Successive phase transitions in the hexagonal-layered Heisenberg antiferromagnets MnX 2 (X = Br, I)

Abstract

Successive magnetic phase transitions in the Heisenberg antiferromagnets MnX 2 (X = Br, 1) have been studied by neutron diffraction. Two ( T N1 = 2.32 K and T N2 = 2.17 K) and three ( T N1 = 3.95 K, T N2 = 3.8 K and T N3 = 3.45 K) transitions were observed in the bromide and the iodide, respectively. In both compounds, transverse sinusoidally modulated structures occur in the intermediate phases (MnBr 2: T N2 < T < T N1 MnI 2: T N3 < T < T N1), whereas the(MnBr 2) and helical (MnI 2) structures occur through first-order transitions in the lowest-temperature phases. The successive transitions are brought about by several competing exchange interactions and the dipole interaction.