Study On Magnetic And Electrical Properties Of Ni-Mn Based Heusler Alloys Bulks And Films

Ni-Mn based Heusler ferromagnetic shape memory alloys have received attention due to their magnetocaloric effect,magnetoresistance,magnetic shape memory effect and exchange bias properties.A series of Mn_46.5Ni₄₃Sn_10.5-xAl_x（x=0.5,1.5,2.5,at%）alloys were prepared by arc-melting under high-purity argon（99.98%）atmosphere.The effect of Al substitution for Sn on on the martensitic transformation,magnetic properties and magnetocaloric effect had been researched,and the reasons of influence were discussed preliminarily.The epitaxial Ni-Mn-Ga thin films grown on Mg O（001）substrate were prepared by DC magnetron sputtering.The microstructure,martensitic transformation,magnetic properties magnetoresistance and anomalous Hall effect were researched and discussioned.A useful attempt was made to further reveal the anomalous Hall effect in Ni-Mn-Ga films.The details are as follows:（1）As the increasing content of Al,the martensitic transformation temperatures of Mn_46.5Ni₄₃Sn_10.5-xAl_x（x=0.5,1.5,2.5）alloys increase rapidly,while Curie temperature shows a small decrease.Meanwhile,With the increase of Al content the（35）M decreases obviously.The martensitic transformation temperaturs were affected by different magnetic fields.As the magnetic field increases,the martensite transformation temperatures move toward a lower temperature.（2）By calculating the different Al content of the alloys of the magnetic entropy change found for x=0.5,around the MT temperature,the maximum values of magnetic entropy change are 11.4,24.5 and 37.7 J/kg K under the field changing of 10 k Oe,20 k Oe and 30 k Oe,respectively.Meanwhile,the values of effective refrigerant capacity were calculated.Mn_46.5Ni₄₃Sn₁₀Al_0.5alloy has the biggest value of effective refrigerant capacity,and the value is 53.6 J/kg during the MT temperature under the field changing of 30 k Oe.The effective refrigerant capacity is not only related to the magnetic entropy change,but also related to the hysteresis loss and the phase change temperature region.Al substitution for Sn can effectively broaden the working temperature range of Mn_46.5Ni₄₃Sn_10.5-xAl_x alloy.（3）The thin films of Ni_46.7Mn_31.7Ga_21.6 and Ni_47.8Mn_30.8Ga_21.4（at%）of the microstructures showed the austenite phase and 7M martensite plates.The film has a martensitic transformation during the heating and cooling,and the austenite phase undergoes a Curie transition from paramagnetic to ferromagnetic.Meanwhile,the Ni_47.8Mn_30.8Ga_21.4 film has lower coercivity than the Ni_46.7Mn_31.7Ga_21.6 film.The lower coercivity helps to reduce the hysteresis loss during the phase transition cycle.（4）For Ni_46.7Mn_31.7Ga_21.6 and Ni_47.8Mn_30.8Ga_21.4films the negative values of MR were about-0.6%and-0.8%during the martensitic transformation under 30k Oe,respectively.The MR near the Curie point of the magnetic transition is about-2.0%and-2.9%,respectively.And The origin of negative MR is mainly due to the reduction of spin-dependent scattering.（5）The anomalous Hall effect measurement of the Ni_47.8Mn_30.8Ga_21.4 film shows the AHE is derived from the interaction of spin orbits,the Hall resistivity measured in our sample is primarily determined by AHE.And the AHE only due to skew scattering.It provides an experimental and theoretical basis for further revealing the AHE in Ni-Mn-Ga films.