Magnetic structure and lattice contraction in Mn₃NiN

The nuclear and magnetic structures of the cubic antiperovskite compound Mn₃NiN are studied using neutron powder diffraction as a function of temperature and applied magnetic field. On cooling in zero field from the paramagnetic state, an anomalous lattice expansion abruptly occurs as the long range antiferromagnetic order first develops below T_N = 262 K. The magnetic structure has lower symmetry than the crystal and can be conveniently described by a rhombohedral R-3 symmetry with dimension of √2a_c × √2a_c × √3a_c, where a _c is the chemical unit cell. In this description, the magnetic moment of the Mn is restricted to the a-b plane, but continuously rotates from 90° to around 38° as the temperature decreases to ≈120 K, below which the spin directions remain fixed. The combined magnetic and structural transition exhibits a very large magnetovolume effect with an entropy change of ΔS ≈ 54 J/kg K, but the application of a 6 T magnetic field has little effect on the magnetic structure or entropy change. The crystallographic results indicate that the properties can be tailored by small changes in the composition and site vacancies, and the results enrich the understanding of the magnetostrictive effects in antiperovskites.