Theoretical Study On Magnetization And Magnetic Entropy Change Of Ferrimagnet

Magnetic entropy change is one significant quantity to characterise refrigeration property of magnetocaloric material.Theoretically,isothermal magnetic entropy change can be calculated according to Maxwell's relations.Experimentally,magnetic entropy change of materials is usually measured by magnetization or heat capacity indirectly,and magnetocaloric effect also can be characterized by adiabatic temperature change.Recently,unique magnetization compensation phenomenon of ferrimagnet has drawn more attention,but there is little theoretical study on magnetic entropy change of ferrimagnet.In this paper,the magnetic entropy change of ferrimagnetic is investigated by using Green's function theory on account of the issue of relation between magnetization compensation phenomenon and magnetic entropy change.Three-dimensional magnetic superlattice and two-dimensional mixed-spin magnetic structure are constructed considering the microstructure of ferrimagnet.The magnetic system is described by Heisenberg model,and Hamiltonian of the system includes the exchange interaction items,the single-ion anisotropy items and the Zeeman items.At first,the equations of motion of Green's functions of spins are set up,spin functions of sublattices are obtained through decoupling to higher-order Green functions generated by the exchange interaction term and the single-ion anisotropy term using random phase approximation and Anderson and Callen's decoupling scheme respectively,and then magnetizations of sublattices are calculated by means of Callen's technique.Finally,magnetic entropy change is calculated according to Maxwell relations.With regard to the three-dimensional magnetic superlattice system composed of spin-1 and 2 and spin-3/2 and 2,the magnetization of the system shows three cases,in absence of magnetization compensation,critical situation and in presence of magnetization compensation.The magnetic entropy change of two superlattice systems is different when the magnetization compensation phenomenon does not exist or is not obvious.In the system of spin-1 and 2,the total magnetic entropy change is negative in overall temperature and shows a positive peak or a big numerical value near the transition temperature,so the system only undergoes normal magnetic entropy change.However,for the system of spin-3/2 and 2,the total magnetic entropy change displays a positive peak before the transition temperature and a negative peak near the transition temperature,namely the system undergoes inverse and normal magnetic entropy change before and near the transition temperature respectively.The magnetic entropy change behaviors of two superlattice systems are similar when the magnetization compensation phenomenon is obvious.The total magnetic entropy change shows a negative peak near the compensation temperature and a positive peak near the transition temperature,namely,the systems undergo normal and inverse magnetic entropy change near the compensation temperature and the transition temperature respectively.As for the two-dimensional mixed-spin system composed of spin-1 and 2 and spin-3/2 and 2,by calculating the magnetization and the total magnetic entropy change of system and magnetic entropy change of sublattices as a function of temperature respectively,we found the results in mixed-spin system are similar to the magnetic superlattice system.Furthermore,comparing to the magnetic superlattice system,we found the compensation temperature and transition temperature of the mixed-spin system are higher,but the interval of the compensation temperature and the transition temperature is smaller and the numerical value of the total magnetic entropy change is smaller.