Spin reorientation in tetragonally distorted spinel oxide NiCo$_2$O$_4$ epitaxial films

TL;DR

This study investigates the spin reorientation transition in NiCo$_2$O$_4$ epitaxial films, revealing temperature-dependent magnetic anisotropy changes and suggesting cation anti-site disorder as a key factor.

Contribution

It provides experimental evidence of spin reorientation in NiCo$_2$O$_4$ films and links the transition to cation anti-site disorder, a novel insight into magnetic anisotropy control.

Findings

Spin reorientation transition observed at a specific temperature.

ECMA cone angles increase up to 45-50 degrees at low temperature.

Cation anti-site disorder, especially Ni$^{3+}$ ions, likely causes ECMA.

Abstract

We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$-axis increased successively at $T_{\mathrm{SR}}$ and approached a maximum of 45-50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the Ni$^{3+}$ ions occupying the $T_d$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation anti-site distribution of Ni$^{3+}$, which is possibly formed during the growth process of the thin films.