On the theory of the magnetic quantum phase transitions of the 1st kind in van Vleck Izing antiferromagnet
T.I. Lyashenko1, V.M. Kalita1,2, and V.M. Loktev1,3
1National Technical University of Ukraine “KPI”, 37 Peremohy Ave., Kyiv 03056, Ukraine
2Institute of Physics of the National Academy of Sciences 46 Nauki Ave., Kiev 03028, Ukraine
3Bogolyubov Institute for Theoretical Physics of the National Academy of Sciences of Ukraine 14-b Metrolohichna Str., Kyiv 03143, Ukraine
Received July 4, 2018, revised October 9, 2018, published online November 26, 2018
Quantum phase transitions induced by an external magnetic field in a Van Vleck paramagnet with ion spin S = 1 and competition of an easy-plane one-particle anisotropy and Ising spin-spin interactions are investigated. To describe the phase transformations we use the Lagrange function, whose minimization is performed by the coefficients of a linear combination of the spin wave functions that determine the magnet ground state. It is shown that such an approach agrees with the Landau theory of phase transitions of the 1st kind. It is obtained that the transition from the Van Vleck paramagnetic (singlet) phase to the ferromagnetic one is realized by forming an intermediate state with only one magnetized spin sublattice.
Key words: magnetic quantum phase transition, magnetic anisotropy, ferro- and antiferromagnetics.