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...

No abstract

No abstract

Ionic and redox control of magnetism can produce large changes to a variety of magnetic properties using a relatively small voltage. A model structure that continues to be of interest is Pt/Co/GdO xH y/Au, where the Co magnetic layer oxidation state and perpendicular magnetic anisotropy can be toggled using voltage control of proton transport through the GdO xH y electrolyte layer. The hydration of the oxide layer to form a hydroxide phase is the key to improve the speed of these magneto-ionic devices, but there is insufficient understanding of protonic defect incorporation and transport during hydration and electrical gating. In this work, we use polarized neutron reflectometry (PNR) to observe the effects of hydration and electrical gating by scanning in an as-grown state, a hydrated state, and in operando during electrical gating. We directly measure the depth profile of hydrogen and confirm the transformation from oxide (Gd2O3) to hydroxide [Gd(OH)3]. We observe the accumulation of H in the Co magnetic layer and the effects of gating on the structure and hydrogen content of the other layers in the device stack. Using PNR and secondary ion mass spectrometry, we find evidence for much more complex chemistry at the Co/GdO xH y interface than was previously assumed, including evidence for persistent CoO phases and CoO xH y phases. We offer insight on using PNR to observe relatively fast proton dynamics in the system and fitting a rather complex set of parameters to achieve a physical result for the fit spectrum and scattering length density profiles.

Solid state ionic materials such as metal oxide ceramics are of interest due to the wide variety of properties that can be controlled through voltage control of ion concentration and transport in these materials, e.g. electrical resistivity, optical properties, and magnetic properties. Ion movement through a solid oxide electrolyte film can be controlled very simply by electrical gating of the oxide, i.e. applying a voltage across the film. The ease of processing and CMOS compatibility of metal oxides makes them strong candidate materials for next-generation devices for information storage and computing, sensing, energy generation (fuel cells), and energy storage (batteries).Hydrogen ions (protons) take the form of (OH)O • defects in oxides and move by hopping between oxygen sites (Grotthuss mechanism); their small size allows for high conductivity.8 Protonic conduction in oxides has been studied in solid oxide fuel cells (SOFCs)1; proton incorporation has also been shown to change the optical4, electrical2,3, and magnetic properties of the oxide5, or a combination of multiple properties e.g. through inducing phase changes in strontium cobalt oxide (SCO)6. However, current work focuses on materials that require high-temperature processing and operation (>200C, or up to 600-1000C for SOFCs), and bulk or microscale films. This work aims to take solid oxide protonic devices down to the ultrathin (10-100nm) scale, to introduce simpler fabrication techniques (sputtering), and to achieve room-temperature operation of ionic devices. We investigate gadolinium oxide (GdOx) as a model material due to its hygroscopic nature, which allows for fast room-temperature proton conduction.7,8 Using variations of a Pt/Co/GdOx/Au device structure, a gate voltage of 2-5V is applied, initiating a hydrolysis reaction at the top electrode and allowing water to be incorporated into the film. The electric field within the GdOx drives protons to the bottom interface, where it can diffuse into the adjacent magnetic layer and affect a variety of properties. Thin film ferromagnetic materials such as cobalt have been shown to undergo dramatic changes in their magnetic anisotropy with voltage gating, due to the incorporation and removal of hydrogen at the interface with the electrolyte and within the magnetic layer.6 Control of the compensation temperature of a thin film ferrimagnet and control of exchange bias in an antiferromagnet have also been demonstrated with this magneto-ionic proton pump mechanism.9,10 Using a gadolinium cobalt (GdCo) magnetic layer inserted into the MOM device stack to detect the presence of hydrogen, we have developed a sensor for use in a novel time-domain proton transport measurement. We apply short voltage pulses to the device and use magneto-optic Kerr effect (MOKE) to observe the resulting changes in the compensation temperature and magnetic state of the sensor layer. This can be used as a more direct measure of the presence of protons at the bottom interface, as compared to frequency domain measurements such as...