Specificities of the Magnetocaloric Effect near the Point of a Second-Order Phase Transition in a Ferromagnet with Biquadratic Exchange

The magnetocaloric effect (change in the magnetic entropy upon isothermal magnetization, ) is investigated in the model of a ferromagnet with bilinear ( ) and biquadratic ( ) exchange interactions between nearest magnetic neighbors for the region of the ratios of the exchange parameters and in which the transition from the paramagnetic to the magnetically ordered state is a second-order phase transition. In the mean-field approximation, the thermodynamic potential of a magnetically ordered state is obtained, which is characterized by two order parameters: dipole (relative magnetization), , and quadrupole, and the temperature and field behavior of the order parameters of this ferroquadrupole state near the Curie point are considered. It was found that the exponents of the power-law dependences of the change in the magnetic entropy on the magnetic field coincide with exponents of the power-law dependence of the change in the entropy of an ordinary ferromagnet with only bilinear exchange; i.e., the change in entropy at the Curie point, , is proportional to slightly lower , ; and, above , At the same time, the coefficients at these power factors in the entropy essentially depend on the ratios of the exchange parameters and : they are minimal at and noticeably increase in absolute value with an increase in the ratio reflecting an increase in the decrease in entropy with an increase in the contribution from the biquadratic exchange.