Ab initio study of Co- and Cu-doped Ni-Mn-Ga alloys

abstract

en

Much attention has been paid to the Ni-Mn-Ga magnetic shape memory (MSM) alloy because it exhibits interesting properties such as a giant magnetic field-induced strain (MFIS). The MFIS occur below the martensitic transformation temperature, when a high-temperature cubic phase, austenite, transforms to a phase with lower symmetry, martensite. The martensitic transformation point can be adjusted by doping. The MFIS of 12% has recently been reported in the nonmodulated martensite with reduced c/a, achieved by simultaneous doping by 4% of Cu and Co. The influence of Co and Cu doping on Ni-Mn-Ga is investigated using the exact mufin-tin orbital method in combination with the coherent-potential approximation. Single element doping and simultaneous doping by both elements are studied in Ni50−xCoxMn25−yGa25−zCuy+z alloys. Doping with Co in the Ni sublattice decreases the c/a ratio of the nonmodulated martensite but it simultaneously increases the cubic phase stability. Doping with Cu in the Mn or in Ga sublattices does not change c/a ratio significantly and it decreases the cubic phase stability. For combined simultaneous doping by Co and Cu sublattices the effects of the individual dopants are independent and about the same as for the single element doping. The described effects can be understood on the basis of a density of states analysis, by studying the states that drive the Jahn-Teller instability that gives rise to the martensitic transformation.

Much attention has been paid to the Ni-Mn-Ga magnetic shape memory (MSM) alloy because it exhibits interesting properties such as a giant magnetic field-induced strain (MFIS). The MFIS occur below the martensitic transformation temperature, when a high-temperature cubic phase, austenite, transforms to a phase with lower symmetry, martensite. The martensitic transformation point can be adjusted by doping. The MFIS of 12% has recently been reported in the nonmodulated martensite with reduced c/a, achieved by simultaneous doping by 4% of Cu and Co. The influence of Co and Cu doping on Ni-Mn-Ga is investigated using the exact mufin-tin orbital method in combination with the coherent-potential approximation. Single element doping and simultaneous doping by both elements are studied in Ni50−xCoxMn25−yGa25−zCuy+z alloys. Doping with Co in the Ni sublattice decreases the c/a ratio of the nonmodulated martensite but it simultaneously increases the cubic phase stability. Doping with Cu in the Mn or in Ga sublattices does not change c/a ratio significantly and it decreases the cubic phase stability. For combined simultaneous doping by Co and Cu sublattices the effects of the individual dopants are independent and about the same as for the single element doping. The described effects can be understood on the basis of a density of states analysis, by studying the states that drive the Jahn-Teller instability that gives rise to the martensitic transformation.

shape memory alloys; Ni2MnGa; doping; ab initio

26.02.2014

ViCoM

Vienna

From Electrons to Phase Transitions

35–35

1