Magnetic properties of coupled ultrathinNiO∕Fe3O4(001) films

Abstract

A strong reduction in the magnetic spin and orbital moments as well as large magnetocrystalline anisotropies are observed in ultrathin $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) films compared to ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (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 $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (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 $1.5\text{\ensuremath{-}}\mathrm{nm}$-thick interfacial $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ compound. Angular-dependent measurements of the effective spin and orbital magnetic moments at the Fe ${L}_{2,3}$ 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 ${L}_{2,3}$ 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 $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (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 ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) substrate.