Magnetic properties of iron doped magneto-electric LiNiPO$_4$ single crystals

Neutron scattering and magnetic susceptibility studies of pure and iron-doped magneto-electric LiNiPO$_4$ single crystals are compared. Elastic neutron scattering of single-crystal LiNiPO$_4$ reveal a spontaneous first-order commensurate- incommensurate magnetic phase transition (at $T_{C-IC}$ = 20.8 K) accompanied by a second transition ($T_{IC}$ = 21.7 K) from long- to short-range IC structure. These transitions are also identified in the magnetic susceptibility measurements. The modulated structure has a predominant antiferromagnetic component giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic peak, and a ferromagnetic component giving rise to peaks at small momentum transfers around the origin $(0,\pm Q,0)$. The wavelength of the modulated magnetic structure varies continuously with temperature. Magnetic susceptibilities of the iron doped crystals show magentic-field dependent properties characteristic of spin-glass systems.