Noncollinear Magnetism in FeO Induced via Site-Selective Rare-Earth Substitution Boosting Its Saturation Magnetization.

Substituting rare-earth Eu ions with a large atomic number into 3d transition metal oxides can precisely control their magnetic properties through significant spin-orbit coupling, leading to noncollinear magnetism. Experimental investigations are combined with density functional theory to explore site-selective Eu substitution as a strategy to enhance the magnetic properties of FeO thin films. The substitution location of Eu, that is, octahedral versus tetrahedral, is confirmed by electrical resistivities and valence band photoemission measurements. Tetrahedrally Eu-substituted FeO exhibited an exceptionally enhanced saturation magnetization M of up to 4.4 μ/f.u. because of noncollinearity, whereas octahedrally Eu-substituted FeO showed significantly reduced M. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements clearly revealed that in the tetrahedrally Eu-substituted FeO, the Eu magnetic moment positively contributed to the orbital magnetic moment that exhibited strong magnetic anisotropy. The deviation of the observed M from the lower value predicted by Néel's theory of collinear ferrimagnetism further supported the role of noncollinearity. These results provide empirical evidence for the spin configuration of tetrahedrally Eu-substituted FeO and a new perspective for designing practical ferrimagnetic 4f compounds with exceptional M.