Oxygen vs. Nitogen Magneto-ionics

Voltage-controlled magnetism, where magnetic properties are controlled via an applied electricfield instead of current, could represent a significant increase in energy savings in future magnetically actuateddevices. Practically, however, this approach faces several important obstacles, such as thickness limitations inelectrically charged metallic films, mechanical failure in strain-mediated piezoelectric/magnetostrictive devices,and a lack of room-temperature multiferroics. Voltage-driven ionic motion (magneto-ionics) may provide a pathforward by avoiding many of these drawbacks, in addition to its own interesting magnetoelectric phenomena.Nevertheless, translating magneto-ionics into real world devices requires significant improvements in magneto-ionic rates, cyclability, and magnetization. Here, we report on the development of magneto-ionics in single-layer, semiconducting transition metal oxides and nitrides, and the subsequent enhancements in theirperformance. We first present electrolyte-gated and defect-mediated O transport in single-layer, paramagneticCo
O
at room temperature (i.e. without thermal assistance), which allows voltage-controlled magneticswitching (referred to here as ON-OFF ferromagnetism: Fig. 1) via internal reduction/oxidation processes
.Negative bias partially reduces Co
O
to Co, resulting in films with Co- and O-rich areas (ferromagnetism: ON).Positive bias re-oxidizes Co back to Co
O
(paramagnetism: OFF). We show that the bias-induced motion of Ois caused by mixed vacancy clusters, with O motion promoted at grain boundaries and assisted by thedevelopment of O-rich diffusion channels. The generated ferromagnetism is shown to be stable, and easilyerased by sufficient positive bias. This voltage-induced process is demonstrated to be compositionally,structurally, and magnetically reversible and self-contained, as no oxygen reservoir besides Co
O
is needed.We then show that room-temperature magneto-ionic effects in electrolyte-gated paramagnetic Co
O
films canbe significantly increased, both in terms of generated magnetization (6 times larger) and speed (35 timesfaster), if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlyingconducting buffer layer: Fig. 2) instead of electric-double-layer transistor-like configuration (placing the electriccontacts at the side of the semiconductor)
. In addition to gains in speed, magnetization measurements showa marked increase in the squareness ratio and a decrease in the switching field distribution of the hysteresisloops in Co
O
biased in the capacitor configuration, the result of the formation of more uniform ferromagneticregions. These results are attributed to the uniform electric field applied throughout the film, as confirmed byCOMSOL simulations. As the measured films are quite thick, further miniaturization promises even greatermagneto-ionic rates. We then demonstrate room-temperature voltage-driven nitrogen magneto-ionics (i.e., Ntransport) by electrolyte-gating of a CoN film
. Nitrogen magneto-ionics in CoN is compared to oxygenmagneto-ionics in Co
O
, in films using an electrochemical capacitor configuration. Both materials are shownto be nanocrystalline (face-centered cubic structure), and show reversible voltage-driven ON-OFFferromagnetism (Fig. 1). Nitrogen transport is found to occur uniformly throughout the film, creating a plane-wave-like migration front, without assistance of diffusion channels. Nitrogen magneto-ionics also requires lowerthreshold voltages and exhibits enhanced rates and cyclability, due to the combination of a lower criticalelectric field required to overcome the energy barrier for ion diffusion and the lower electronegativity of nitrogenwith respect to oxygen, consistent with
ab initio
calculations contrasting N vs. O motion in cobalt stacks. Theseresults place nitrogen magneto-ionics as a robust alternative for efficient voltage-driven effects and, along withoxygen magneto-ionics, may enable the use of magneto-ionics in devices that require endurance and moderate speeds of operation, such as brain-inspired/stochastic computing or magnetic micro-electro-mechanical systems.
References:
[1] A. Quintana, E. Menéndez, M. O. Liedke et al., ACS Nano, Vol. 12, p. 10291 (2018)
[2] J. de Rojas, A. Quintana, A. Lopeandía et al., Advanced Functional Materials, Vol. 30, p. 2003704 (2020)
[3] J. de Rojas, A. Quintana, A. Lopeandía et al., Nature Communications, Vol. 11, p. 5871 (2020)
KEYWORDS:
magneto-ionics, voltage-controlled magnetism, oxygen, nitrogen.
IMAGE CAPTION:
Fig. 1. Hysteresis loops (M vs. H) of as-prepared, negatively-biased, and positively-biased CoN films atmagneto-ionic activation voltages.
Fig. 2. A schematic of the electrochemical capacitor configuration used to bias cobalt-oxide (Co
O
) andcobalt-nitride (CoN) films.