2National Technical University of Ukraine “KPI”, 37 Peremogy Ave., Kiev 03056, Ukraine
3Institute of Physics of the National Academy of Sciences 46 Nauki Ave., Kiev 03028, Ukraine
4Bogolyubov Institute for Theoretical Physics, 14-b Metrologicheskaya Str., Kiev 03680, Ukraine
Received October 10, 2017
A magnetic field-induced 1st-order phase transition in an isotropic non-Heisenberg ferromagnet with a fourth-order exchange interaction in spin at temperatures above the Curie point is described. The field behavior of the magnetization is analyzed, the temperature dependences of the critical fields of the stability of the paramagnetic and ferromagnetic phases, as well as the critical field of the phase transition between them, are determined. It is shown that this 1st-order phase transition with a finite magnitude of the magnetization jump can occur in small magnetic fields. An H–T phase diagram is obtained. It is found that the magnitude of the entropy jump under the magnetic field-induced phase transition from the paramagnetic phase to the ferromagnetic one can exceed by more than an order of magnitude the entropy change of an isotropic Heisenberg ferromagnet when it is magnetized at the Curie point. It is shown that the expansion of the model free energy up to the 6th power in terms of magnetization can only qualitatively describe the entropy behavior in the phase transition between the paramagnetic and ferromagnetic phases.
PACS: 65.40.gd Entropy; PACS: 75.30.Sg Magnetocaloric effect, magnetic cooling; PACS: 75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects.