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With a high Curie temperature and low entropy change, the magnetic-field-induced martensitic transformation has been obtained in ferromagnetic shape memory alloys MnNiGa by doping a small amount of Co. Due to the ferromagnetic activation effect of Co, a large amount of antiferromagnetically aligned Mn moments are turned into ferromagnetic ordering, which is verified by our electronic structural calculation and experimental observation. Consequently, the magnetization rises up to 70 emu/g and the magnetization difference between two phases increases about ten times, resulting in a considerable dT/dH of 4 K/T and a well-defined reversed transformation induced by a magnetic field. [ABSTRACT FROM AUTHOR]

Effect of a magnetic field on martensitic transformation in the NiCoMnGa alloys was investigated. A field-induced reversible martensitic transformation from the martensitic phase of low magnetization to the parent phase of high magnetization has been realized. The substitution of Co for Ni atoms has turned the magnetic ordering of the parent phase from partially antiferromagnetic to ferromagnetic, resulting in a large magnetization change across the transformation, which dramatically enhances the magnetic field driving force. The transformation temperature can be downshifted by magnetic field at a rate up to 14 K/T in Ni37Co13Mn32Ga18. Other mechanism details were also discussed. [ABSTRACT FROM AUTHOR]

Magnetic field-induced martensitic transformation was realized in Ni50-xCoxMn39Sb11 alloys. The partial substitution of Co for Ni has turned the antiferromagnetically aligned Mn moments in the starting material Ni50Mn39Sb11 into a ferromagnetic ordering, raising the magnetization at room temperature from 8 emu/g for NiMnSb to ∼110 emu/g for Ni41Co9Mn39Sb11. In the same quaternary sample, a magnetization difference up to 80 emu/g was measured across the martensitic transformation, and the transformation temperature (T0=259 K) could be lowered by 35 K under a field of 10 T. Also a magnetoresistance over 70% was observed through this field-induced transformation. [ABSTRACT FROM AUTHOR]

Variation of electrical resistance in single-crystalline Ni50Mn50-xInx alloys (x=14–16) upon martensitic transformation was investigated. In Ni50Mn35In15 with Tm∼295 K, a negative magnetoresistance (MR) over 60% is attainable at moderate field strengths; in Ni50Mn34In16 with Tm∼190 K, the MR can exceed 70% over a temperature of approximately 100 K. The significant change in electric resistance upon martensitic transformation originates primarily from the altered electronic structure, while the large effect of a magnetic field follows its ability to manipulate the transformation in materials of low Tm and large ΔM/ΔS. The extremely large MR promises more innovative applications for these important alloys. [ABSTRACT FROM AUTHOR]