ontrolling the magnetic state of devices by electrical means is critical for spin-based data storage and logic 1,2 . One of the key technological challenges is to achieve efficient 180° magnetic switching by electrical means. Current methods are mostly based on local magnetic fields or spin torques 3,4 . Due to a much lower energy consumption 5,6 , voltage-controlled magnetization switching is desirable. However, it is inherently difficult because electric fields do not induce the required time-reversal symmetry breaking for 180° magnetic switching. Many methods, such as using piezoelectric and multiferroic materials 5,[7][8][9][10][11] , are being explored for voltage-controlled magnetization switching. However, these methods involve either high voltages for inducing enough strain, or difficult fabrication procedures.Multi-sublattice materials present unique opportunities for voltage control of magnetism 12,13 , with ferrimagnets being promising for achieving 180° switching owing to their multi-sublattice configuration with magnetic moments of different magnitudes opposing each other. By tuning the relative sublattice magnetization magnitudes, the net magnetization can be reversed. Moreover, compared with ferromagnets, ferrimagnets offer technological advantages as they allow for small spin textures 14 , fast spin dynamics [14][15][16] and ultrafast optical switching 17 . However, the conventional approaches to controlling the compensation of ferrimagnets, such as varying the composition at fabrication 18 , annealing 19,20 , heating or cooling 21 and hydrogen gas exposure 22,23 , do not allow for localized electrical actuation. Ultrashort light pulses have been shown to enable all-optical switching of ferrimagnets 17,24,25 , however, the need for an ultrafast laser source may complicate device designs and the optical paths may be difficult to scale.Here, we show the reversible control of the dominant sublattice of a rare earth-transition metal (RE-TM) alloy ferrimagnet (GdCo) by a gate voltage (V G ) using a solid-state hydrogen pump 26 . The control originates from the injection of hydrogen, sourced from ambient moisture through hydrolysis, into GdCo, which tunes the relative sublattice magnetizations and hence the degree of compensation. By applying a small V G , the compensation temperature (T M ) can be shifted by >100 K, and the dominant sublattice can be reversibly switched under ambient, isothermal conditions. Element-specific X-ray magnetic circular dichroism (XMCD) revealed that hydrogenation reduces the sublattice magnetization of Gd substantially, but only modestly reduces that of Co. Mean-field modelling of the experimental data combined with ab initio calculations suggest that this results from hydrogen-induced reduction of the inter-sublattice exchange coupling strength that is largely responsible for the Gd sublattice order. We demonstrate here that the dominant sublattice can be toggled using pulses as short as 50 μs at room temperature, and that the devices show no degradation after >10 4 gatin...

Electrochemistry-mediated voltage control of magnetization close to ON/OFF switching is realized in electrodeposited oxide/metal nanoislands.

actuation devices. [1,2] The electric currentinduced switching of magnetization is one approach, but requires large spin polarized currents. For energy efficient operation, however, the involved dissipative electric currents should be minimal. The electric field control of magnetism is a promising low-power alternative, and increasing research efforts in this direction resulted in the discovery of various mechanisms in single-and two-phase multiferroics, magnetic semiconductors, and at gated metal surfaces allowing for voltage control of magnetic properties. [2,3] The exchange bias (EB), discovered decades ago, became an important property in modern thin-film magnetism, as it offers an elegant way to prepare thermally stable artificial domains and electrodes with a predefined macroscopic magnetization alignment. [4,5] The EB is manifested by a shift of the hysteresis curve of the ferromagnetic layer along the magnetic field axis and is based on a quantum mechanical exchange interaction occurring at the common interface between a thin ferromagnetic (FM) and antiferromagnetic (AFM) layer. EB thin films are extensively used to control the direction of magnetization in magnetic memories, spintronic, and magnetophoretic devices. [6] In the latter, for example, in-plane EB systems are utilized to generate artificial magnetic domain patterns and associated reconfigurable magnetic stray field landscapes, which can be used to locally guide the motion of magnetic micro-and nanoparticles. [7] Consequently, tailoring and control of EB has become a central objective in these research branches. Chemical, mechanical, thermal, and light ion bombardment-based techniques have been developed to tailor the EB, mainly by introducing irreversible material modifications at the AFM/ FM interface, for example, by changing the interface roughness or incorporating impurities. [8] The modification of the EB is not reversible for all of these methods. Moreover, most of them require an additional magnetic field and a sophisticated apparatus, such as vacuum equipment.The electric control of EB would provide an easily integrable alternative, ideally allowing for operando and reversible EB modification. Indeed, reversible control can be achieved by electric current-induced torque in the AFM layer, but Joule heating limits the energy-efficiency. [9] Alternative low power mechanisms for voltage-control of EB are therefore of great Electric manipulation of exchange bias (EB) systems is highly attractive for the development of modern spintronic and magnetophoretic devices. To date, electric control of the EB has mainly been based on multiferroic or resistive switching behavior in specific antiferromagnets, which limits the material choice and accessible EB states. In addition, the effects are mostly volatile, requiring constant voltage application. The continuous and nonvolatile tuning of the EB via electrochemical manipulation of the ferromagnetic layer is presented. In FeO x /Fe/IrMn systems, large changes in the EB field of fully shifted magneti...

