Interfacial Magnetization in Ultrathin NiCo₂O₄ Thin Films

NiCo₂O₄ (NCO) films are well-known to exhibit strong perpendicular magnetic anisotropy [1] on account of the epitaxial strain provided by a substrate with small lattice mismatch, e.g. MgAl₂O₄ (MAO). The conductivity of NCO also lends itself to potential uses in spin-electronic devices, as has already been studied for a NCO/MAO/MAO magnetic tunnel junction [2]. In these cases, the magnetic behavior of the interface is critical to realizing the full potential of such devices. To this end, we have studied the magnetic behavior of NCO films of varying thickness using x-ray magnetic circular dichroism (XMCD). Ultrathin films (t < 4 nm) show features which, through comparison of electron yield and fluorescence yield measurement modes, can be attributed to magnetization at the film-vacuum interface. Further, the surface magnetization persists at a lower transition temperature than for the bulk. Physical mechanisms for this effect will be discussed. 

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

[2] Y. Shen, D. Kan, I.-C. Lin, M.-W. Chu, I. Suzuki, and Y. Shimakawa, Appl. Phys. Lett. 117, 042408 (2020).