Anomalous spin current anisotropy in a noncollinear antiferromagnet.

Cubic materials host high crystal symmetry and hence are not expected to support anisotropy in transport phenomena. In contrast to this common expectation, here we report an anomalous anisotropy of spin current can emerge in the (001) film of Mn3Pt, a noncollinear antiferromagnetic spin source with face-centered cubic structure. Such spin current anisotropy originates from the intertwined time reversal-odd ([Formula: see text]-odd) and time reversal-even ([Formula: see text]-even) spin Hall effects. Based on symmetry analyses and experimental characterizations of the current-induced spin torques in Mn3Pt-based heterostructures, we find that the spin current generated by Mn3Pt (001) exhibits exotic dependences on the current direction for all the spin components, deviating from that in conventional cubic systems. We also demonstrate that such an anisotropic spin current can be used to realize low-power spintronic applications such as the efficient field-free switching of the perpendicular magnetizations.

Efficient Tuning of the Spin-Orbit Torque via the Magnetic Phase Transition of FeRh.

The understanding and control of the spin-orbit torque (SOT) are central to antiferromagnetic spintronics. Despite the fact that a giant SOT efficiency has been achieved in numerous materials, its efficient tuning in a given material has not been established. Materials with magnetic phase transitions (MPTs) offer a new perspective, as the SOT efficiency may vary significantly for the different magnetic orderings across the transition, and the transition itself can be readily tuned by various control parameters. This work reports that the SOT efficiency of a FeRh-based perpendicular magnetized heterostructure can be significantly tuned by varying the temperature across the MPT. The SOT efficiency exhibits a temperature hysteresis associated with the first-order nature of the MPT, and its value in the ferromagnetic phase is seen to be enhanced by ∼450%, simply by a lowering of temperature to drive FeRh into the antiferromagnetic phase. Furthermore, current-induced magnetization switching can be achieved without an assistant magnetic field for both ferromagnetic and antiferromagnetic FeRh, with a low critical switching current density for the latter. These results not only directly establish FeRh as an efficient spin generator but also present a strategy to dynamically tune SOT via varying the temperature across MPTs.

Recent developments on the magnetic and electrical transport properties of FeRh- and Rh-based heterostructures.

It is fascinating how the binary alloy FeRh has been the subject of a vast number of studies almost solely for a single-phase transition. This is, however, reasonable, considering how various degrees of freedom are intertwined around this phase transition. Furthermore, the tunability of this phase transition-the large response to tuning parameters, such as electric field and strain-endows FeRh huge potential in applications. Compared to the bulk counterpart, FeRh in the thin-film form is superior in many aspects: materials in thin-film form are often more technologically relevant in the first place; in addition, the substrates add extra dimensions to the tunability, especially when the substrate itself is multiferroic. Here we review recent developments on the magnetic and transport properties of heterostructures based on FeRh and its end-member Rh, with the latter providing a new route to exploiting spin-orbit interactions in functional spintronic heterostructures other than the more often employed 5dmetals. The methods utilized in the investigation of the physical properties in these systems, and the design principles employed in the engineering thereof may conceivably be extended to similar phase transitions to other magnetic materials.

Stretching-Tunable High-Frequency Magnetic Properties of Wrinkled CoFeB Films Grown on PDMS.

We demonstrated a convenient method via applying uniaxial tensile strains to continuously tune the high-frequency properties of flexible magnetic films. CoFeB films were magnetron sputtered onto prestretched polydimethylsiloxane (PDMS) membranes. They exhibit a self-assembled periodic wrinkling surface structure because of the large mismatch of Young's moduli between the elastomeric PDMS substrates and the metal layers. The wrinkling morphology and the residual tensile stress caused by the Poisson effect can be continuously tuned by a uniaxial stretching strain less than the growth prestrain, which consequently results in changes in high-frequency performance. The initial permeability and the ferromagnetic resonance frequency of flexible CoFeB thin films can be monotonously tuned in wide ranges of about hundreds and 1 GHz, respectively. A good repeatability over thousands of stretching-relaxing cycles has been demonstrated without any obvious reduced high-frequency properties. This flexible CoFeB films with excellent stretching-tunable high-frequency performances are promising for application in flexible and tunable microwave devices.

