Magnetocaloric Effect And Critical Behavior Of Composition Modulated Weyl Semimetal Co₃Sn₂S₂

In recent years,Kagome?crystals have become popular research materials in the fields of basic science,and technical application deserves further exploration.The special frustrated structure and interesting energy band characteristics make the Kagome?lattice produce many novel physical properties.The Weyl semimetal Co₃Sn₂S₂ is a typical system in the Kagome?crystal,which has many unique properties,such as anomalous Hall effect,giant magneto-thermal effect,etc.For magnetic properties,its paramagnetic-ferromagnetic transition temperature is about T_c=175 K.However,the nature of the magnetic ground state in the temperature range below T_c has been controversial in recent years.The work published in Nature Communications in 2020,the one observed the coexistence of ferromagnetic state and antiferromagnetic state（phase separation state）by muon spin rotation method.Moreover,the volume ratio of antiferromagnetic state reaches 80%when it approaches T_c with the increase of temperature.However,it is observed that there are spin glass state and exchange bias phenomenon below T_c in another work.In this paper,the single crystals of Co₃Sn₂S₂ with Kagoméstructure were selected to study their crystal structure,elemental components,electrical/thermal transport properties,magnetic properties,magnetocaloric effects and critical behavior by compositional modulation of the Co and S sites.The influence of composition on the critical behavior is studied based on two reasons.On the one hand,the critical behavior of component regulation has not been reported for Co₃Sn₂S₂.On the other hand,the study of critical behavior is an effective methods to reveal the nature of magnetic interaction.The main contents of this paper are given as follows:1.The single crystal Co₃Sn₂S₂ was prepared by chemical vapor transport method.The magnetocaloric effect and critical behavior of Co₃Sn₂S₂ single crystals have been studied based on the results of crystal structure,magnetic and thermal transport.The main conclusions are as follows:From the results of temperature-dependent magnetization,it is found that the magnetization in the temperature region near T_c has a thermal hysteresis phenomenon under a small magnetic field,which can be attributed to the coexistence of ferromagnetism and antiferromagnetism or/and spin glass phase.The magnetic entropy change of the magnetic field H along the easy axis（c-axis）is much greater than that in the H//ab plane.The magnetic phase transition along the c-axis direction is a second-order phase transition,however,the magnetic phase transition in the ab-plane is not a second-order phase transition and there may be a first-order phase transition.The critical parameters（β=0.337,γ=1.178 andδ=4.594）in the H//c axis are not consistent with the mean field model,but closer to the 3D-Ising model.Based on the results of the exchange integral function J（r）∝^-4.744,it is considered that the long-range magnetic interaction and the short-range magnetic interaction coexist in the ferromagnetic phase,and the possible magnetic phase is also given.2.The single crystal of Co_2.5Fe_0.5Sn₂S₂ was prepared by flux method.It is found that Fe doping causes significant changes in magnetic properties,magnetocaloric effect and critical behavior compared with the results of undoped Co₃Sn₂S₂.The M（T）has a negative value at 77K for the H//c axis,which is believed to be due to the anti-parallel orientation of the magnetic moment of Fe to the c-axis,and one of the reasons for Fe doping inhibits ferromagnetism based on the first-principles calculation.Another reason for Fe doping inhibits the ferromagnetism is the doped element Fe has one less 3d electron than the substituted element Co.The magnetic transition in the c-axis direction is second-order magnetic transition,while the magnetic transition in the ab-plane is non-ideal second-order phase transformation.The critical exponent（β=0.473,γ=1.102 andδ=3.302）and exchange integral function(（）≈^-4.6469)of Co_2.5Fe_0.5Sn₂S₂ conform to the mean-field model,not the 3D-Ising model of Co₃Sn₂S₂.The above results mean that there are long-range ferromagnetic interactions in the ferromagnetic phase of Co_2.5Fe_0.5Sn₂S₂.3.Annealing the single crystal of Co₃Sn₂S₂ for 5 hours at 500℃in a mixed atmosphere of 10%H₂ and Ar argon to obtain a hydrogen-annealed single crystal of Co₃Sn₂S₂.The results show that the magnetization of H//c and H//ab is greater than that of Co₃Sn₂S₂,which is attributed to the enhancement of magnetic coupling between layers and within layers by H₂annealing.The magnetic transition along the c-axis is a second-order phase transition,but the magnetic transition in the ab-plane is not a second-order phase transition.The critical exponents（β=0.3207,γ=0.9320 andδ=3.9888）are between the 3D-Ising model and the mean field model,which is closer to the 3D-Ising model.The results of the critical behavior indicate that the anisotropic short-range magnetic interaction dominates in the ferromagnetic phase below T_c.The exchange integral function J（r）∝^-4.3703 is closer to the mean field model,which indicates the existence of long-range ferromagnetic interaction in the sample.The magnetic interaction of hydrogen annealed samples is different from the 3D-Ising model of Co₃Sn₂S₂ and the mean field model of Co_2.5Fe_0.5Sn₂S₂.4.The results of the spin glass states of Co₃Sn₂S₂ and Co_2.5Fe_0.5Sn₂S₂ single crystals were investigated as follows.Based on the variable field M（T）,AC susceptibility and remanent magnetization,the main contents are observed:The analysis results show that with the increase of applied magnetic field H,the freezing temperature T_wi shifts to low temperature in Co₃Sn₂S₂and Co_2.5Fe_0.5Sn₂S₂ single crystals,which represents the characteristics of spin glass.Both the real partχ’（T）and imaginary partχ’（T）of AC susceptibility show strong dependence on frequency,the peaks ofχ’（T）andχ’（T）shift to higher temperature with the increase of frequency,which is also a typical feature of spin glass.The decay rate of residual magnetization is very slow and basically presents a horizontal linear behavior with the increase of decay time,which is caused by the frustration state in the sample.The above analysis results confirm the existence of spin glass state in Co₃Sn₂S₂ and Co_2.5Fe_0.5Sn₂S₂ single crystal samples.