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

抄録

Neutron diffraction experiments on the Heisenberg antiferromagnet MnBr₂ 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 {1em}0 {1em}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.