Lattice and antiferromagnetic symmetries in disorder-free spinel oxide ZnFe₂O_4.

Pyrochlore-structured spinel magnets are of broad theoretical and experimental interest because of their potential to host frustrated spin configurations. However, many 3d spinel oxides are plagued by a high level of disorder, expressed as both off-stoichiometry and site mixing. As magnetism is highly sensitive to those disorders, the antiferromagnetic structure of ZnFe₂O₄ has not been fully resolved; two neutron powder diffraction refinements in the 1970s failed to provide a consistent model of the spin arrangement. A large amount of short-range magnetic diffuse scattering has had an adverse effect on the refinement of this presumed anitferromagnet. In addition, the spin structure with a wavevector k= (1/2, 1, 0) was assumed to be non-collinear, which brings an intrinsic difficulty of refinement solely using powder diffraction. Recently, we resolved both the site disorder and stoichiometric issues in this spinel magnet and took neutron magnetic diffraction data on a piece of clean-limit single crystal at CORELLI [1]. Our lattice diffraction study has revealed that the symmetry group is no longer the central-symmetric type of Fd-3m, but instead is of space group F-43m with a broken inversion symmetry. All these have allowed a refreshed perspective of the spin structure in ZnFe₂O₄. We also discuss how the spin structure helps to understand the magnetic behavior in the H-T magnetic phase diagram.