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Frustrated magnets in high magnetic fields

Speaker: Ben Bryant (Radboud University)
Date: Wednesday 21 November 2018
Time: 15:00
Venue: Queens Buildings N/3.28

Materials that exhibit magnetic frustration often have complex non-collinear spin ground states that can give rise to multiferroicity. The antiferromagnetic spinel CdCr2O4 shows strong spin frustration, and when a high magnetic field is applied to the antiferromagnetic state, a magnetostructural phase transition occurs to a fractional magnetization plateau with half the saturation magnetization. As well as spin frustration, CdCr2O4 exhibits strong spin-lattice coupling, so thermal expansion measurements can provide valuable insight into the magnetostructural phases. We performed magnetostriction and, for the first time, thermal expansion measurements in magnetic fields up to 30 T, enabling us to precisely map the entire low-temperature phase diagram of CdCr2O4. Above the transition to the plateau phase we observe a distinct type of negative thermal expansion. The experimental phase diagram strongly resembles its theoretical counterpart, derived from a magnetoelastic theory, but diverges in the greater thermal stability of the plateau state. The negative thermal expansion in the plateau state, and its enhanced thermal stability, are both characteristic of an exceptionally strong spin-lattice coupling in this phase. To go beyond bulk measurements and into spatially-resolved studies, we employed our unique high field Atomic Force Microscope to obtain electrostatic images at up to 30 T. We find evidence for a magnetic field-induced electric polarization, which increases with field up to the magnetization plateau, where it collapses to zero. The field-induced polarization may arise due to the spin spiral ground state.