Study On Magnetic Phase Transition And Magnetocaloric Effect Of Mn-based Alloys

Magnetic refrigeration technology is a new refrigeration technology based on the magnetocaloric effect of magnetic materials.Compared with the traditional gas compression refrigeration technology,magnetic refrigeration technology has attracted wide attention due to its many advantages,such as environmental friendliness,high efficiency,stability and reliability.Exploring and developing new magnetic refrigeration materials with large magnetocaloric effect is one of the keys to achieve magnetic refrigeration technology.Mn-based alloys were cheap,easily to prepare and possessed considerable magnetocaloric effect,considering as a potential magnetic refrigeration material.Many Mn-based alloys also have high magnetocrystalline anisotropy.But few Mn-based materials having large magnetocaloric effect and high magnetocrystalline anisotropy simultaneously.Therefore,the Mn2Sb alloy was mainly studied in this thesis.The structure,magnetic properties and magnetocaloric effect of the alloys were systematically studied by means of substitution,doping and hydrostatic pressure.At the same time,the uniaxial magnetocrystalline anisotropy and coercivitv of the related materials were studied.The main work and research results are listed below:(1)Mn2-xVxSb(x=0.05,0.1,0.15)alloys were prepared by arc melting.Mn1.95V0.05SbLix(x=0.15,0.225)alloys were obtained by induction melting.All alloys possessed the tetragonal Cu2Sb-type structure,space group was P4/nmm.(2)The first-order magnetic phase transition from the anti ferromagnetic state to the ferrimagnetic state was achieved by the substitution of V,and the spin reorientation phase transition was observed.Increasing the doping amount of the Li element and removing the static pressure caused lattice expansion,the temperature of the transformation phase transition decreased,and the thermal hysteresis gradually increased.When the appropriate hydrostatic pressure was 0.36kbar,the maximum of magnetic entropy change of Mn1.95V0.05Sb alloy increased from 6.9 J/kg K to 8.2 J/kg K under the magnetic field change of 5T.The decreased in the slope of the critical field and temperature curve show a weak anti ferromagnetic coupling.The magnetic transition of the four-corner Mn1.95V0.05Sb alloy was sensitively dependent on the interatomic distance between the four Mn11 sites and electron density obtained around the Mn11 site.(3)Mn2Sb1-xSnx(x=0.1,0.2,0.3)alloys were prepared by arc melting.All alloys possessed the tetragonal Cu2Sb-type structure.The phase transition temperature increased with the increase of Sn substitution.And the lattice parameter a decreased while c increased.The first-order magnetic phase transition of antiferromagnetic state to ferrimagnetic state realized by the substitution of Sn and observed the spin reorientation phase transition.When a hydrostatic pressure of 2.5kbar was applied,the phase transition temperature decreased and the magnetic phase transition became slow.The maximum of magnetic entropy change of Mn2Sb1-xSnx(x=0.1,0.2,0.3)alloys under magnetic field change of 7T were 4.4 J/kg K,4.8 J/kg K and 2.8 J/kg K,respectively.When a hydrostatic pressure of 2.5kbar were applied to the alloys,the magnetic entropy of the Mn2Sb0,9Sn0.1 alloy and the Mn2Sb0.8Sn0.2 alloy increased to 5.2 J/kg K and 5.5 J/kg K,respectively.(4)Mn2Sb1-xSnx(x=0.1,0.2,0.3)powders and ribbons were prepared by conventional melt-spinning.The direction of easy magnetization was changed from ab basis plane to c axis before and after the spin reorientation phase transition.The decreased axis ratio c/a caused the increased magnetocrystalline anisotropy field with increasing temperature.And the coercivity was abnormally increased with increasing temperature.