Guangheng Wu

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.

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets

The magnetostructural coupling between the structural and the magnetic transition has a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here we demonstrate the feasibility of obtaining a stable magnetostructural coupling over a broad temperature window from 350 to 70 K, in combination with tunable magnetoresponsive effects, in MnNiGe:Fe alloys. The alloy exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

Martensitic transformation and shape memory effect in ferromagnetic Heusler alloy Ni2FeGa

We have synthesized ferromagnetic Heusler alloy Ni2FeGa using the melt-spinning technique. The Ni2FeGa ribbon, having a high chemical ordering L21 structure, exhibits a thermoelastic martensitic transformation from cubic to orthorhombic structure at 142 K and a premartensitic transformation. The alloy has a relatively high Curie temperature of 430 K, a magnetization of 73 Am2/kg, and a low saturated field of 0.6 T. The textured samples with preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3% upon the thermoelastic martensitic transformation.

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.

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31 °C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.

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.

Martensitic transformation and shape memory effect in a ferromagnetic shape memory alloy: Mn2NiGa

Heusler alloy Mn2NiGa has been developed by synthesizing a series of ferromagnetic shape memory alloys Mn25+xNi50−xGa25 (x=0–25). Mn2NiGa exhibits a martensitic transformation around room temperature with a large thermal hysteresis up to 50 K and a lattice distortion as large as 21.3% and has a quite high Curie temperature of 588 K. The martensite shows a high-saturated field up to 2 T. The excellent two-way shape memory behavior with a strain of 1.7% was observed in the single crystal Mn2NiGa. The magnetic-field-controlled effect created a total strain up to 4.0% and changed the sign of the shape deformation effectively.

Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn1-xCoGe alloys

It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0≤x≤0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field–induced martensitic transformation and a giant magnetocaloric effect in the Mn1−xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.

Magnetic entropy change in Fe-based compound LaFe10.6Si2.4

A large magnetic entropy change has been observed in an intermetallic compound LaFe10.6Si2.4. The maximum −ΔSM≈3.2 J/kg K was found at its Curie temperature, ∼242 K, upon a 2 T magnetic field change. Although the entropy change is slightly smaller than that of pure Gd metal, such Fe-rich compounds still appear to be very attractive candidates since (a) the raw materials are much cheaper than pure Gd metal; (b) the Curie temperature can be easily shifted by tuning the composition; (c) the materials are much more chemically stable than pure Gd metal.

Magnetic properties and martensitic transformation in quaternary heusler alloy of NiMnFeGa

Quaternary Heusler alloy Ni2(Mn,Fe)Ga has been studied systematically for the structure, martensitic transformation, and magnetic properties in two systems of Ni50.5Mn25−xFexGa24.5 and Ni50.4Mn28−xFexGa21.6. Substituting Fe for Mn up to about 70%, the pure L21 phase and the thermoelastic martensitic transformation still can be observed in these quaternary systems. Iron doping dropped the martensitic transformation temperature from 220 to 140 K, increased the Curie temperature from 351 to 429 K, and broadened the thermal hysteresis from about 7 to 18 K. Magnetic analysis revealed that Fe atoms contribute to the net magnetization of the material with a moment lower than that of Mn. The temperature dependence of magnetic-field-induced strains has been improved by this doping method.