Linear magnetoelectric coupling and type-II multiferroic order in NiMn2O4

We report an unexplored type-II multiferroic order in NiMn2O4, exhibiting strong linear magnetoelectric coupling above liquid-nitrogen (LN) temperature. The compound becomes ferroelectric at similar to 100 K, coinciding with ferrimagnetic ordering, with a polarization value of similar to 535 mu C/m(2) for a poling field of 5 kV/cm. At LN temperature, the polarization value increases linearly (similar to 21%) with a magnetic field up to 30 kOe. Rietveld refinement of neutron diffraction patterns reveals a ferrimagnetic model with antiparallel moments at tetrahedral and octahedral sites, as well as a canting of octahedral moment persisting up to similar to 100 K. Low-temperature synchrotron diffraction confirms a step-like oxygen displacement during multiferroic ordering, suggesting that the Dzyaloshinskii-Moriya interaction polarizes the intervening oxygen atoms through magnetostriction, providing a microscopic mechanism for spontaneous electric polarization in this linear magnetoelectric multiferroic compound.