Study On The Multi-caloric Effect In Intermetallic Compounds

The magnetocaloric effect refers to the phenomenon that the magnetic material absorbs or releases heat under the change of the external magnetic field.Generally,for the material in which the spin and the lattice are coupled,the ferroic order parameter is likely to be strongly coupled to the lattice,so the change of the external field will not only cause an order change in its ferroelectric order parameters,but also change its lattice symmetry and thus brings huge caloric effects.The pressure field?such as uniaxial stress or hydrostatic pressure?can also drive the phase transition due to changes in unit cell volume during the phase transition.The multi-field drived caloric effect,that is,the multicaloric effect,is expected to solve many problems faced by solid-state refrigeration at present,such as narrow phase transition temperature range,low entropy value,and large hysteresis loss,which has become a hot research topic.Ni-Mn-In metamagnetic shape memory alloys exhibit various important functional properties during their martensitic transformation,such as inverse magnetocaloric effect,metamagnetic shape memory,magnetic superelasticity and giant magnetoresistance,whic has attracted an increasing attention from researchers.In this thesis,the metamagnetic Heusler alloy Ni-Mn-In and a hexagonal MM'X?M,M':transition group element;X:main group element?system with magnetic-structure characteristics are selected as the research object.The alloy was prepared by arc melting and high temperature quenching process.The magnetic properties were measured by magnetic measurement,thermal measurement,variable temperature XRD measurement and magnetic measurement under hydrostatic pressure.The magnetic properties and caloric effect of the martensitic transformation process were studied.The main results achieved are as follows:1.Theoretical studies have shown that the caloric effect of multiple fields driven multiferroic materials in the phase transition is not given by the sum of each monocaloric response,and the cross-coupling term?i.e.the coupled caloric effect?plays an important role.The research based on the Landau phase transition theory indicates that the enhanced thermal effect in multiple fields is due to the enhancement of the coupling of magnetic and structural degrees of freedom.Attempts have been made to obtain a means of enhancing thermal effects by analyzing the coupling heat,but few experimental studies have reported coupling caloric effect.In this paper,the multicaloric effect of the metamagnetic shape memory alloy Ni₅₀Mn₃₅In₁₅ in the magnetic field and hydrostatic pressure is studied quantitatively for the first time by magnetic measurement under pressure.Through theoretical analysis,it is found that the expression of the coupled-caloric effect is closely related to the magnetic volume coupling coefficient₁₂,and the magnetocaloric effect at a certain pressure is equivalent to the magnetocaloric effect at ambient pressure adjusted by the coupled caloric effect,so the coupling term will directly affect the magnetocaloric effect under pressure.It is shown that when a pressure of 9.995 GPa is applied,the peak value of entropy change can be as high as?35?S?25.7 J kg^-1K^-11 upon a magnetic field change of5-0 T,which increases by 8%compared to that of ambient pressure.This result is consistent with the calculated result of the coupled caloric effect.Under the continuous pressure regulation of 0-0.995 GPa,the magnetostructural phase transition temperature is continuously adjustable in the temperature range of 264 K-294 K.By further quantitative analysis of the magnetic volume coupling coefficient₁₂ and the coupledcaloric effect,we reveal that the pressure increases the magnetocaloric effect by enlarging the magnetostructural coupling strength in the material.This work shows that the quantitative analysis of the cross-coupling term of the two-field drive reveals the nature of the pressure-enhanced magnetocaloric effect and helps to design new materials based on magnetostructural coupling.2.At present,most of the research on magnetostructural coupling materials under hydrostatic pressure focuses on the regulation of pressure on phase transition temperature and magnetocaloric effect.The effects of pressure on the physical properties and the microstructure,especially the magnetic properties of martensite and austenite phases before and after phase transformation are rarely reported.However,the pressure changes the coupling effect by changing the local environment of the magnetic atoms in the two phases,which is the reason for the pressure-regulated magnetostructural phase transition temperature.In this paper,Ni₅₀Mn₃₅In₁₅ alloy was studied as the research object.The influence of pressure on microstructure and magnetization before and after the martensite transformation of Heusler alloy with Mn was systematically studied.The results show that at a low temperature of 5 K,the increase of pressure leads to a decrease in magnetization,which is as high as dlnM_M?5K?/dP=-53×10^-3GPa^-1.This phenomenon is attributed to the enhancement of the antiferromagnetic interaction between the Mn atom occupying the In site and the nearest neighbor Mn atom as the external pressure decreases the unit cell volume.As the temperature increases,the lattice thermal expansion begins to act in the opposite direction to the hydrostatic pressure,and the pressure induced decrease in magnetization to begin to decrease.When the temperature is raised to 200 K,the pressure-induced magnetization reduction is dlnM_M?200 K?