Spin canting and exchange in the two-dimensional antiferromagnets (C 3H 7NH 3) 2 and (C 3H 7NH 3) 2MnBr 4

Abstract

The compounds (C 3H 7NH 3) 2MnCl 4 and (C 3H 7NH 3) 2MnBr 4 are good approximations of the two-dimensional quadratic Heisenberg antiferromagnet. From measurements of the magnetic susceptibility the exchange constants at 80 K have been obtained, J/ k = −4.45(5) K and −4.50(5) K, respectively. These exchange constants are compared with theoretical results for the MnXMn superexchange path. A slight temperature dependence of J is observed, resulting in values which are about 5% lower at 300 K. The chloride is found to show weak ferromagnetism below T c = 39.2(2) K, which is ascribed to an antisymmetric exchange term. The canting angle of the spins is only 0.05°. The bromide compound orders at 47(3) K but shows no sign of weak ferromagnetism.