled degrees of freedom and tunable magneto-ionic properties. In this Perspective, we consider oxide spintronics as a knot of physics and chemistry and mainly discuss two current hot topics: spin-charge interconversion and magneto-ionics. First, spin-charge interconversion is focused on oxide films and heterostructures including 4d/5d heavy metal oxides (e.g., SrIrO₃) and two-dimensional electron gases. Based on spin-charge interconversion, charge currents can be transformed to spin currents and generate spin-orbit torque in oxide/metal and all-oxide heterostructures. Additionally, the voltage control of magnetism in TMOs by the magneto-ionic pathway has rapidly accelerated during the past few years due to the versatile advantages of effective control, nonvolatile nature, low power cost, etc. Typical magneto-ionic oxide systems and corresponding physicochemical mechanisms will be discussed. Finally, further developments of oxide spintronics are envisioned, including material discovery, physics exploration, device design, and manipulation methods.

Recent development and challenges in fuel cells and water electrolyzer reactions: an overview

Water electrolysis is the most promising method for the production of large scalable hydrogen (H2), which can fulfill the global energy demand of modern society. H2-based fuel cell transportation has been operating with zero greenhouse emission to improve both indoor and outdoor air quality, in addition to the development of economically viable sustainable green energy for widespread electrochemical applications. Many countries have been eagerly focusing on the development of renewable as well as H2-based energy storage infrastructure to fulfill their growing energy demands and sustainable goals. This review article mainly discusses the development of different kinds of fuel cell electrocatalysts, and their application in H2 production through various processes (chemical, refining, and electrochemical). The fuel cell parameters such as redox properties, cost-effectiveness, ecofriendlyness, conductivity, and better electrode stability have also been highlighted. In particular, a detailed discussion has been carried out with sufficient insights into the sustainable development of future green energy economy.

Rashba-Edelstein Effect in the h-BN Van Der Waals Interface for Magnetization Switching

Van der Waals materials are attracting great attention in the field of spintronics due to their novel physical properties. For example, they are utilized as spin-current generating materials in spin-orbit torque (SOT) devices, which offers an electrical way to control the magnetic state and is promising for future low-power electronics. However, SOTs have mostly been demonstrated in vdW materials with strong spin-orbit coupling (SOC). Here, the observation of a current-induced SOT in the h-BN/SrRuO₃ bilayer structure is reported, where the vdW material (h-BN) is an insulator with negligible SOC. Importantly, this SOT is strong enough to induce the switching of the perpendicular magnetization in SrRuO₃ . First-principles calculations suggest a giant Rashba effect at the interface between vdW material and SrRuO₃ (110)_pc thin film, which leads to the observed SOT based on a simplified tight-binding model. Furthermore, it is demonstrated that the current-induced magnetization switching can be modulated by the electric field. This study paves the way for exploring the current-induced SOT and magnetization switching by integrating vdW materials with ferromagnets.

Observation of topological Hall torque exerted on a domain wall in the ferromagnetic oxide SrRuO3

In a ferromagnetic Weyl metal SrRuO₃, a large effective magnetic field H_eff exerted on a magnetic domain wall (DW) by current has been reported. We show that the ratio of H_eff to current density exhibits nonmonotonic temperature dependence and surpasses those of conventional spin-transfer torques and spin-orbit torques. This enhancement is described well by topological Hall torque (THT), which is exerted on a DW by Weyl electrons emerging around Weyl points when an electric field is applied across the DW. The ratio of the H_eff arising from the THT to current density is over one order of magnitude higher than that originating from spin-transfer torques and spin-orbit torques reported in metallic systems, showing that the THT may provide a better way for energy-efficient manipulation of magnetization in spintronics devices.

Modulation of Spin-Orbit Torque from SrRuO3 by Epitaxial-Strain-Induced Octahedral Rotation

Spin-orbit torque (SOT), which arises from the spin-orbit coupling of conduction electrons, is believed to be the key route for developing low-power, high-speed, and nonvolatile memory devices. Despite the theoretical prediction of pronounced Berry phase curvatures in certain transition-metal perovskite oxides, which lead to considerable intrinsic spin Hall conductivity, SOT from this class of materials has rarely been reported until recently. Here, the SOT generated by epitaxial SrRuO₃ of three different crystal structures is systematically studied. The results of both spin-torque ferromagnetic resonance and in-plane harmonic Hall voltage measurements concurrently reveal that the intrinsic SOT efficiency of SrRuO₃ decreases when the epitaxial strain changes from tensile to compressive. The X-ray diffraction data demonstrate a strong correlation between the magnitude of SOT and octahedral rotation around the in-plane axes of SrRuO₃ , consistent with the theoretical prediction. This work offers new possibilities of tuning SOT with crystal structures and novel opportunities of integrating the unique properties of perovskite oxides with spintronic functionalities.

Novel Spin-Orbit Torque Generation at Room Temperature in an All-Oxide Epitaxial La0.7 Sr0.3 MnO3 /SrIrO3 System

Spin-orbit torques (SOTs) that arise from materials with large spin-orbit coupling offer a new pathway for energy-efficient and fast magnetic information storage. SOTs in conventional heavy metals and topological insulators are explored extensively, while 5d transition metal oxides, which also host ions with strong spin-orbit coupling, are a relatively new territory in the field of spintronics. An all-oxide, SrTiO₃ (STO)//La_0.7 Sr_0.3 MnO₃ (LSMO)/SrIrO₃ (SIO) heterostructure with lattice-matched crystal structure is synthesized, exhibiting an epitaxial and atomically sharp interface between the ferromagnetic LSMO and the high spin-orbit-coupled metal SIO. Spin-torque ferromagnetic resonance (ST-FMR) is used to probe the effective magnetization and the SOT efficiency in LSMO/SIO heterostructures grown on STO substrates. Remarkably, epitaxial LSMO/SIO exhibits a large SOT efficiency, ξ_||  = 1, while retaining a reasonably low shunting factor and increasing the effective magnetization of LSMO by ≈50%. The findings highlight the significance of epitaxy as a powerful tool to achieve a high SOT efficiency, explore the rich physics at the epitaxial interface, and open up a new pathway for designing next-generation energy-efficient spintronic devices.

Independence of the Inverse Spin Hall Effect with the Magnetic Phase in Thin NiCu Films

Large spin Hall angles have been observed in 3d ferromagnets, but their origin, and especially their link with the ferromagnetic order, remain unclear. Here, we investigate the evolution of the inverse spin Hall effect of Ni_{60}Cu_{40} and Ni_{50}Cu_{50} across their Curie temperatures using spin-pumping experiments. We show that the inverse spin Hall effect in these samples is comparable to that of platinum, and that it is insensitive to the magnetic order. These results point toward a Heisenberg localized model of the transition and suggest that the large spin Hall effects in 3d ferromagnets can be independent of the magnetic phase.

Effects of Anisotropic Strain on Spin-Orbit Torque Produced by the Dirac Nodal Line Semimetal IrO2

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξ_DL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO₂ thin films. IrO₂ possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σ_SH. We find that the (001) films do exhibit exceptionally high ξ_DL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σ_SH to be 1.9 × 10⁵ and 3.75 × 10⁵ Ω^-1 m^-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξ_DL and σ_SH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σ_SH near the DNLs.

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO3 Thin Film

SrRuO₃ (SRO) thin films and their heterostructure have attracted much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft X-ray scattering (RSXS) at the oxygen K edge to investigate the spin configuration of a four-unit-cell SRO film that was grown epitaxially on a single-crystal SrTiO₃. The RSXS signal under a magnetic field (∼0.4 tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.