Quantum fluctuations in antiferromagnets of the BX2 family (abstract)

Abstract

The hexagonal antiferromagnets of the BX2 family, where B is a magnetic ion, and X a halogen, are characterized by an in plane exchange interaction J much stronger than the exchange interaction J′ along the c axis, so that some features of the triangular antiferromagnet (TA) could survive in these real compounds. In particular we are interested to investigate a possible ‘‘planar’’ phase with the spins lying in the c plane supported by quantum fluctuations when an external magnetic field H is applied perpendicular to the c axis. We find that such a phase is stable for sufficiently small interplane coupling owing to the zero point motion energy, whereas an ‘‘umbrella’’ phase would be expected on the basis of the classical approximation for any nonzero interplane coupling. Notice that for the TA model the planar and umbrella configurations are degenerate in classical approximation for any H and infinite isoenergetic planar configurations exist. Quantum fluctuations select a planar configuration with a spin over three opposite to the field. We find that the classical scenario itself is substantially different for any j=J′/J≠0, because only a planar configuration with a spin over three nearly perpendicular to the field and the umbrella configuration minimize the energy of the model and the umbrella is stable. However, quantum fluctuations stabilize the planar configuration for any H below a critical interplane coupling j*. For intermediate j the planar configuration is stable for low and high fields, whereas only the umbrella phase is stable of j large enough.