Neutron Diffraction Study of Successive Phase Transitions in the Heisenberg Antiferromagnet MnBr₂

Neutron diffraction experiments on the Heisenberg antiferromagnet MnBr 2 were performed to study the successive magnetic phase transitions ( T N1 =2.32 K, T N2 =2.17 K). The magnetic structure in the intermediate phase is found to be a transverse sinusoidally-modulated structure with an incommensurate modulation vector q ic ≃(0.207, 0, 0.387), which varies as a function of temperature. At T N2 a first-order transition takes place from the incommensurate structure to the commensurate ( q c =(1/4 0 1/4)) structure. These phase transitions are successfully accounted for using mean-field theory based on a Hamiltonian which consists of six exchange interactions and the dipole interaction.