Study On Structure,magnetism And Magnetocaloric Effect Of Mn-based Alloys

Energy efficiency and sustainability have become the focus of attention and research in today’s world.Under this background,a refrigeration technology,namely magnetic refrigeration,based on the magnetocaloric effect of new magnetic materials.As an environmentally friendly,non-polluting and efficient refrigerant,it has been widely used in many fields.But there are still many difficult problems to overcome in its research,which has become the driving force of our research.As a kind of ferromagnetic material,Mn₅Ge₃ has remarkable saturation magnetization,favorable magnetic ordering temperature and the advantages of rare earth-free materials,which make it a research hotspot.At present,the research on it is generally focused on improving its refrigeration performance by substituting atoms and doping interstitial atoms.In order to improve its refrigeration efficiency,the performance of Ag and Ti doped at Ge site is investigated in this paper.The first and second chapters of this paper mainly introduce the related principles of magnetic refrigeration,magnetic refrigeration,and some basic theories of magnetic refrigeration.At the same time,some main experimental instruments needed for sample preparation,storage and testing,as well as the use methods and precautions of each experimental instrument are also introduced.Chapter three deals with some properties of Mn₅Ge₃ alloy itself.It is a hexagonal D8₈ structure（space group p6₃/mcm）,in which Mn atoms occupy two sites:4d（Mn1）and 6g（Mn2）,with magnetic moments of 1.96μ_B and3.23μ_B,respectively,and Curie temperatures ranging from 296 to 304 K.Then the Curie temperature calculated by MT curves is 299.8 K.The isothermal magnetization curves of the samples show that the saturation magnetization of the materials changes obviously near the phase transition temperature.Subsequently,the Arrott curves are obtained by using the measured isothermal magnetization curves.The positive slopes of the Arrott curves indicate that the materials undergo the second-order phase transition.And the refrigeration capacity of samples is much higher than that of many alloys,which indicates that Mn₅Ge₃ can be an ideal room temperature magnetic refrigeration material.Finally,the relationship between the maximum magnetic entropy and magnetic field shows that it is strictly linear with（μ₀H）^0.633.In the fourth chapter,the properties of Mn₅Ge₃ alloys doped with Ag were investigated.It was found that the Curie temperature of the material did not change significantly,which still remained near room temperature.At the same time,compared with element doping proposed in other literatures,under the same conditions,the change of magnetic entropy obtained by us is the largest.The phase transition analysis of the materials shows that our samples undergo the second-order phase transition,which provides a basis for its wide application.The critical behavior of the materials was also studied.It was pointed out that when the Ag content was 0.5,the sample conforms to the 3D Heisenberg model.In the fifth chapter,the doping of element Ti is studied.The analysis of phase transition and critical exponents are basically the same as that of Chapter four.Simultaneously,the magnetocaloric effect is slightly lower than that of Ag,but it shows some other interesting phenomena.It was found that the diffraction peaks of all samples shifted to the left relative to Mn₅Ge₃,and the intensity of the peaks decreased.The MT curves of the samples show that Curie temperature decreases obviously with the increase of doping content.Meanwhile,the materials undergo two phase transitions when the doping content is 1 and 1.5.In Chapter six,a one-step hydrothermal method in different high dc magnetic fields was used to prepare the Fe₃O₄ nanoparticles,which can greatly optimize its structure and magnetic properties.Under the magnetic field,the average particle size decreased from 72.9 to 41.6 nm,meanwhile,the particle crystallinity is also greatly improved.The magnetic field enhances its saturation magnetization and coercivity which is benefit for the magnetic recording material.The high magnetic field induce another magnetic structure whose magnetic moment increased.At room temperature,these nanoparticles exhibit superparamagnetism whose critical size(D_sp)is about 26 nm.An obvious Verwey transition is observed in the vicinity of 120 K of Fe₃O₄nanoparticles,where the magnetic easy axis switches from the<111>to<100>direction.The effective magnetic anisotropy decreases with the increase of the test temperature,which can be attributed to the value of H_c decreasing with the increase of the test temperature.