| Room-temperature magnetic refrigerant materials exhibiting a first-order magnetic phase transition(FOMT)have recently received a large interest due to their potential applications.But the researches mainly focus on characterizing the properties and structures,the understanding of the origin of the FOMT in these materials is still limited.Especially the mechanism of interplay between magnetic,electronic and structural degrees of freedom during the FOMT needs more insight and is significant for further development of the room-temperature magnetic refrigerant materials.This thesis has took a systemic study on the composition,structures,phase transition process,and thermal properties of Fe2P-based compounds,revealed the correlations between them.The mechanism of FOMT was deeply disgussed in terms of the interplays between magnetic/structural/electronic degrees of freedom.The main results are listed as follows:Neutron powder diffraction,differential scanning calorimetry,and magnetization measurements have been used to determine the effects of the Mn content and crystallographic location,and of Ge content on the crystal structure,magnetic moment,magnetic entropy change,hysteresis,and saturation field of the first-order magnetic transition(FOMT)from paramagnetic(PM)to ferromagnetic(FM)states in the MnFePGe system.The results indicate that the Mn atoms located on the 3f site enhance the desirable properties.A correlation between applied magnetic field to complete the transition and the temperature range of the PM and FM states coexistence was established.On the basis of these results a pathway to understand the nature and to optimize the MCE(magnetocaloric effect)properties of this system for magnetic refrigeration applications have been identified.Magnetization measurements and X-ray diffraction have been used to determine the magnetocaloric and structural properties of Fe2P.The mechanism of FOMT for Fe2P has been studied in aspect of electron density distribution by using maximum entropy method and first principle calculations.The results indicate that Fe2P has a small entropy change with a weaker FOMT.The magnetism is coupled with the lattice parameter a,the enhancement of which during the FOMT leads to the abnormal change of lattice parameter a.It is found that there are two turning temperature points of magnetism when access to Tc in the PM regime,from which the magnetism of Fe1(3f)and Fe2(3g)atoms starts to generate,respectively.The temperature that the magnetism generation of Fe1(3f)atom start with is higher than that of Fe2(3g)atom,but only when the magnetism of Fe2(3g)atom start to generate the PM start to transform into FM.Besides,the reasons for the moment of Fe2(3g)atom larger than Fe1(3g)atom and the abnormal magnetic susceptibility in the PM regime are interpreted in terms of electron density.The first principle calculations and measurements of neutron and X-ray diffractions show that the Mn and Fe atoms preferentially occupy the 3g and 3f site,respectively,while the Ge atoms should only occupy the 2c site.The influence of atomic site occupations on the magnetocaloric properties are also discussed.It shows that enhancing the magnetism of atoms at 3f site contribute to the decrease of hysteresis,while enhancing the magnetism of atoms at 3g site gives rise to the increase of hysteresis.The results of neutron diffraction show that decreasing the occupancy of Mn atoms at 3f site can change the magnetic moment orientation of atoms at 3f site.And Ge entering the 2c site changes the electronic structures and enhances the Fe and Mn 3d exchange splitting across the Fermi level,consequently leading to a linear increase in the transition temperature with increasing Ge content.Scanning electron microscope and energy-dispersive spectroscopy reveal the inhomogeneous distribution of Ge in grains,which makes the grains with larger Ge content transform from PM to FM phase first when cooling and thus causes the phase coexistence.Diminishing the variances in covalent bonding strengths across the FOMT gives rise to an exponential decay in the heat hysteresis when increasing the Ge occupancy at the 2c site.Based on the atomic site occupation,X-ray diffraction patterns for MnFePo.74Geo.26 compound are refined by using the Rietveld method to determine its structural parameters,and its mechanism of FOMT has also been discussed in aspect of electron density distribution by using maximum entropy method and first principle calculations.The results indicate that the nature of FOMT is much stronger than Fe2P,and consequently a larger entropy change.The lattice parameter a is also coupled with the magnetism in MnFePo.74Geo.26 compound,and the abnormal change of lattice parameter a shows that there exist three turning temperature points for the change of magnetism above Tc.Two turning points are the temperatures that the magnetism generations of Fe(3f)and Mn(3g)atoms start with.As in Fe2P,the temperature that the magnetism generation of Fe(3f)atom start with is higher than that of Mn(3g)atom.But after the beginning of magnetism generation for Mn(3g)atom,the FOMT from PM to FM does not take place instantly but until to the magnetism of Fe(3f)and Mn(3g)atom enhances enough.The effects of Mn content on the structures of Fe-rich and Mn-rich compounds MnFePGe in FM states have been discussed,and the origin of the magnetoelastic effect has also been investigated by using maximum entropy method and first principle calculations.It is found that the Mn atoms on the 3g site and 3f site have completely different effects on the structures of FM state.The change of magnetism gives rise to opposite change trends for the lattice parameters a and c,reflecting the magnetoelastic effect resulted from the variation of Mn content.This magnetoelastic effect is caused by the distribution of magnetic elctrons for 3g site in the ab plane and the total electron-density distributions for 3g and 3f site along the c axis.The changes in electrons across the phase transtion has been studied by using the quantum theory and verified by the experimental results.The results show that the total charge is conserved during the phase transition,and the redistribution of electrons in the PM-FM phase transition of Fe2P based materials is the result of creation and annihilation of electrons. |
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