The Effects Of B And C-doping On The Crystal Structure And Magnetic Properties Of Heusler Alloys

This thesis mainly studied the influences of B-doping or C-doping on the electronic structure,phase transition and magnetic properties of Heusler alloys by first-principles calculation and experiment.Increasing B content of Mn₅₀Ni₄₀In_10-x0-x B_x?x=0-4?alloys leads to a contraction of cell volume and obvious increase of the martensitic transformation temperature.The doping of B also strengthens the antiferromagnetic coupling in martensitic Mn₅₀Ni₄₀In_10-xB_x and reduces the Curie temperature of the austenite.First-principles calculations on Mn₂Ni_1.5In_0.5and Mn₂Ni_1.5In_0.25B_0.25.25 have been performed.The extra Ni atoms substituting for In prefer entering Mn?A?site and push Mn to the In?D?site.The substitution of B for In can increase the energy difference?E_M between the austenite and martensite,which leads to the increase of phase transition temperature.The calculated results agree well with the experimental results.As the B content increases,the Curie temperature of Fe₂MnSi_1-xB_x?x=0-0.4?alloys increases and the spin reorientation temperature T_R decreases and disappears eventually.As the B content increases,the saturation magnetization approaches the Slater-Pauling curve.The saturation magnetization decreases when x is higher than 0.4,which due to the appearance of the second phase in the alloy.Fe₂Mn_1-x-x Cr_xSi?x=0.125,0.375,0.5,0.625,0.75,0.875?alloys were prepared by melt-spinning method and investigated.With increasing Cr content,the saturation magnetization of the alloys approaches the Slater-Pauling curve and the Curie temperature increases gradually.Increasing B content of Fe₂MnGaB_x?x=0,0.02,0.04,0.06?alloys leads to an expansion of cell volume,so the B atom locates in the interstitial position of the face-centered cubic lattice.There is a FM-AFM transition at²80 K for Fe₂MnGa.As the B content increases,this transition temperature decreases gradually.The martensitic transformation temperature gradually decreases with increasing C content of Mn_0.76Fe_0.24NiGeC_x?x=0-0.06,0.1?alloys.And the martensitic transformation disappears when x is higher than 0.05.As the C content increases,the saturation magnetization decreases and finally the alloys become antiferromagnetic at 5 K.In addition,we synthesized Mn_1-x-x V_xNiGe?x=0,0.1,0.15,0.2,0.25,0.3,0.4?alloys and investigated their structure,phase transition and magnetic properties.It is found that the phase transition temperature of Mn_1-xV_xNiGe alloys increases with increasing V content x.The doping of V also decreases the Curie temperature of martensite phase.