Surface and interface effects on the electronic and magnetic properties of NiCo₂O₄ thin films

NiCo₂O₄ (NCO) thin film surfaces are found to be Ni-rich and there exist surface-to-bulk core level shifts in binding energies of the Ni 2p_3/2 and Co 2p_3/2 core levels [1]. While the Ni-rich conducting NCO thin film surface undergoes irreversible metallic to non-metallic (dielectric) phase transition, the highly reduced surface of the NCO thin film can undergo a reversible phase transition from highly dielectric to a much diminished dielectric (or highly enhanced metallic) character with temperature [1]. Spin-polarized inverse photoemission spectroscopy (SPIPES) and angle-resolved x-ray magnetic circular dichroism (XMCD) of the Ni-rich NCO thin film show that the surface spin moments of the film are canted, although the same film in the bulk is known to possess perpendicular magnetic anisotropy (PMA) [2]. When a thin platinum (Pt) layer was deposited on the NCO film, the spin moment canting at the interface of Pt/NCO increased dramatically, leading to significantly different spin ordering at the interface compared to the bulk spin ordering (PMA) of the NCO thin film. The reversibility in electronic phase transition can ultimately lead to two controlled, programmable and non-volatile electronic phase states, whereas spin-canting surfaces and interfaces make NCO thin films interesting for spintronics.



[1] A. Subedi, D. Yang, X. Xu, and P. A. Dowben, J. Phys. D: Appl. Phys. 57, 495301 (2024).

[2] C. Mellinger, J. Waybright, X. Zhang, C. Schmidt, and X. Xu, Phys. Rev. B 101, 014413 (2020).