Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn₂O₄

Spinel compounds AB₂X₄ consist of both tetrahedral (AX₄) and octahedral (BX₆) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn₂O₄ has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn₂O₄ by employing the high-resolution neutron powder diffraction. The cation distribution is found to be (Mn_0.9²⁺Fe_0.1³⁺)(Mn³⁺Fe_0.9³⁺Mn_0.1²⁺)O₄. While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at T_S ≈ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn³⁺ cation. Strong magnetoelastic coupling is unveiled at T ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to T ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn³⁺/Fe³⁺ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ∼25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O-B-O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn₂O₄. Our study indicates that FeMn₂O₄ is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.