Magnetic structure and magnetoelectric properties of the spin-flop phase in LiFePO4

We investigate the magnetic structure and magnetoelectric(ME) effect in the high-field phase of the antiferromagnet LiFePO4 above the critical field of 31T. A neutron diffraction study in pulsed magnetic fields reveals the propagation vector to be q=0 for the high-field magnetic structure. Pulsed-field electric polarization measurements show that, at the critical field, the low-field off-diagonal ME coupling αab is partially suppressed, and the diagonal element αbb emerges. These results are consistent with a spin-flop transition where the spin direction changes from primarily being along the easy b axis below the transition to being along a above. The persistence of off-diagonal ME tensor elements above the critical field suggests a lowering of the magnetic point-group symmetry and hence a more complex magnetic structure in the high-field phase. In addition, neutron diffraction measurements in low magnetic fields show no observable field-induced spin canting, which indicates a negligible Dzyaloshinskii-Moriya interaction. The observed spin-flop field supports the Hamiltonian recently deduced from inelastic neutron studies and indicates that the system is less frustrated and with a larger single-ion anisotropy than originally thought. Our results demonstrate the effectiveness of combining pulsed-field neutron diffraction and electric polarization measurements to elucidate the magnetic structures and symmetries at the highest attainable field strengths.