Spin and orbital magnetic moments in perpendicularly magnetized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">i</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">o</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo><mml:mi>y</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn><mml:mo>−</mml:mo><mml:mi>z</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> epitaxial thin films: Effects of site-dependent cation valence states
Daisuke Kan, Masaichiro Mizumaki, Miho Kitamura, Yoshinori Kotani, Yufan Shen, Ikumi Suzuki, Koji Horiba, Yuichi Shimakawa
Abstract
We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in inverse-spinel ferrimagnet $\mathrm{N}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{2+y}{\mathrm{O}}_{4\ensuremath{-}z}$ (NCO) epitaxial films with perpendicular magnetic anisotropy. We show that the oxygen pressure ${P}_{\mathrm{O}2}$ during film growth by pulsed laser deposition influences not only the cation stoichiometry (site occupation) but also the cation valence state. Our XAS results show that the Ni in the ${O}_{h}$-site is in the intermediate valence state between +2 and +3, $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ ($0<\ensuremath{\delta}<1$), whose nominal valence state (the \ensuremath{\delta} value) varies depending on ${P}_{\mathrm{O}2}$. On the other hand, the Co in the octahedral (${O}_{h}$) and tetrahedral (${T}_{d}$) sites, respectively, have a valence state close to +3 and +2. We also find that the XMCD signals originate mainly from the ${T}_{d}$-site $\mathrm{C}{\mathrm{o}}^{2+}$ ($\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$) and ${O}_{h}$-site $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ ($\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$), indicating that these cation valence states are the key to determining the magnetic and transport properties of NCO films. Interestingly, the valence state of $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ that gives rise to the XMCD signal remains unchanged independent of ${P}_{\mathrm{O}2}$. The electronic structure of $\mathrm{N}{\mathrm{i}}^{(2+\ensuremath{\delta})+}$ that is responsible for the magnetic moment and electrical conduction differs from those of $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{N}{\mathrm{i}}^{3+}$. In addition, the orbital magnetic moment originating from $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ is as large as $0.14\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$ and parallel to the magnetization, while the $\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ orbital moment is as small as $0.07\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{N}{\mathrm{i}}_{\mathrm{Oh}}$ and is rather isotropic. $\mathrm{C}{\mathrm{o}}_{\mathrm{Td}}$, therefore, plays the key role in the perpendicular magnetic anisotropy of the films. Our results demonstrate the significance of the site-dependent cation valence states for the magnetic and transport properties of $\mathrm{NiC}{\mathrm{o}}_{2}{\mathrm{O}}_{4}$ films.