Theoretical investigation on the magnetocaloric effect of BiFeO3

Abstract

The magnetic entropy change (MEC) properties of BiFeO3 (BFO) were investigated theoretically by means of quantum microscopic calculations based on the Green's function, the classical thermodynamical theory and Maxwell's relation. The MEC increase as the temperature rises, while decline steeply from max to zero at the magnetic phase transition temperature TN. This anomaly of the MEC can be explained as the antiferromagnetic phase transition. The temperature dependence of MECs shows two kinks. One kink appears at the temperature T1 = 425K, which is interpreted as the result of the dependence of the spin cycloid anharmonicity on temperature. Another abnormal kink of MEC appears at nearly T2 = 550 K, which is associated with the onset of structural changes. Besides, it is found that TN shifts to higher temperature as the exchange coupling constants (J1) between one magnetic spin and its nearest neighbor, the exchange coupling constants (J2) between the two next- nearest neighbor magnetic spins, the exchange coupling constant (I) between the nearest neighbor pseudo-spin and magnetoelectric coupling constant (g) increase. What's more, it is also found that the magnetic entropy is decrease as the J1 and J2 increase. At last but not least, the I and g affect magnetic entropy only in the vicinity of TN. Those achieved conclusions are in good accordance with the experimental results and other theory findings.