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 (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)similar or equal to(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.