Integration of Both Invariable and Tunable Microwave Magnetisms in a Single Flexible La0.67Sr0.33MnO3 Thin Film.

High-quality flexible magnetic oxide thin films have promoted a wide range of potential applications in spintronic devices due to their unique physical properties. To obtain the optimized microwave magnetism for future all-oxide-based spintronic applications, high-quality oxide materials with excellent epitaxial quality as well as specific bending properties related to ferromagnetic resonance are high in demand. Here, (001)-oriented La0.67Sr0.33MnO3 epitaxial thin films with different thicknesses have been grown and subsequently transferred onto flexible poly(dimethylsiloxane) substrates. The microwave magnetisms of these film samples have been investigated under various bending states. Under bending, the ferromagnetic resonance lineshape of the film gradually transits from a single mode to a superposition of multimodes, possibly because of the uneven distribution of magnetization in the bending film at X-band. This phenomenon is more apparent when the direction of the applied magnetic field goes close to the out-of-plane of the film. Hence, an integration of invariable and continuous tuning of ferromagnetic resonance field under various mechanical bending can be achieved in one same sample by just tuning the direction of the applied magnetic field, which reveals that the flexible La0.67Sr0.33MnO3 thin films have huge potential in the applications in future flexible multifunctional devices.

A Strategy to Modulate the Bending Coupled Microwave Magnetism in Nanoscale Epitaxial Lithium Ferrite for Flexible Spintronic Devices.

With the development of flexible electronics, the mechanical flexibility of functional materials is becoming one of the most important factors that needs to be considered in materials selection. Recently, flexible epitaxial nanoscale magnetic materials have attracted increasing attention for flexible spintronics. However, the knowledge of the bending coupled dynamic magnetic properties is poor when integrating the materials in flexible devices, which calls for further quantitative analysis. Herein, a series of epitaxial LiFe5O8 (LFO) nanostructures are produced as research models, whose dynamic magnetic properties are characterized by ferromagnetic resonance (FMR) measurements. LFO films with different crystalline orientations are discussed to determine the influence from magnetocrystalline anisotropy. Moreover, LFO nanopillar arrays are grown on flexible substrates to reveal the contribution from the nanoscale morphology. It reveals that the bending tunability of the FMR spectra highly depends on the demagnetization field energy of the sample, which is decided by the magnetism and the shape factor in the nanostructure. Following this result, LFO film with high bending tunability of microwave magnetic properties, and LFO nanopillar arrays with stable properties under bending are obtained. This work shows guiding significances for the design of future flexible tunable/stable microwave magnetic devices.

Flexible Lithium Ferrite Nanopillar Arrays for Bending Stable Microwave Magnetism.

Recent development in magnetic nanostructures has promoted flexible electronics into the application of integrated devices. However, the magnetic properties of flexible devices strongly depend on the bending states. In order to realize the design of new flexible devices driven by an external field, the first step is to make the magnetic properties insensitive to the bending. Herein, a series of LiFe5O8 nanopillar arrays were fabricated, whose microwave magnetic properties can be modulated by tuning the nanostructure. This work demonstrates that nanostructure engineering is useful to control the bending sensitivity of microwave magnetism and further design stable flexible devices.

Electric-field control of non-volatile 180° switching of the unidirectional anisotropy field in a multiferroic heterostructure.

We investigate the room-temperature, electric-field-mediated, non-volatile 180° switching of the unidirectional anisotropy field in an IrMn/CoFeB/Ta/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure. The variation in exchange bias under different electric fields appears clearly in the magnetic hysteresis loops. The remnant magnetization as a function of electric field, as determined by static magnetic measurements, exhibits a non-volatile behavior, which is consistent with the results of the ferromagnetic resonance field as a function of electric field. Moreover, the measured ferromagnetic resonance shows that the uniaxial magnetic anisotropy field is non-volatile and the unidirectional anisotropy field undergoes 180° switching that can be acquired and separated distinctly. This result is attributed to the piezo-strain effect. The electric-field-mediated non-volatile 180° switching of the unidirectional anisotropy field paves the way for sensors and other spintronic devices.