Noncollinear Magnetism in Fe3O4 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 Eu3+ substitution as a strategy to enhance the magnetic properties of Fe3O4 thin films. The substitution location of Eu3+, that is, octahedral versus tetrahedral, is confirmed by electrical resistivities and valence band photoemission measurements. Tetrahedrally Eu-substituted Fe3O4 exhibited an exceptionally enhanced saturation magnetization MS of up to 4.4 μB/f.u. because of noncollinearity, whereas octahedrally Eu-substituted Fe3O4 showed significantly reduced MS. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements clearly revealed that in the tetrahedrally Eu-substituted Fe3O4, the Eu3+ magnetic moment positively contributed to the orbital magnetic moment that exhibited strong magnetic anisotropy. The deviation of the observed MS 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 Fe3O4 and a new perspective for designing practical ferrimagnetic 4f compounds with exceptional MS.