Incommensurate Magnetic Structures in Triangular Antiferromagnets Caused by the Dipole-Dipole Interaction: a Realization of Conical-Point Instability

The dipole-dipole interaction is important in the phase transitions of RbFeCl 3 -type XY-like antiferromagnets. It can realize incommensurate states, which are related to the conical-point instability first pointed out by Ishibashi and Dvorak in connection with the structural phase transition. Several consequences of the conical-point instability including the in-plane-field effect are studied by using the Landau-type free energy. A recent neutron scattering on RbFeCl 3 by Wada, Ubukoshi and Hirakawa is discussed from the viewpoint of the conical-point instability caused by the dipole-dipole interaction.