Magnetic properties of coupled ultrathin NiO/Fe3O4 (001) films

A strong reduction in the magnetic spin and orbital moments as well as large magnetocrystalline anisotropies are observed in ultrathin NiO/Fe3O4 (001) films compared to Fe3O4 (001). Combining x-ray absorption spectroscopy, x-ray magnetic circular and linear dichroism (XMCD-XMLD), and high-resolution transmission electron microscopy (HRTEM) the magnetic anisotropy of ultrathin NiO/Fe3O4 (001) films was studied and compared to structural anisotropy at the interface. The evolution of the XMCD signal at the Fe octahedral sites allows a quantitative description of the interface. A simple model is proposed where uncompensated spins are located in a 1.5-nm-thick interfacial NiFe2O4 compound. Angular-dependent measurements of the effective spin and orbital magnetic moments at the Fe L-2,L-3 edges reveal the presence of a large magnetocrystalline anisotropy, appearing for a particular growth temperature of the films. The magnetocrystalline anisotropy measured at the Fe L-2,L-3 edges is attributed to the specific anisotropic structure developed at the interface and to crystalline ordering of a strained compound. HRTEM confirms the presence of an ordered anisotropic structure at the NiO/Fe3O4 (001) interface. The in-plane strained NiO(001) films promote antiferromagnetic order where the spins are oriented parallel to the '100' in-plane direction, collinear to the magnetization direction in the Fe3O4 (001) substrate.