Magnetic Properties And Magnetocaloric Effect Of The Cr-based Spinel Sulfides ACr₂S₄

Transition-metal chalcogenide spinels are typical strongly correlated electron materials.On the one hand,since the strong coupling among spin,charge,orbital and lattice degrees off reedom,this system displays rich physical properties such as colossal magnet or esistance?CMR?,colossal magneto capacitance?CMC?,gigantic Kerr rotation,charge ordering,spin-dimerization and magnetoelectric effect etc..On the other hand,the existence off frustration?geometric or bond frustration?increases complexity of the system,yields complex behaviors such as spin-ice,spin-orbital liquid,orbital glasses and so on.Depending on external perturbation such as magnetic field,electric field,chemical substitution,disorder and soon,different quantum states are subtle balanced,increasing difficulty for understanding the intrinsic physics of this system.In this thesis,we have selectedCo_1-xCu_xCr₂S₄ as object,and studied their physical properties such as magnetocapacitance,magnetic frustration,spin-phonon coupling and soon.The main contents are summarized as follows:1)With the first-principles method based on density functional theory?DFT?,the basic properties of CuCr₂S₄ are studied,such as lattice structure,charge density,density of electronic states,band structure,effective atomic charge,and mechanical properties.Based on first-principles calculation program Vienna ab initio simulation package?VASP?,we get that the lattice constants are 9.856?,which is close to the experimental data.With the eletronic density of states analysis of the materials,we found that the magnetic moment of Cr induces spin polarizations in Cu 3d electrons and S s and p electrons in Cr atoms,which show the reverse magnetic moment.2)Co_1-xCu_xCr₂S₄?x=0,0.2,0.4,0.6,0.8?compounds were synthesized by solid state reaction.The powder x-ray diffraction?XRD?patterns of Co_1-xCu_xCr₂S₄ were recorded on a Bruker D8 x-ray diffractometer equipped with a Cu-Ka radiation source operated at 40 kV and 40 mA.These XRD data were analyzed using Rietveld refinement method by GSAS-EXPGUI soft-ware package.The reasonable refinement results verify that the pattern a pure spinel phase.From the refined lattice parameters a,b,c and the calculated unit-cell volume found that the lattice constants a,b,c and the unit cell volume V decrease linearly with an increase of x in accordance with the Vegard's law,indicating that our samples are good solid solutions.3)The magnetization of Co_1-xCu_xCr₂S₄?x=0–0.8?as a function of the external magnetic field was taken at 5 K to determine the spontaneous magnetization.Research has shown that the spontaneous magnetization also increases linearly from 1.9 ?B/f.u.for x=0 to 4.1 ?B /f.u.for x=0.8,which is approximately in agreement with the previous works.Therefore,one can infer that the moments of the Co and Cr atoms are antiparallel align-ment with the values of 2.750 ?B and 2.325 ?B,respectively,with Cu atom moments are too small will not much affect the system,we suppose that non-magnetic of Cu atom.4)The Curie temperatures TC defined as the maximum slope in the M–T curves,obtained by analyzing the first order derivative of the M-T curve.It is worth to note that TC of 303 K for x=0.6,quite near to RT,implies that it might be a potential candidate used as a RT magnetic refrigeration material.From the isothermal magnetization curves of the selected Co_1-xCu_xCr₂S₄?x=0,0.6,0.8?were recorded at temperatures near their TC's with applied magnetic field up to 5.0 T,The accuracy of ?Sm calculated using thermodynamical theory,The maximum ?S values,close to their corresponding Tc,are 3.34,2.57 and 2.04 J/kg·K for x=0,0.6,0.8,respectively.The ?Sm exhibits ?-shaped peak around the Curie temperature,in accord with the nature of a second order phase transition.