Noncollinear Magnetism in Fe ₃ O ₄ Induced via Site‐Selective Rare‐Earth Substitution Boosting Its Saturation Magnetization

Substituting rare‐earth Eu ions with a large atomic number into 3 d 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 3+ substitution as a strategy to enhance the magnetic properties of Fe 3 O 4 thin films. The substitution location of Eu 3+ , that is, octahedral versus tetrahedral, is confirmed by electrical resistivities and valence band photoemission measurements. Tetrahedrally Eu‐substituted Fe 3 O 4 exhibited an exceptionally enhanced saturation magnetization M S of up to 4.4 μ B /f.u. because of noncollinearity, whereas octahedrally Eu‐substituted Fe 3 O 4 showed significantly reduced M S . X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism measurements clearly revealed that in the tetrahedrally Eu‐substituted Fe 3 O 4 , the Eu 3+ magnetic moment positively contributed to the orbital magnetic moment that exhibited strong magnetic anisotropy. The deviation of the observed M S 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 Fe 3 O 4 and a new perspective for designing practical ferrimagnetic 4 f compounds with exceptional M S .