| Mn2-based Heusler ferromagnetic shape memory alloys have attracted wide attention from materials scientists and researchers in related fields,due to its unique physical properties,such as giant magnetoresistance,large magnetic entropy and large exchange bias field etc.In this paper,the Mn2-based Heusler alloy and Co-V-Ga alloy are used as research objects.The former uses experiments to study its phase transition,magnetic properties,magnetocaloric effects,and exchange bias effects,while the latter used the first-principles calculations to study the structure and phase transition mechanism of magnetic structures,and obtains the following research results:Mn50-xCrxNi42Sn8(x=0,0.4,0.6,0.8)polycrystalline samples have been synthesized by arc melting and have been studied.It is found that all the alloys have non-modulated tetragonal martensite structure at room temperature.The transformation temperature decreases with the increase of Cr content.The M-T curves under the effect of 20 kOe show that the magnetic properties of these alloys are weak.The maximum magnetization difference between martensite and austenite phases is ?M=7.61 emu/g.The change of magnetic properties is mainly related to the change of Mn-Mn distance and the hybridization strength between Ni(A)-Mn(D).At low temperatures,the magnetization of the martensite phase increases with the increase of Cr content.The exchange bias field of up to 2624 Oe was observed in Mn50Ni42Sn8 alloy after cooling from room temperature to 5 K in 500 Oe magnetic field.Along with the increase of Cr content,the exchange bias field decreases gradually.When the content of Cr is x=0.8,the exchange bias field increases first and then followed by a decrease with the increase of the cooling field.The exchange bias field is the largest when the cooling field is 500 Oe.This is mainly attributed to the change of the interface exchange coupling between the spin glass state and antiferromagnetic region.Two sets of Mn50-xCrxNi40In10(x=0,1,2)polycrystalline samples were prepared by arc melting method.The phase transformation,magnetic properties and magnetic entropy changes of the samples were studied by changing the cooling rate of heat treatment and the substitution of Cr.It is shown that the martensitic transformation temperature of the slow-cooling alloy is higher than that of the quenched sample.This is mainly related to the atomic order degree of the alloy.As the Cr content increases,the magnetization of austenite gradually decreases,while the magnetization of martensite gradually increases.Compared with the quenched sample,the magnetization of the slow-cooled sample for both austenite and martensite states are enhanced.It is due to the atomic order is improved in slow-cooled sample,which leads to the ferromagnetic exchange become strong.In addition,when a 3 T magnetic field was applied,a large entropy change was observed in the Mn49Cr1Ni40In10 alloy,which is about 13 Jkg-1K-1.First-principles calculations have been performed on Heusler alloy Co2V1.5Ga0.5 to reveal the mechanism for magnetostructural transition.It has been found that the alloy prefers to crystallize into ferromagnetic L21 type structure at austenite.A potential of tetragonal distortion from ferromagnetic cubic structure to non-magnetic tetragonal structure has been predicted from the view of energetically favorable state.The stability of tetragonal Co2V1.5Ga0.5 is further confirmed by the phonon spectrum.The peaks of dx2-y2 and dz2 states for Co and V(D)3d states near the Fermi level for the cubic structure split at the tetragonal structure,implying the structural transition is mainly attributed to the band Jahn-Teller effect.The hybridization between the Co 3d states and the 3d states of V at D site plays an important role in the martensitic transformation.A volume contraction of 1.3%is obtained accompanying with the magnetostructural transition. |
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