Linear magnetoelectric coupling and type-II multiferroic order in NiMn₂O₄

We report an unexplored type-II multiferroic order in NiMn $2$O $4$⁠, exhibiting strong linear magnetoelectric coupling above liquid-nitrogen (LN) temperature. The compound becomes ferroelectric at $∼100$ K, coinciding with ferrimagnetic ordering, with a polarization value of $∼535$  $μ$C/m $2$ for a poling field of 5 kV/cm. At LN temperature, the polarization value increases linearly (⁠ $∼21$%) 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 $∼100$ 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.