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 LiFePO 4 above the critical field of 31 T. 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 alpha ab is partially suppressed, and the diagonal element alpha bb 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.