Effect Of Ni/Fe Doping On Martensitic Transformation And Magnetic Properties For Co-V-Ga Heusler Alloy

Heusler alloys have attracted much attention due to their rich physical properties,such as half-metallicity,magnetocaloric effect（MCE）and ferromagnetic shape memory effect,etc.Recently,many Co-based Heusler alloys with martensitic transformation（MT）have been found,such as Co₂Cr（Ga,Si）,Co₂Cr（Al,Si）,Co-V-Si,Co-V-Ga and Co₂VGa_0.5Sb_0.5 alloys.Before 2017,though some of off-stoichiometric Co-V-Ga alloys have been found to present L2₁–D0₂₂MT,metamagnetic MT have never been found in this system,which can be ascribed to the fact that the Curie temperature（T_C）is much lower than the MT temperature.Until 2018,Liu et al.have prepared V-rich Co₅₀V₃₄Ga₁₆ polycrystalline alloy,and they observed metamagnetic MT by tuning the ratio of V and Ga to reduce the MT temperature effectively.In addition to adjusting the proportion of their constituent to control MT,it is also possible to substitute some elements to change the MT temperature.Therefore we have studied the effect of doping on the Co-V-Ga alloy,and the main contents are as follows:（1）The physical properties and research progress of Heusler alloy are reviewed,including the crystal structure,MT,and MCE of Heusler alloy.（2）The main experimental methods and principles related to this thesis are described,including the preparation of samples,the measurement of sample structure,phase transition and magnetism.（3）We investigated the crystalline structure,MT and magnetic properties of Co₂V_1-xNi_xGa（x=0.1,0.3,0.5,0.7）alloys.Results show that the alloys exhibit MT as the Ni content increases up to a certain value（x=0.3）.Based on the above results,we have synthesized Co₅₀V_25-xNi_xGa₂₅（x=5,6,7,8,9,12.5）alloys with partial substitution V for Ni in Ga₅₀V₂₅Ga₂₅,while the ratio of Co and Ga maintains invariant.It was found that the alloys exhibit MT as the Ni content increases up to a certain value（x=7）.However,the magnetic change before and after the phase transition is not pronounced.In order to tune the MT and magnetic transition in the same temperature range,we prepared the Co-V-Ni-Ga alloy with change of the content of Ga and V.Further experiments indicate that the magnetic structure transition with large magnetization change occurs in the polycrystalline Co₅₀V₁₇Ni₇Ga₂₆ specimen prepared by slightly changing the ratio of V and Ga with the constant proportion of Ni elements（x=7）.Co₅₀V₁₇Ni₇Ga₂₆ alloy with the further decrease of the e/a would result in the MT temperature and the T_C to be regulated in the same temperature range leading to the appearance of a magnetic structural transition with a large magnetic change.In addition,the MCE was obtained with a value of 11.5 J/kg·K around 94 K for the magnetic field changes of 7 T in the Co₅₀V₁₇Ni₇Ga₂₆ alloy.（4）We have synthesized Co₅₀V₂₅Ga_25-xFe_x（x=11,12,12.5,13,14）alloys with partial substitution Ga for Fe in Ga₅₀V₂₅Ga₂₅,while the ratio of Co and V maintains invariant.Then,we investigated the crystalline structure,MT and magnetic properties of Co₅₀V₂₅Ga_25-x Fex（x = 11,12,12.5,13,14）alloys.Results show that the Co₅₀V₂₅Ga_12.5Fe_12.5 alloy which has not been annealed exhibits MT from the ferromagnetic austenitic phase with high-magnetization to the ferromagnetic martensite phase with low-magnetization near room temperature.And the MCE was obtained with a value of 4 J/kg·K around 343 K for the magnetic field changes of 3 T in the Co₅₀V₂₅Ga_12.5Fe_12.5 alloy which has not been annealed.