Physical properties and x-ray diffraction of a NiMn2O4 single crystal below and above the ferrimagnetic transition at T-c=145 K

The magnetic and calorimetric measurements of stoichiometric NiMn2O4 showed a phase transition at T-c = 145 K. The single crystal X-ray diffraction investigations pointed to the purely magnetic nature of the transition. The magnetostriction effect, observed in the temperature dependence of the unit cell volume and persisting above T-c, suggested a short-range spin ordering. The cation distribution over the tetrahedral and octahedral sites in the spinel structure has been determined; the inversion parameter nu = 0.87. The role of Mn cations in the magnetic coupling process has been discussed. Our results allow the formula to be written with the Mn cations in two oxidation states on the octahedral site: (Ni-1-nu(2+)) Mn-nu(2+) [Ni-nu(2+) Mn-2-2 nu(3+) Mn-nu(4+)] O-4 (C) 1997 Elsevier Science Ltd.