| NiMn based Heusler compounds are remarkable classes of Ni-Mn-Z(Z = In,Sn,Sb,…)typed intermetallic materials with 1:1:1 or 2:1:1 stoichiometry,which have attracted extensive attention of many researchers owing to their substantial optics,magnetic and electro transport properties like magneto-resistance effects,exchange bias effects,magnetocaloric effects,magnetoelastic effects and so on.Among all these alloys,researchers have devoted much efforts to investigate various magnetic field induced structural transitions in Ni-Mn-In alloys.However,lack of studies specific on the microscopic mechanisms of the metamagnetic transitions of the NiMn based Heusler alloys strongly limit the understanding of these alloys on the basic magnetism point of view,which even creats controversies over fundamental conceptions.Therefore,in order to deepen the understanding of the magnetization behaviors and reveal the mechanism of the metamagnetic transitions of these alloys,the magneto-structural transitions as well as the magnetocaloric effects have been intensively investigated by in situ imaging methods.The main contents are listed as following:1.The thermal and magnetic field-induced structural transitions as well as the time dependent isothermal growth of Ni50Mn34In15.5Al0.5 and Ni50Mn34In15 Al samples have been studied to investigate the mechanism of the reversibility of the transitions.By comparing the effects of Al doping among these samples,it is suggested that Al doping has positive effect on the nucleation and growth of the martensite in these samples,which has pinning effect during the heating process.2.The temperature and external magnetic field dependent magnetization behaviors of the Ni-Mn-In sample have been discussed.And the thermal and magnetic field induced structural transitions have also been introduced to discover the relations between sample temperature and reversibility of the field-induced transitions.3.In situ observation of the thermal and magnetic field induced transition of group II Ni-Mn-Ga sample have been carried out.As a result,seven-layered modulated martensite and non-modulated martensite have been observed during the phase transition of the austenitic parent phase in the cooling process,which lead to the discovery of the different magnetic response between two kind of martensite.4.The magnetocaloric effects of the above samples have been studied under various external magnetic fields.It suggests that the spin reorientation and the magnetic field induced transition have different contributions on the magnetocaloric effects,and Al doping affects the magnetocaloric effects as well.5.The magnetic field induced adiabatic temperature changes of the Ni-Mn-Ga sample have been studied,and the direct measurement of the adiabatic temperature changes under pulsed magnetic field have been explored.As a result,the intrinsic magnetic scattering and the spin scattering between twin-boundaries present negtive correlation with the magneto-resistance of the NiMn based alloys,while the pinning effects between twin-doundaries by Al-doping are the main mechanism over tuning the magnetic peoperties of the samples,which cause the shift of the characteristic temperatures towards high temperature region and enlarge the magnetocaloric effects of the samples as well.Moreover,Ni-Mn-In-Al alloys have inverse magnetocaloric effects,while Ni-Mn-In and Ni-Mn-Ga alloys have conventional magnetocaloric effects.Meanwhile,magnetic field-induced structural transitions play the dominating role in the magnetocaloric effects,while in Ni-Mn-In alloy,spin reorientation contributes the most part of the magnetocaloric effects. |
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