Magnetic properties of the weak ferromagnet NH 4MnF 3

Abstract

The magnetic properties of NH 4MnF 3 have been studied by means of heat capacity, a.c. susceptibility at zero field and at H=10 kOe as a function of temperature, and at T= 4.2 K as afunction of field. Moreover, magnetization measurements were carried out at T=4.2 K as a function of field up to 20 kOe in a vibrating sample magnetometer and up to 400 kOe in a pulsed-field magnet. The compound has the simple cubic perovskite structure at room temperature. At T c =182.1±0.1 K a structural transition to a pseudo-tetragonal symmetry occurs, which gives rise to a small change in the paramagnetic susceptibility. The onset of antiferromagnetic ordering takes place at T N =75.1±0.1 K at zero field, and a spin-flop transition at T=4.2 K is detected at H SF =3.7±0.2 kOe. Evidence for the presence of a small weak-ferromagnetic moment is found from an anomaly in χ and from the presence of magnetic absorption near T N. From a comparison of the data to predictions for the s.c. Heisenberg antiferromagnetic model, a value of J/ k = -3.11 K is obtained for the exchange interaction.