The possibility of tuning magnetic material properties by ionic means is exciting both for basic science and, especially in view of the excellent energy efficiency and room temperature operation, for potential applications. In this perspective, we shortly introduce the functionality of magneto-ionic materials and focus on important recent advances in this field. We present a comparative overview of state-of-the-art magneto-ionic materials considering the achieved magnetoelectric voltage coefficients for magnetization and coercivity and the demonstrated time scales for magneto-ionic switching. Furthermore, the application perspectives of magneto-ionic materials in data storage and computing, magnetic actuation, and sensing are evaluated. Finally, we propose potential research directions to push this field forward and tackle the challenges related to future applications.

No abstract

We demonstrate reversible voltage control of perpendicular exchange bias via H + pumping in a NiO/Pd/Co/Pd/Gd(OH) 3 /Au heterostructure at room temperature. The perpendicular exchange bias results from a tailored layer structure consisting of an antiferromagnetic NiO layer and a ferromagnetic Co layer, stabilized by an ultrathin Pd interlayer. Voltage mediated H + pumping through the Gd(OH) 3 layer and subsequent H absorption at the Pd/Co interface leads to a decrease in the perpendicular anisotropy. In consequence, also the perpendicular exchange bias vanishes upon voltage application (3V). During voltage switch-off, this process reverses and perpendicular exchange bias recovers. The first voltage switching cycle shows relatively slow kinetics and an inverse relation of exchange bias and coercivity changes. We discuss these features with regard to an H-induced crystallization of the initially amorphous Pd/Co/Pd trilayer, which is revealed by transmission electron microscopy. With subsequent voltage switching steps, a decrease of the exchange bias field in the voltage switch-off state is observed, which levels off with increasing cycle numbers. A reversible setting of exchange bias field values is achieved when a magnetic field (+/− 2 kOe) is superposed during the H loading step. In this case, the shift of the exchange bias field can be controlled by the direction of the applied magnetic field. These results open an innovative route to electrically control exchange bias.

No abstract

The perspective of energy-efficient and tunable functional magnetic nanostructures has triggered research efforts in the fields of voltage control of magnetism and spintronics. We investigate the magnetotransport properties of nanocomposite iron oxide/iron thin films with a nominal iron thickness of 5−50 nm and find a positive magnetoresistance at small thicknesses. The highest magnetoresistance was found for 30 nm Fe with +1.1% at 3 T. This anomalous behavior is attributed to the presence of Fe 3 O 4 −Fe nanocomposite regions due to grain boundary oxidation. At the Fe 3 O 4 /Fe interfaces, spin-polarized electrons in the magnetite can be scattered and reoriented. A crossover to negative magnetoresistance (−0.11%) is achieved at a larger thickness (>40 nm) when interface scattering effects become negligible as more current flows through the iron layer. Electrolytic gating of this system induces voltage-triggered redox reactions in the Fe 3 O 4 regions and thereby enables voltage-tuning of the magnetoresistance with the locally oxidized regions as the active tuning elements. In the low-magnetic-field region (<1 T), a crossover from positive to negative magnetoresistance is achieved by a voltage change of only 1.72 V. At 3 T, a relative change of magnetoresistance about −45% during reduction was achieved for the 30 nm Fe sample. The present low-voltage approach signifies a step forward to practical and tunable room-temperature magnetoresistance-based nanodevices, which can boost the development of nanoscale and energy-efficient magnetic field sensors with high sensitivity, magnetic memories, and magnetoelectric devices in general.