/d P=-39×10^-3GPa^-1.However,when the alloy undergoes a magnetostructural phase transition to a ferromagnetic austenite phase,its magnetization increases with increasing pressure.This is due to the reason that the crystal symmetry changes after the phase martensite transformation,and ferromagnetic coupling occurs due to the super-exchange of Mn atoms through the 3d electrons of Ni atmos.And as the atomic distance decreases,the ferromagnetic exchange effect increases and thus the magnetization increases.In addition,the difference in the energy band structure of the Ni atom in the two phases is also one of the reasons why the magnetization varies with the pressure representation:For the high temperature cubic phase,the unpaired electrons in the Ni atom occupy the3d orbital isomorphism,however,for the low temperature tetragonal phase,due to the Jahn-Teller distortion,the 3d orbital state of Ni is split into two orbits of energy,and the unpaired electrons are redistributed in the lower energy orbit.Since the Mn atoms achieve super-exchange through the 3d electrons of Ni,the influence of pressure on the3d electron orbitals of Ni leads to a change in the magnetization of the alloy.In the phase transition temperature region,due to the increase of pressure,the antiferromagnetic effect in the low temperature phase is enhanced,so the contribution of magnetic to Gibbs free energy changes,causing its phase transition temperature to move to the high temperature region.This effect is consistent with the principle that when the In content is lowered?that is,the increase in Mn for the In content?causes the phase transition temperature to rise:The increase in the substitution of Mn for In content results in an increase in the antiferromagnetic interaction between the Mn atom occupying In and the nearest neighbor atom,resulting in a decrease in the Gibbs free energy of the magnetic phase portion,and thus an increase in the phase transition temperature.It can be seen from the above studies that the influence of pressure on the magnetic properties and microstructure of the two phases before and after the martensitic transformation of Ni-Mn-In alloy causes the phase transition temperature to move,thus revealing the occurrence of pressure-driven magnetostructural coupling materials.The study of the magnetic and microstructure of Heusler alloy by hydrostatic pressure helps us to understand its basic properties and understand the deep physical mechanism of pressure-regulating phase transition temperature.3.The use of Maxwell's relationship to calculate the entropy change in the first-order phase transition is widely used in the study of magnetocaloric effects.However,for the first-order phase transition system,due to the existence of hysteresis,when using the Maxwell relationship to calculate the entropy change in the phase transition process through the isothermal magnetization curves,it is necessary to be cautious,otherwise the calculation result will be far from the actual value.Taking Ni₅₀Mn₃₅In₁₅ as the research object,the isothermal magnetization curve of the phase change temperature region of the cooling process was measured by the traditional standard measurement mode?standard measurement method?and loop measurement method,and the entropy change was calculated by Maxwell relationship.The results show that the peaks of entropy change are 30 J kg^-1K^-1 and 25 J kg^-1K^-1 under the variation of 0-5 T magnetic field during the two methods in cooling and ascending magnetic field process.For the process of cooling and descending magnetic field,the results of the two methods are consistent under the change of 5 T-0 magnetic field,and the peak value of entropy is kept at 30 J kg^-1K^-1.It shows that for the system,the measurement results of the loop measurement method in the process of cooling and ascending magnetic field are slightly different from the standard measurement method,but the results of the two methods in the process of cooling and descending magnetic field keep consistent.The analysis shows that due to the hysteresis effect of the phase transition,the thermal history and magnetic history are the fundamental reasons for the standard measurement method and the loop measurement method to be different in the cooling and ascending magnetic field process,and the measurement results of the cooling and descending field processes are consistent.In addition,using the Clausius-Clapeyron equation to calculate the entropy to be 25.3 J kg^-1K^-1,when the magnetic field changes to 0-5 T during the cooling and ascending magnetic field process,which is consistent with the results of the loop measurement method of the same measurement process.Furthermore,the relationship between magnetic field cycle stability and superelasticity before and after material phase transformation was studied.This work has important guiding significance for the reasonable calculation and analysis of the magnetocaloric effect of the metamagnetic Heusler alloy,and it is also useful for other similar systems.4.The effect of hydrostatic pressure on the magneto-structural phase transition in MnCoGe_0.99In_0.01 alloy was investigated.First,a low magnetic field driven magnetostructural phase transition is achieved for the first time by substitute Ge with a minority of larger atomic radius of In.This process is related to the change in the atomic local environment of manganese after the replacement of Ge.For the MnCoGe_0.99In_0.01alloy studied in this thesis,the melting method,annealing time and quenching rate in the preparation process are all technical factors that cause the magnetic hysteresis of the sample.For MnCoGe_0.99In_0.01 alloy,the phase transition temperature is around 330K at ambient pressure,and the peak value of entropy can reach 17.9 J kg^-1K^-1 under the magnetic field change of 0-5 T.The hydrostatic pressure can drive transition temperature move toward the low temperature values.The phase transition temperature decreases to about 280 K under 0.24 GPa pressure,and the peak entropy change can still be maintained at 15.9 J kg^-1K^-1 under the change of 5 T magnetic field,which indicates that the magnetocaloric effect including room temperature can be achieved in the system by low pressure regulation.When the pressure reaches 0.53 GPa,the magnetic structure decoupling occurs,so the entropy change during the phase transition is sharply reduced to 10.4 J kg^-1K^-1.The pressure-driven phase transition temperature is continuously adjustable at room temperature,indicating that the cooling efficiency can be improved by multifield regulation.