Critical Role of Electrical Resistivity in Magnetoionics

Abstract

The utility of electrical resistivity as an indicator of magnetoionic performance in stoichiometrically and structurally similar thin-film systems is demonstrated. A series of highly nanocrystalline cobalt nitride (Co-N) thin films (85 nm thick) with a broad range of electrical properties exhibit markedly different magnetoionic behaviors. Semiconducting, near stoichiometric CoN films show the best performance, better than their metallic and insulating counterparts. Resistivity reflects the interplay between atomic bonding, carrier localization, and structural defects, and in turn determines the strength and distribution of applied electric fields inside the actuated films. This fact, generally overlooked, reveals that resistivity can be used to quickly evaluate the potential of a system to exhibit optimal magnetoionic effects, while also opening interesting challenges.

Magneto-Ionics in Single-Layer Transition Metal Nitrides

“…The use of a nonaqueous, aprotic polar liquid electrolyte will prevent electrochemical oxidation during voltage biasing. 5,10,12,24 This configuration generates an electric double layer (EDL) at the film surface which applies a uniform, out-of-plane electric field. 5,10,58−62 In-plane measurements of M−H hysteresis loops, spanning the range between −20 and 20 kOe, were measured using a vibrating sample magnetometer.…”

“…11 Recent studies of the magneto-ionic properties of singlelayer thin films with structural oxygen (Co 3 O 4 ) or nitrogen (CoN) ions, present in the as-prepared thin-film state, have demonstrated that fully reversible and cyclable magnetic transitions between a nonferromagnetic (OFF) and a ferromagnetic state (ON) are indeed possible. 5,12,24 Interestingly, and in contrast to the diffusion channels observed in Co 3 O 4 , CoN films transport nitrogen via a planar ion migration front and possess both superior cyclability and lower operating voltages than Co 3 O 4 , 24 hinting that metal nitrides may compare favorably with their metal oxide counterparts. Previous ab initio calculations of the enthalpy of formation of CoN have predicted values of ΔH f ≈ −50 kJ mol −1 , 25 significantly higher (i.e., less negative) than experimental estimates conducted on CoO and Co 3 O 4 of ΔH f ≈ −237.9 and −910.02 kJ mol −1 , respectively.…”

Voltage control of magnetism with magneto-ionic approaches: Beyond voltage-driven oxygen ion migration

“…Spin textures, such as skyrmions, 112,113 chiral domain walls, 114,115 and spin spirals, 116,117 are stabilized under the Dzyaloshinskii-Moriya interaction (DMI), which have been shown to be tunable using magnetoionics, [118][119][120][121] with possible applications in spin-orbitronics. 122 Continued fundamental research has led to further exploration of different structures, materials, and mobile ions, with successful demonstration of magneto-ionic effects using hydrogen, 118,[123][124][125] lithium, [126][127][128][129][130] nitrogen, [131][132][133] and flourine. 134 The last two years have seen a flurry of exciting activity, a few of which are highlighted in Fig.…”

“…It must be noted here that this planar front is a unique trait, one which can be imagined to allow the imprinting of magnetic shapes parallel to the applied field, and potentially allowing for the toggling of related magnetic and stoichiometrically controlled crystalline states. This planar front is currently attributed 131 to the highly nanostructured nature of the films, isotropic grains, and a defect gradient along the depth of the film, with more defects near the top and fewer near the working electrode, resulting in faster ionic motion occurring near the surface under voltage, while relatively fewer defects result in lower ionic rates deeper in the film. This, along with the smaller, isotropic grains compared with Co 3 O 4 and lower electrical resistivity effectively makes the ionic front penetration depth a voltage dependent quantity.…”

“…This process is known to allow for tuning of structural defects and may affect ionic pathways and the electric field generated inside the target material. 131 The structural ion approach allows for tuning ion vacancies or enriching regions of the target material and consequently modifying magneto-ionic performance, as recently demonstrated by Jensen et al in Gd/NiCoO. 162 Indeed, structural ion materials are still far from being fully optimized.…”