Magnetic Phase Transition, Entropy Change, And Permanent Magnetic Properties Of Several Magnetic Materials

In this thesis,the magnetic phase transition,entropy change and interaction mechanism of several non-collinear magnetic materials,as well as the preparation process and permanent magnetic properties of high abudunce rare earth Co-based nanoflakes powders and sintered magnets are investigated.Those substances,with the so-called non-collinear magnetic structures,which are assumed to be the sources of novel functional devices,are strongly influenced by many factors,such as composition,anisotropy,electric or magnetic field,temperature,and fabrication,etc.,besides a variety of magnetic properties presented.Here,the phase transition,entropy change,reversal magnetization processes,and interaction mechanism of Mn_32.5Ni_32.5Ga₃₅5 alloy,HoMn_6-xFe_xSn₆?x=0,0.3,0.5,0.6,0.7,0.8?and DyCoGe compounds,are studied in detail,under the modulation of external magnetic field,temperature,chemical compositions,and preparation conditions.Rare-earth elements are important and indispensable elements of magnetic materials.In our country,not less than 40%of rare-earth raw materials are used to prepare permanent magnetic materials.The most direct and fundamental solution to the problem of large amounts of high abundance rare earth elements such as La and Ce which are caused by the sharp increase in the consumption of rare earth permanent magnet materials of Nd and Pr is the development of La and Ce high abundances permanent magnetic materials.The two most critical problems of permanent magnetic materials of rare earth elements are the Curie temperature and the coercivity determined by the exchange and magnetocrystalline anisotropy,respectively.In this paper,CaCu₅ light rare-earth-cobalt-based compounds with high Curie temperature and strong uniaxial anisotropy are studied,and the magnetic powders and sintered magnets are prepared by surfactant-assisted ball milling and spark plasma sintering?SPS?.The microstructure and process parameters of the nano-sized magnetic powders were regulated,and the realization path and permanent magnetic mechanism of the newly practical high abundance rare-earth permanent magnetic materials were explored.The essentialresults are concluded as follows:The magnetic structure,magnetic phase transition,and magnetic entropy change of the Mn_32.5Ni_32.5Ga₃₅ alloy were studied systematically,especially the dependence of magnetocrystalline anisotropy on temperature and composition,and its influence on macroscopic domain structure.The Mn_32.5Ni_32.5Ga₃₅ alloy is Ni₂In type crystal structure,and the space group is P63/mmc;the transition temperature from paramagnetism to the helical magnetic structure is 350 K.Under the magnetic field,the topological magnetism of biskyrmion can be generated below 341 K.The maximum entropy change values under the 1 T and 2 T magnetic fields are 0.7J/?kg·K?and 1.4 J/?kg·K?,respectively.Because the crystal grains are dominated by the magnetostatic interaction,the magnetocrystalline anisotropy has an important influence on the macroscopic magnetic properties,especially the magnetic domain structure.With the decrease of temperature,the uniaxial magnetocrystalline anisotropy constant K_u1 of Mn_32.5Ni_32.5Ga₃₅ alloy increases,and the slope changes at 102 K and200 K are consistent with the spin reorientation characteristics of the thermomagnetic curve.The magnetic entropy change curve shows a distinct peak at these two temperatures.The first-order reversal curve?FORC?was used to analyze the reversible and irreversible components in the remagnetization process.It was found that the macroscopic geometry of the sample had a significant effect on the magnetization and magnetization reversal of Mn_32.5Ni_32.5Ga₃₅.When the thickness is 0.5 mm,the reversible and irreversible components coexist in the sample,but when the thickness is 0.08 mm,there is no irreversible component in the sample.The strongest peak splitting characteristic of the FORC plots found only in samples with a thickness of0.5 mm indicates that a certain magnetic domain structure was generated and quenched in the sample during the magnetic field change.It can be known that the presence of irreversible component in the Mn_32.5Ni_32.5Ga₃₅ sample is not fovor to the existence of biskyrmion.The type of magnetic structure of HoMn_6-xFe_xSn₆?x=0,0.3,0.5,0.6,0.7,0.8?materials stacked in the order of Mn-Ho-Mn-Mn-Ho-Mn layered depends on the Fe content.When x?0.5,the magnetic structure is mainly determined by the interaction between the Mn?Fe?-Mn?Fe?layers.When x?0.5,the interaction between the Ho-Mn?Fe?layers is mainly affected magnetic structure.When x=0.6,the helical-ferrimagnetic transition disappears,and as the temperature rises,the magnetic state directly changes from the helicalmagnetic to paramagnetic.When x is 0.7 and0.8,the HoMn_6-xFe_xSn₆?x=0.7,0.8?sample undergoes spin reorientation in the helical magnetic structure.When the external magnetic field is 2 T,the maximum entropy changes corresponding to x=0,0.3,0.5,0.6,0.7 and 0.8 samples are 0.6,0.1,0.3,0.2,0.5 and 0.2 J/?kg?K?,respectively.By using the hot-press sintering process of SPS,it is proved that the phase transition of the thermomagnetic curve of DyCo Ge at 160 K is due to the change of magnetic anisotropy during preparation.The sample obtained by the SPS sintering process has no distinct texture and is therefore more completely isotropic.Compared with the magnetocaloric effect of samples not sintered by SPS,the maximum magnetic entropy change of isotropic sample also decreases.When the magnetic field is 1 T and 2 T,the maximum magnetic entropy change of DyCoGe material prepared by SPS sintering process are 1.8 J/?kg?K?and 4.7 J/?kg?K?,respectively.The?La,Ce?Co₅ and MMCo₅ nanoflakes were prepared by a surfactant-assisted ball milling method.The thickness of the nanoflakes was asymmetrical Gaussian distribution.The number of particles below the average particle size was large,and the number of particles above the average particle size was small.The minimum of thicknesses were 45 nm and 20 nm,respectively;the coercive forces at room temperature were 437 kA/m and 469 kA/m,respectively,and the maximum remanence ratios were 0.75 and 0.71,respectively.Using this nanoflakes,?La,Ce?Co₅and MMCo₅ sintered magnets were prepared by SPS sintering process,in which the?La,Co?Co₅ sintered magnet had a coercive force of only 127 kA/m,and the remanence ratio was 0.27,while the coercive force of the MMCo₅ sintered magnet is less than 40 kA/m and the remanence ratio is 0.2.These results show that more practical exploration of high abundance rare-earth cobalt-based permanent magnetic materials can be made.