Anisotropic Magnetic And Magnetocaloric Effects In Single Crystal TbMn₂Ge₂ And Mn₃Si₂Se₆

In the pursuit of high efficiency,green and pollution-free,more and more new technologies are being applied,in which materials with special properties and functions play an important role.These materials not only have the basic physical properties of traditional materials,but also have special electrical,magnetic,optical,acoustic and thermal properties,which can respond to external stimuli and achieve various functions.Magnetocaloric materials can convert energy through the thermodynamic effect of a magnetic field and are used in a wide range of applications such as refrigeration and temperature control.This thesis focuses on two new magnetocaloric materials,TbMn₂Ge₂ single crystal and Mn₃Si₂Se₆ single crystal,and investigates their magnetic and magnetocaloric effects in depth.The main findings are as follows:Firstly,TbMn₂Ge₂ single crystal samples were prepared using the flux method.The prepared samples were of high purity and had a tetragonal Th Cr₂Si₂ structure.The results of the magnetic study show that the material has a Curie temperature of95 K and the crystals exhibit significant magnetic anisotropy.Above the Curie temperature T_C,a magnetic field-induced variable magnetic phase transition is found,which originates from the spin flip of the Mn magnetic moment.In addition,the variable magnetic phase transition is accompanied by a pronounced magnetostriction effect,indicating the presence of spin-lattice coupling in the sample.TbMn₂Ge₂single crystals are first-order magnetic phase transition materials,and the maximum value of the magnetic entropy change at the transition temperature T_C is 24.02J/kg·K under an external field of 7 T.This result is found to be the largest magnetic entropy change ever reported for the RMn₂Ge₂ family by comparison with materials of the same series.The corresponding relative cooling power and average temperature entropy change results for the material are 378.4 J/kg and 21.39 J/kg·K.In addition,the sample exhibits a plateau-like magnetocaloric effect in the temperature range 93～108 K for a magnetic field of 7 T.The above results indicate that TbMn₂Ge₂ single crystals have applications in the field of magnetic refrigeration and are potential magnetic refrigeration materials.Secondly,The Mn₃Si₂Se₆ single crystals were prepared by chemical vapor phase transport method.According to the X-ray diffraction results,the samples belong to the p3 1c space group and the prepared samples are pure phases.The samples exhibit ferromagnetism at temperatures below 69 K and antiferromagnetic interactions in the paramagnetic phase at temperatures above 69 K.The sample has a clear magnetic anisotropic behavior with the easy magnetization axis located in the ab-plane of the sample.The Mn₃Si₂Se₆ single crystal is a second order magnetic phase change material and the maximum value of the magnetic entropy change obtained for the sample at the Curie temperature T_C is 3.66 J/kg·K when the external field is parallel to the ab-plane of the sample at 7 T,while the maximum value of the magnetic entropy change obtained is 2.39 J/kg·K when the external field is parallel to the c-axis of the sample,indicating that there is magnetic anisotropy in the magnetocaloric effect of the material.The critical behavior of the samples was analyzed using MAP theory and the Kouvel-Fisher method,and the critical indices obtained by the two methods were very close.The critical indices obtained by the Kouvel-Fisher method areβ=0.4934,γ=1.098 andδ=3.2254,from which the magnetic exchange coefficient J（r）≈r^-4.641 is obtained,indicating the presence of both long-range mean-field effects and short-range 3D Heisenberg effects within the sample.