Magnetization and magnetostriction of Van Vleck antiferromagnets with magnetic anisotropy of “easy-plane” type

Abstract

The magnetic properties of the antiferromagnetic state induced in the Van Vleck magnet by longitudinal magnetic field are described within thermodynamic Landau approach. We consider the case of a magnet with a single-ion anisotropy of ``easy-plane'' type and ion spin $S=1$. It is shown that quantum phase transition to antiferromagnetic state is related to appearance of the spontaneous spin polarization parallel to an ``easy-plane.'' Application of longitudinal field gives rise to the tilt of sublattice magnetizations toward the field direction and increase in their modula as well. Linear field dependence of the net magnetization (similar to that observed in the classical antiferromagnets) can be attained only in the limit of large interion ``easy-plane'' anisotropy comparable to the single-ion one. We also study peculiarities of the induced magnetostriction of the Van Vleck antiferromagnet and show that: (i) like the magnetization, magnetostriction has a singularity at the phase-transition point and (ii) the sign of magnetostriction can be reversed during the magnetization process due to the influence of the spin tilt. An attempt is made to qualitatively compare the obtained results with the available experimental data.