Bromine NMR Study of Cupric Compounds (C_nH_2n+1NH₃)₂CuBr₄ (n=1, 2)

Abstract

The spin structure of (C_nH_2n+1NH₃)₂CuBr₄ (n=1, 2) is studied by Br NMR at 1.7 K. In (C₂H₅NH₃)₂CuBr₄, the magnetic space group is Pb′c′a type and the moment direction inclines at an angle of 49.5° with the c-axis. A weak ferromagnetic moment appears along to the c-axis. The anisotropic exchange field is estimated to be 2.5 kOe. The inter-layer exchange field and the out-of-layer anisotropy field are much smaller than previously reported values. The parameters of Br nuclear spin Hamiltonian are also obtained. In (CH₃NH₃)₂CuBr₄, the moment direction inclines at 27° with the c-axis. It is difficult to interpret the NMR results by the two-sublattice model. A model of the four-sublattice antiferromagnet is developped.