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

Neutron diffraction experiments on the Heisenberg antiferromagnet MnBr2 were performed to study the successive magnetic phase transitions (TN1 = 2.32 K, TN2 = 2.17 K). The magnetic structure in the intermediate phase is found to be a transverse sinusoidally-modulated structure with an incommensurate modulation vector qic ≃ (0.207, 0, 0.387), which varies as a function of temperature. At TN2 a first-order transition takes place from the incommensurate structure to the commensurate (qc=(l/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.