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MA: Fachverband Magnetismus

MA 5: Spin Structures and Magnetic Phase Transitions I

MA 5.11: Talk

Monday, March 18, 2024, 12:15–12:30, EB 202

On the valence of chalcogen spinels — •Vinícius Estevo Silva Frehse, Aleksander Sukhanov, Elaheh Sadrollahi, and Marein Rahn — IFMP, Dresden, Germany

The valence characteristics in the metallic ferromagnetic spinels CuCr2X4 (X = S, Se, Te) has long been subject to debate. At the heart of this controversy lies the ambiguity between two scenarios, proposed by F. K. Lotgering and J. B. Goodenough: According to Lotgering, monovalent non-magnetic Cu ions (3d10) exist next to mixed-valent (3+/4+) Cr with a reduced magnetic moment. Conversely, in the Goodenough model, Cu is divalent (3d9), with an anti-parallel spin-polarization that partially compensates the Cr3+ magnetism. In the light of potentially competing or almost degenerate valence distributions on a frustrated lattice, it would be of great interest to clarify the order parameter and mechanism of a pronounced low-temperature phase transition that has recently been observed by local and bulk magnetic probes (µSR, NMR and Mössbauer spectroscopy). Surprisingly, we find that magnetic neutron powder diffraction is not sensitive to this transition. A review of past and present experimental evidence provides some constraints on the unusual scenarios that could reconcile this apparent contradiction.

Keywords: Ferromagnetic spinel; Mixed valence