Condensed Matter Physics, 2008, vol. 11, No. 3(55), p. 509-522

Title: Quantum phase transition: Van Vleck antiferromagnet in a magnetic field
  I.M.Ivanova (National National Technical University of Ukraine "KPI", Prosp. Pobedy 37, Kyiv 03056, Ukraine) ,
  V.M.Kalita (Institute of Physics of the National Academy of Science of Ukraine, Prosp. Nauki 46, Kyiv 03680, Ukraine) ,
  V.O.Pashkov (National Aviation University, Kosmonavta Komarova Ave. 1, Kyiv 03058, Ukraine) ,
  V.M.Loktev (Bogolyubov Institute for Theoretical Physics of the National Academy of Science of Ukraine, Metrolohichna Str. 14, Kyiv 03680, Ukraine)

Theoretical description of magnetic properties of antiferromagnetic state, induced by the longitudinal magnetic field, in the Van Vleck - singlet - magnet with single-ion anisotropy of "easy-plane" type and ion spin S=1 is proposed. It is shown that the quantum phase transition to the antiferromagnetic state is connected with the spontaneous appearance of spin polarization in the easy plane. Spin polarization of the ground non-degenerated state proves to be the order parameter of such a transition and the Landau thermodynamic approach can be employed for its (transition) description. The magnetic properties which include the field behavior of magnetization and magnetic susceptibility of the antiferromagnetic phase in the fields of different directions are studied. An attempt is made to qualitatively compare the obtained results with the available experimental data.

Key words: quantum magnetic phase transition, single-ion anisotropy, spin polarization, Van-Vleck antiferromagnet
PACS: 75.10.-b, 75.10.Jm, 75.30.Gw, 75.30.Kz, 75.50.Ee

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