Study On The Magnetic And Magnetocaloric Properties Of PrNi₂ And Tb₃Ga₅O₁₂

Nowadays, much attention is paid to Pr Ni2 and Tb3Ga5O12(TGG) crystals due to their excellent magnetocaloric behavior and the technical application in refrigerators. The excellent properties of the materials are induced by the crystal field effect, the exchange interaction between different magnetic ions and the spin fluctuations of magnetic ions. However, the exchange interaction and the spin fluctuations have not been considered sufficiently in the previous studies. The exchange interaction-effective field was simply equaled to a linear function MH =lM. It led to the self-consistent method must be used in the calculations. However, the calculated results are still not satisfying, and the applied magnetic field and temperature dependences of the exchange interaction are screened consequently. In this article, we study the effective field by analyzing the experimental data of magnetic moment, and obtain the quantitative form of the field, which indicates that the effective field is correlated with the applied field and temperature. Using the effective field, the magnetic moment, the susceptibility, the reciprocal susceptibility, the magnetic heat capacity and the magnetic entropy change have been calculated. The calculated values are in good agreement with the experimental values. The main studied results are presented as follow:The first accomplished work is the investigation of magnetic and magnetocaloric properties of Pr Ni2 crystal. We modify the CF parameters according to the experimental data of zero-field magnetic heat capacity, and successfully obtain the crystal field energy levels and the wave functions by using the modified parameters. Based on the calculated CF energy levels, the zero-field magnetic heat capacity has been calculated. The calculated values are in good agreement with the measured values. By considering the exchange interaction between different magnetic ions and the spin fluctuations, the exchange interaction-effective field of Pr Ni2 is given as: nMe THT)1(0Mwbg-++=. Using the effective field, the magnetic moment and the reciprocal susceptibility in different temperature are calculated. The calculated values are in good agreement with the experimental values, and it has proved the correctness of the effective field put out by us. Under this exchange interaction model, we give the theoretical values of the magnetic entropy change in the applied field range of 0-5T.The second accomplished work is the study of magnetic and magnetocaloric properties of Tb3Ga5O12(TGG) crystal. The CF energy levels and the wave functions of Tb3+ ions in TGG are calculated based on the quantum mechanics theory. By analyzing the exchange interaction between different Tb3+ ions and the spin fluctuations of Tb3+ ions, the exchange interaction-effective field of TGG is given as:. The expression reveals the applied field and temperature dependence of the exchange interaction in TGG crystal. By using the effective field, the Zeeman splitting levels, the wave functions, the magnetic moment and the susceptibility are calculated. The calculated values are in good agreement with the experimental data. In order to examine the correctness of the effective field put out by us, we calculated the isothermal magnetic entropy change of TGG, and the calculated results are in good agreement with the measured values.