Toward On‐and‐Off Magnetism: Reversible Electrochemistry to Control Magnetic Phase Transitions in Spinel Ferrites

Abstract

The magnetoelectric effect, i.e., electric-fi eld control of magnetism in artifi cial heterostructures is usually limited to surface/interface atoms of the magnetic materials. In order to attain electrical control of magnetism in bulk ferromagnets, this study proposes to extend the defi nition of magnetoelectric phenomena to include reversible, chemistry-controlled magnetization switching. A large and reversible change in the room temperature magnetization in strong ferromagnets is reported, with electrochemistry-driven Li-ion exchange; carefully chosen spinel ferrites demonstrate a reversible magnetization variation up to 50% for CuFe 2 O 4 and 70% for ZnFe 2 O 4 . In case of CuFe 2 O 4 , the magnetization variation is predominantly associated with the preferential reduction of Cu 2+ to Cu + ions, and, hence, abides a nearly one-to-one relationship with the amount of injected Li-ions. In addition, the reduction of Cu 2+ also annihilates the Fe 3+  O  Cu 2+ magnetic interaction, resulting in a marked decrease in the Neél temperature of CuFe 2 O 4 . In contrast, the electrical tuning of superexchange interactions is found to play the decisive role in ZnFe 2 O 4 , where the simple electrochemical reduction model of magnetic cations can only explain a nominal fraction of the total magnetization variation, and indeed an electrochemically controlled reversible change in transition temperature is found necessary to account for the large magnetization variation observed.

Hybrid supercapacitors for reversible control of magnetism

“…The first involves non-faradaic electrostatic doping, where the charge carriers are electrostatically separated at the interface between a magnetic material and a ferroelectric78910, a dielectric11 or an electrolyte1213, in analogy to a parallel plate capacitor. The second implies faradaic electrochemical doping, where redox reactions with exchange of charge carriers14151617 occur across the interface between a magnetic material and a different chemical species, resembling the behaviour of an electrochemical cell.…”

Voltage‐Controlled On/Off Switching of Ferromagnetism in Manganite Supercapacitors

“…Electric field control of magnetism has been investigated in a variety of magnetoelectric (ME) systems, including single‐phase multiferroics, composite ME heterostructures, and magnetoionics . Giant ME effects were successfully attained by applying an external voltage to modify the strain, charge, or chemical state of a magnetic component. Nonetheless, the quest for realizing a working ME device, where a high magnetic on/off ratio is accompanied with a suitable reversibility, operating voltage, switching speed, and working temperature, remains still open.…”

“…In the past few years electrolyte gating of magnetic materials has been established as a powerful method to manipulate magnetism . Electrolytes allow for reversible modulation of appreciably higher charge carrier densities (>10 14 cm −2 ) than high‐κ dielectrics and ferroelectrics using lower voltages.…”

“…Electrolytes allow for reversible modulation of appreciably higher charge carrier densities (>10 14 cm −2 ) than high‐κ dielectrics and ferroelectrics using lower voltages. ME coupling has been realized at the interface of solid/liquid devices via surface electrostatic doping (as in electric double‐layer (EDL) capacitors) or via bulk ionic migration (akin to electrochemical batteries). Recently, robust ME effect was achieved in magnetic supercapacitors via a surface electrochemical mechanism called pseudocapacitance .…”

Magneto‐Ionic Switching of Superparamagnetism

“…Besides multiferroic materials or diluted magnetic semiconductors, electrochemical setups have recently been investigated for that purpose . One can distinguish three fundamentally different processes at the electrode–electrolyte interface, which can be exploited for the electrochemical control of magnetism: I) capacitive or pseudo‐capacitive double‐layer charging, II) redox surface reactions, or III) bulk intercalation triggered by chemical reactions (magneto‐ionic effect) . As high surface‐to‐volume ratios are beneficial for all three tuning approaches, recent studies have mainly focused on ultra‐thin films and nanoporous materials …”