Field driven magnetostructural domain flip in frustrated GeCo2O4

Orateur : Virginie SIMONET (Institut Néel)

Spinel compounds, of generic formula AB2O4 (cubic Fd-3m), combine spin, orbit and lattice degrees of freedom. They can accommodate a magnetic atom on the B sites, which form a pyrochlore lattice, the archetype of frustrated network consisting of tetrahedra joined by their vertices. In Ge spinel compounds, the frustration effects are not straightforward, in particular not solely driven by the first neighbor interactions. In the GeCo2O4 spinel, the magnetic Co2+ ions are on the B site and possess an orbital degree of freedom. A structural distortion towards a lower symmetry phase occurs at the onset of an antiferromagnetic order. Besides, a rich H-T phase diagram was probed by ultrasound, powder neutron diffraction and, electron spin resonance measurements, evidencing several field-induced anomalies whose nature remained unclear. I will present the results of combined single crystal neutron diffraction and synchrotron X-ray diffraction under magnetic field up to 12 T applied along the 2-fold axis of the high temperature cubic structure. These experiments allowed us to unveil the magneto-structural nature of the field-ind uced processes, disentangling the modification of the domain population from spin reorientations. We show in particular the presence of a remarkable phase transition occurring at 9.5 T where a field-induced change in the Co2+ magnetic anisotropy produces a spin-reorientation correlated to a flip of the coupled magnetic and structural domains.

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