Magnetocaloric Effect And Magnetic Phase Transition Of Layered Compounds TbMn_2-xCo_xSi₂ And LaMn₂Ge₂

With the rapid development of information technology,electric power and electronic equipment,manufacturing and other fields,magnetic functional materials widely used in these fields are playing a more and more important role in our daily life,modern magnetic materials research and development has also made great progress.Among them are the discovery of magnetic refrigeration materials.Compared with common gas compression technology,magnetic refrigeration technology based on magnetic thermal effect has great potential and advantages in energy conservation and environmental protection,which has attracted extensive attention of researchers in recent years.The ternary intermetallic compounds of the RT₂X₂ series(R=rare earth,T=transition metal,and X=Si or Ge)which mainly crystallize in the Th Cr₂Si₂-type structure(space group I4/mmm)with a layered crystal structure,have been found to have large magnetothermal effects and small hysteresis losses,showing a broad prospect in magnetic cooling.Among them,the RMn₂X₂ compounds(X=Ge or Si)have been found both the magnitude of the Mn moments and the magnetic state of the Mn sublattice depend strongly on the distance between Mn-Mn atoms,and their structure and magnetic behavior can be controlled via substitution of R,Mn,and X atoms on the 2a,4d,and 4e sites respectively.In recent years,the application of magnetothermal effects of these RMn₂X₂ compounds in magnetic refrigeration has attracted extensive attention,but less insight has been gained on the relationship between magnetoelastic coupling and the magnetic entropy at magnetic transitions,these compounds have been systematically studied in this paper.This approach has enabled us to establish the relationship between magnetic state,physical properties and anisotropic magneto-elastic response.The major research contents and results obtained are as follows:(1)Polycrystalline samples with nominal compositions Tb Mn_2-xCo_xSi₂(x=0,0.1,0.2,0.3,and 0.4)were prepared by argon arc melting method and subsequent annealing process,we have carried out a detailed investigation on the magnetic structure and magnetic phase transition in Tb Mn_2-xCo_xSi₂ compounds.Rietveld refinements of the X-ray and neutron diffraction patterns confirm that the Tb Mn_2-xCo_xSi₂ samples have a body-centred tetragonal structure.The change of lattice constants a and c with Co concentration indicates the existence of anisotropic contraction.There are two magnetic phase transitions T_C1 and T_C2 in Tb Mn_2-xCo_xSi₂ samples at room temperature,the presence of strong anisotropic distortion of the unit cell as a result of magneto-elastic coupling around T_C1 and T_C2.Detailed neutron diffraction investigations have established the formation of a tripled magnetic unit cell structure at the temperature range between two magnetic transitions.The triple magnetic unit cell structure results in the different response of the two continuous magnetic phase transition temperatures to the applied magnetic field,the overlapping entropy curves near the two transition temperatures results in a plateau-like magnetocaloric effect and enhanced refrigerant capacity.(2)Structural and magnetic properties of the La Mn₂Ge₂ compounds in as-casted bulk and melt-spun ribbon form have been studied in detail by combining x-ray,neutron powder diffraction,magnetic measurement and heat capacity study,we found that in this system bulk samples have more advantages in refrigerant capacity than ribbon sample.With decreasing temperature,our neutron diffraction study demonstrates that magnetic state changes first from paramagnetic to incommensurate canted antiferromagnetism AFfs at T_N and then it gives way to incommensurate canted ferromagnetism Fmi below T_C.The critical exponent analysis indicates that the magnetic interactions in La Mn₂Ge₂ are long range.The isothermal magnetic entropy changes around the Curie temperatures suggest that these materials allow scope for use as magnetic refrigeration materials close to room temperature.