Classical effective spin hamiltonian and symmetry groups of the multispin-axis model of CoO

Abstract

The multispin axis model recently proposed by van Laar for the magnetic structure of CoO reflects the tetragonal nature of the crystal structure below the Néel temperature. Starting from simple symmetry arguments we construct an effective anisotropic spin hamiltonian H an of order two in the spins. The minimization of H an leads directly to the multispin axis model. An essential feature of H an is to show that Dzialoshinski-Moriya coupling is mainly responsible for the 90° canting invoked by the van Laar model for the spin components in the Oxyplane. This is in agreement with the large spinorbit coupling usually observed in bivalent Co. The magnetic structure transforms according to irreducible representations of several space groups which leave H an invariant. Possible Shubnikov groups are also indicated.