Voltage-driven motion of nitrogen ions: a new paradigm for magneto-ionics

Abstract

Magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ions. Here, we demonstrate room-temperature voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a CoN film. Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4. Both materials are nanocrystalline (face-centered cubic structure) and show reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Remarkably, nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results may open new avenues in applications such as brain-inspired computing or iontronics in general.

Critical Role of Electrical Resistivity in Magnetoionics

“…However, such FM/reservoir bilayer systems often suffer from poor cyclability due to irreversible structural changes undergone by the FM metal in the oxidation (reduction) process, which involves the formation (destruction) of interfacial oxide phases. An alternative is the use of single-layer target materials whose crystal structure already contains the ions to be transported (such as oxygen or nitrogen) in the asprepared state (e.g., Co 3 O 4 or CoN) [8,10,24,25]. Such target materials can undergo fully reversible transformations from a nonferromagnetic (off ) to a ferromagnetic (on) state and vice versa while experiencing fewer detrimental structural changes than the previously mentioned magnetoionic approaches, possibly due to Co 3 O 4 or CoN structures providing "ready-made" lattice sites into which ions can be driven, leading to increased endurance.…”

“…Such target materials can undergo fully reversible transformations from a nonferromagnetic (off ) to a ferromagnetic (on) state and vice versa while experiencing fewer detrimental structural changes than the previously mentioned magnetoionic approaches, possibly due to Co 3 O 4 or CoN structures providing "ready-made" lattice sites into which ions can be driven, leading to increased endurance. In contrast to Co 3 O 4 , where oxygen migration is assisted by the formation of filamentary channels [10], roomtemperature on-off ferromagnetism in CoN films operates via frontlike plane-wave ionic motion at lower applied voltages and with enhanced cyclability [24]. Thus, transition metal nitrides compare favorably with their transition metal oxide counterparts for magnetoionic applications.…”

Electrically Enhanced Exchange Bias via Solid-State Magneto-ionics

“…In this regard, atomic-scale control of interfaces via ionic migration in solid-state heterostructures − as well as electrolyte-based systems ,− has emerged as an effective tool to modify materials properties, which can be further controlled by an electric field. To date, a variety of magneto-ionically controlled functionalities have been demonstrated, including magnetic anisotropy, ,, antiferromagnetism, ferromagnetism, ,,,− superconductivity, , Dzyaloshinskii–Moriya interaction, spin textures, − and so on. Many of the magneto-ionic studies have focused on the interfacial electrostatic effect, ,− where the charge build-up across interfaces under an electric field modifies the electronic structures and materials characteristics.…”

Voltage‐Induced ON Switching of Magnetism in Ordered Arrays of Non‐Ferrimagnetic Nanoporous Iron Oxide Microdisks

“…Among various magnetoelectric mechanisms that allow for voltage control of magnetism, magneto‐ionics (i.e., electric‐field‐driven ion migration) has been recognized as one of the most scalable. [ 22 ] In certain semiconducting and dielectric materials, electric field induces ion migration of, for example, O 2− , [ 27–29 ] Li + , [ 30 ] H + , [ 31 ] or N 2− , [ 32 ] from the material of interest toward an ion reservoir or vice versa, depending on the voltage polarity. Diffusion of ions induces changes in the oxidation state of the metal, thus leading to large changes in magnetization.…”

“…Room‐temperature ON/OFF switching of magnetization remains rather elusive. Specifically, this has been shown in electrolyte‐gated Co 3 O 4 [ 29,35 ] and CoN [ 32 ] continuous (non‐patterned) films in which voltage was used to change the oxidation state of cobalt, taking advantage of defect‐assisted magneto‐ionics. Likewise, room‐temperature ON/OFF switching of magnetization has been realized via Li + intercalation/de‐intercalation in α‐Fe 2 O 3 nanoparticles loaded onto a Cu foil cathode in a lithium ion battery.…”