Critical Role of Electrical Resistivity in Magnetoionics

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 ($\mathrm{Co}\text{−}\mathrm{N}$) thin films (85 nm thick) with a broad range of electrical properties exhibit markedly different magnetoionic behaviors. Semiconducting, near stoichiometric $\mathrm{Co}\mathrm{N}$ 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.

Figure 1

(a) θ/2θ XRD diffraction patterns of the as-prepared cobalt nitride films. (b) and (c) Enlarged θ/2θ XRD diffraction patterns of near-stoichiometric $\mathrm{Co}\mathrm{N}$ films (card number PDF 00-016-0116) and ${\mathrm{Co}}_3\mathrm{N}$ [40], respectively. (d) Resistivity ρ measured as a function of temperature from 20 to 300 K, for all cobalt nitride as-prepared samples. (e) and (f) positron lifetime components τ_i=1–2 and relative intensities I_i=1–2, respectively, as a function of positron implantation energy E_P for all as-prepared cobalt nitride samples.

Figure 2

(a) Schematic of the condenserlike structure used in each film (electrochemical capacitor configuration). (b) First hysteresis loops under −50 V bias for cobalt nitride films. (c) Saturation magnetization (M_S) measured as a function of time for cobalt nitride films.

Figure 3

Debye screening lengths calculated for charge carrier densities ranging from n = 10¹⁵–10²¹ (least to most insulating cases), calculated for a typical relative permittivity for transition metal nitrides of ε_r = 10 and normalized to the resistive case. Simple schematics above show representative expected field extensions in the ideal insulating (left), semiconducting (center), and metallic cases (right).