Magnetocaloric Effect In Pure And Doped Magnetic Relaxor Ferroelectrics CdCr₂S₄

Magnetic relaxor ferroelectric materials are compounds in which the relaxor ferroelectricity and ferromagnetic (antiferromagnetic) order coexist simultaneously in certain temperature range. Due to the coexistence of different order parameters,they can produce new features, such as intrinsic magnetoelectric coupling effect, namely it will generate electrical polarization when material is under a certain applied magnetic field, or it will generate the magnetic polarization under a certain applied electric field, which could realize mutual control between ferroelectricity and magnetism. Experimental researches have confirmed that this kind of magnetic relaxor ferroelectrics exhibits a variety of excellent physical properties, such as colossal magnetocapacitance, colossal magnetoresistance and gigantic Kerr rotation. Recently, it has been reported that the magnetic relaxor ferroelectrics shows giant magnetocaloric effect, which has great potential application in the magnetic refrigeration. Magnetocaloric (electrocaloric) effect occurs when a magnetic (electric) field applied under reversible and adiabatic conditions changes the temperature of a polarizable material.The discovery of large magnetocaloric effect in Gd₅Si₂Ge₂ alloy and the giant electrocaloric effect in thin film PbZr0.95Ti0.05O₃ has stimulated many experimental studies on this subject for the material researchers. However, Much effort has been focused on the traditional ferromagnet and ferroelectricity. As we all know, the magnetic field can not only change the magnetization but also change the polarization by means of magnetoelectric coupling, so magnetocaloric effect of multiferroic materials has become a hot topic. Although the magnetocaloric effect in BiFeO₃ ceramics and spinel CdCr₂S₄ was reported experimetally, the theoretical research on the magnetocaloric effect in multiferroic materials has not been reported. Therefore, the study of the magnetocaloric effect in magnetic relaxor ferroelectrics is not only of theoretical importance but also valuable for the technological application in material physics, condensed matter physics and microelectrics.The purpose of our work is to investigate the adiabatic temperature change and the isothermal entropy change of magnetic relaxor ferroelectrics. The main results of our study are listed as follows:1. Magnetocaloric effect in magnetic relaxor ferroelectrics CdCr₂S₄.The adiabatic temperature change and the isothermal entropy change of magnetic relaxor ferroelectrics CdCr₂S₄ has been explored by an appropriate theoretical model. The system CdCr₂S₄ consists of two subsysterms, including the relaxor ferroelectronic subsystem and magnetic subsystem, where the spherical random-bond-random-field model (SRBRF) model and Heisenberg model can be applied to describe the two subsystems, respectively. In addition, the strong magnetoelectric coupling between the two order parameters should be taken into account. Calculated results showed that both the entropy change and the adiabatic temperature change show the maximum near the magnetic phase transition temperature, which is in good agreement with the experimental results.2. Impact of doping Fe2+ on the magnetocaloric behavior in the doped magnetic relaxor ferroelectrics Cd_1-xFe_x Cr₂S₄.Doping can change the properties of the multiferroic material. Experimental observations of doping magnetic ions such as Fe²⁺ ions, can change the magnetic phase transition temperature of relaxor ferroelectric materials. Moreover, there is giant magnetocaloric effect near the magnetic phase transition temperature, which inspired us to study the magnetocaloric effect in doped relaxor ferroelectric materials. Doping can change the spin magnetic moment of materials, then this will definitely have influence on the magnetic coupling strength of material. In the doped magnetic relaxor ferroelectric materials Cd_1-xFe_xCr₂S₄, we use a modified spherical random-bond-random-field model (SRBRF) and the site-dilution model based on the Heisenberg model to study the magnetocaloric effect near magnetic phase transition temperature of different concentration of Fe²⁺ ions doping on the material. By calculating the isothermal entropy change and adiabatic temperature change, we find that theoretical results are in accordance with the experimental phenomena. The simulation results show that doping can change the magnetocaloric effect, also remind us that the different magnetic ions doping can increase the correspond temperature of the maximum entropy change of materials. This is benefit to develop room temperature materials in magnetic cooling.