S. Y. Yu

Large magnetoresistance in single-crystalline Ni50Mn50-xInx alloys (x=14-16) upon martensitic transformation

Variation of electrical resistance in single-crystalline Ni50Mn50−xInx alloys (x=14–16) upon martensitic transformation was investigated. In Ni50Mn35In15 with Tm∼295K, a negative magnetoresistance (MR) over 60% is attainable at moderate field strengths; in Ni50Mn34In16 with Tm∼190K, the MR can exceed 70% over a temperature of approximately 100K. 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.

Magnetic field-induced martensitic transformation and large magnetoresistance in NiCoMnSb alloys

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 8emu∕g for NiMnSb to ∼110emu∕g for Ni41Co9Mn39Sb11. In the same quaternary sample, a magnetization difference up to 80emu∕g was measured across the martensitic transformation, and the transformation temperature (T0=259K) could be lowered by 35K under a field of 10T. Also a magnetoresistance over 70% was observed through this field-induced transformation.

Realization of magnetic field-induced reversible martensitic transformation in NiCoMnGa alloys

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 14K∕T in Ni37Co13Mn32Ga18. Other mechanism details were also discussed.

Giant magnetothermal conductivity in the Ni-Mn-In ferromagnetic shape memory alloys

In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn single crystals. Upon cooling, a martensitic transformation is accompanied by a ferromagnetic metal→ferrimagnetic poor-metal transition. Most strikingly, this transition can be shifted to lower temperature and even totally suppressed by a magnetic field. The magnetic field-induced phase transition leads to a large magnetoresistance and a large magnetothermal conductivity up to 70% and 120%, respectively. The specific heat measurements indicate that the large magnetotransport properties are due to the increasing the density of free electrons, suggesting existence of superzone gap in the low-temperature, ferrimagnetic martensite.

Investigation on ferromagnetic shape memory alloys

Abstract Ferromagnetic shape memory alloys, exhibiting large recoverable strain and rapid frequency response, appear to be promising shape memory actuator material. These materials exhibit large shape memory effect associating with martensitic transformation, and magnetic-field-induced strain in the martensite state. The recent development in researches on NiMnGa, NiFeGa, and CoNiGa in our group is briefly reviewed. The perspectives of the ferromagnetic shape memory alloy are also described.