Magnetoelectric phases in the LiMPO4 systems

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• Rasmus Toft-Petersen (Helmholtz-Zentrum, Berlin)
• Monday 14 March 2016, 14:00-15:00
• Physics East 217.

If you have a question about this talk, please contact Vincent Boyer.

The magnetic (H, T) phase diagrams of the frustrated 2D antiferromagnetic LiMPO4 (M = Ni, Co, Fe, Mn) systems are rich and surprisingly similar. The prime example is XY-like LiNiPO4, exhibiting 7 magnetic phases below 55 T longitudinal field. Using epithermal neutrons with pulsed magnetic fields, we successfully probed the magnetic structures in LiNiPO4 in longitudinal fields up to 30 T. At 19 T, we find a counterintuitive phase transition between an IC spiral structure – consistent with XY-anisotropy – and a re-entrant magnetoelectric (ME) commensurate order polarized along the applied field. This magnetized variant of the zero field structure is confirmed by a microscopic model quantitatively describing both the ME response between 19-21 T and that in zero field. Neutron diffraction data was obtained for fields up to 40.5 T. In the sister compound LiFePO4, we recently discovered weak canting components in the magnetic structure in zero field. These indicate that the crystal symmetry is lower than previously established. Through the Dzyaloshinky-Moriya interaction, this canting can be shown to give rise to a ME response that even describes the temperature dependency of the ME tensor components. It can generally be said that the single anisotropy terms of the LiMPO4 systems are quite strong and varied, but with similar exchange constants. This makes them ideal for synthesizing mixed anisotropy magnets – where exchange interactions and single ion anisotropies compete. In the case of the (Co, Ni) substitution, this leads to a field induced phase transition from long to short range order. Substituting Fe for Ni on the other hand, gives rise to an additional spontaneous ME order parameter and offers the possibility of tailoring the magnetoelectric response.

This talk is part of the Cond. Mat. seminar series.

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• Cond. Mat. seminar
• Physics East 217

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