High entropy oxide epitaxial films with interface perpendicular magnetic anisotropy and tunnel magnetoresistance effect toward spintronic applications

High entropy materials, usually composed of five or more constituent elements with a high mixing entropy have attracted increasing attention due to the marked development of new phases of multicomponent structural and functional materials. In particular, high entropy oxides (HEOs) are expected to realize their potential for electronic functionalities in spintronics, since the oxygen lattice required for achieving the functionalities remains besides the cation site disorder. In this study, we explored the HEO thin films with a rock-salt-like structure of LiTiMgAlGaO (L5O) for perpendicular magnetic anisotropy (PMA), which is induced at an interface with the CoFeB ferromagnet. The atomically homogeneous cation distributions in the 10–20 nm thick L5O films were achieved by atomic sputtering lamination on a MgO(001) single crystal substrate. The films were grown with a highly (001)-oriented epitaxial growth and have an atomically flat surface with an average roughness of 0.07 nm. We observed perpendicular magnetization of CoFeB on the L5O layer after 250–350 °C post-annealing, revealing that introduction of significantly large PMA at HEO/ferromagnet interfaces. A large interface PMA energy of up to ∼ 0.8 erg/cm² at the interface was observed due to the achievement of structurally stable epitaxial layers with high crystallinity and sharp interfacial flatness of L5O and CoFeB interfaces. We also demonstrated that a tunnel magnetoresistance (TMR) ratio of up to 84 % at room temperature in epitaxial Fe/L5O/Fe(001) magnetic tunnel junctions (MTJs) with ultrathin MgO insertions at the L5O interfaces, indicating that the spin-dependent coherent tunneling mechanism is also observed in HEO-based MTJs. In addition, the L5O barrier exhibits low barrier heights less than 1 eV due to the bandgap reduction caused by the five cations. Our results of the high interface PMA energy, the relatively large TMR ratio, and the low barrier height show that the HEO materials can be a promising material family of ultra-thin barriers of MTJs for the next generation of spintronic devices such as ultra-high-density memory and spin artificial intelligence devices.