Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

摘要

We study planar ferromagnetic spin-chain systems with weak antiferromagneticinter-chain interaction and dipole-dipole interaction. The ground state dependssensitively on the relative strengths of antiferromagnetic exchange and dipoleenergies kappa=J'a^2c/(g_L\mu_B)^2. For increasing values of \kappa, the groundstate changes from a ferromagnetic via a collinear antiferromagnetic and anincommensurate phase to a 120^o structure for very large antiferromagneticenergy. Investigation of the magnetic phase diagram of the collinear phase, asrealized in CsNiF_3, shows that the structure of the spin order dependssensitivly on the direction of the magnetic field in the hexagonal plane. Forcertain angular domains of the field incommensurate phases appear which areseparated by commensurate phases. When rotating the field, the wave vectorcharacterizing the structure changes continuously in the incommensurate phase,whereas in the commensurate phase the wave vector is locked to a fixed valuedescribing a two-sublattice structure. This is a result of the competitionbetween the exchange and the dipole-dipole interaction.