Spin and orbital magnetic moments in perpendicularly magnetized Ni${}_{1-x}$Co${}_{2+y}$O${}_{4-z}$ epitaxial thin films: Effects of site-dependent cation valence states

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 the inverse-spinel ferrimagnet Ni${}_{1-x}$Co${}_{2+y}$O${}_{4-z}$ (NCO) epitaxial films with the perpendicular magnetic anisotropy. We show that the oxygen pressure P${}_{O2}$ during the 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, Ni${}^{(2+\delta )+}$ (0<$\delta$<1), whose nominal valence state (the $\delta$ value) varies depending on P${}_{O2}$. On the other hand, the Co in the octahedral (O${}_h$) and tetrahedral (T${}_d$) sites respectively have the valence state close to +3 and +2. We also find that the XMCD signals originate mainly from the T${}_d$-site Co${}^{2+}$ (Co${}_{Td}$) and O${}_h$-site Ni${}^{(2+\delta )+}$ (Ni${}_{Oh}$), indicating that these cation valence states are the key in determining the magnetic and transport properties of NCO films. Interestingly, the valence state of Ni${}^{(2+\delta )+}$ that gives rise to the XMCD signal remains unchanged independent of P${}_{O2}$. The electronic structure of Ni${}^{(2+\delta )+}$ that is responsible for the magnetic moment and electrical conduction differs from those of Ni${}^{2+}$ and Ni${}^{3+}$. In addition, the orbital magnetic moment originating from Co${}_{Td}$ is as large as 0.14 ${\mu }_B/C{o}_{Td}$ and parallel to the magnetization while the Ni${}_{Oh}$ orbital moment is as small as 0.07 ${\mu }_B/N{i}_{Oh}$ and is rather isotropic. The Co${}_{Td}$ therefore plays the key role in the perpendicular magnetic anisotropy of the films. Our results demonstrate the significance of the site-dependent cations valence states for the magnetic and transport properties of NCO films.

References in corpus (3)

• A different look at the spin state of Co${}^{3+}$ ions in CoO${}_5$ pyramidal coordination
• Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
• Perpendicular Magnetic Anisotropy in Conducting NiCo2O4 Films from Spin-Lattice Coupling

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• All-optical helicity-dependent switching in NiCo${}_2$O${}_4$ thin films
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• Type-II-like ultrafast demagnetization behavior in NiCo2O4 thin films