Nonvolatile Electric Control of Exchange Bias by a Redox Transformation of the Ferromagnetic Layer

Abstract

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...

Voltage‐Controlled Deblocking of Magnetization Reversal in Thin Films by Tunable Domain Wall Interactions and Pinning Sites

“…This increase of domain size could be caused by an increase in domainwall energy, which results in an energysaving decrease in the number of domain walls. We calculate the change in the domainwall energy upon reduction by accounting for an increase in ironlayer thickness from 5 to 7 nm, which is expected during the reduction pro cess, [14,60] and by considering the measured changes in K u (see Section 2.3 and S10, Supporting Information,). This calculation shows that an increase in domainwall energy of 40% can be expected during the reduction process, which could explain the decrease in the number of domain walls and the associated increase in the equilibrium domain size.…”

“…Electrochemical and Magnetic Characterization in an In Situ Kerr Microscopy Setup: An electrochemical cell built in house, compatible with the Kerr microscopy setup (see Figure 3a and further details in ref. [14]) was used. The electrolyte compartment hosted 200 μL of electrolyte solution, and a circular area of 38.5 mm 2 of the FeO x /Fe thin film sample, defined by a sealing ring, was exposed to the electrolyte.…”

“…Energy loss in common magnetic systems, result, magnetoionic materials have emerged as prominent energyefficient candidates for voltageprogrammable mate rials with potential applications in neuromorphic computing, domainwall logic, magnetic random access memory, and magnetbased labonachip technologies. [3,12,14,19,20] To date, studies on magnetoionic effects in thin metal films have focused primarily on changes in the interfaceperpendicular magnetic anisotropy (PMA) [21][22][23][24][25] and the Dzyaloshinskii-Moriya interaction. [26] There is growing recognition that voltageinduced changes in valency and phase, inherent to magnetoionic effects, can be applied to the magnetic layer beyond the interface.…”

“…[31][32][33][34] For such materials, voltageswitching between antiferromagnetic and ferromagnetic or ferrimagnetic states is demonstrated. [32,35] Further, studies on layered composites and nanoporous metal alloys indicate that also many other exciting magnetic phe nomena, such as the exchange bias, [14] spin reorientation, [16,36] and the RKKY interaction [37] length are tunable by electrolytic gating. Although films with uniaxial inplane anisotropy hold great potential for switchable artificial magnetic stray field landscapes for labonachip [38] or for giant magnetoresistance sensors, [39] there are no reports of magnetoionic control of such films yet.…”

“…[ 5–10 ] In contrast, magneto‐ionic mechanisms involve voltage‐induced ion migration and electrochemical reactions, [ 2,3,11–13 ] and can therefore induce very large and nonvolatile magnetic property changes. [ 12,14–18 ] As a result, magneto‐ionic materials have emerged as prominent energy‐efficient candidates for voltage‐programmable materials with potential applications in neuromorphic computing, domain‐wall logic, magnetic random access memory, and magnet‐based lab‐on‐a‐chip technologies. [ 3,12,14,19,20 ]…”