G. D. Liu

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.

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.

Magnetic field-controlled two-way shape memory in CoNiGa single crystals

A two-way magnetic field controlled shape memory effect has been observed in single crystals of CoNiGa with martensitic transformation temperature ranging from 205 to 341 K. Two-way shape memory with −2.3% strain has been obtained in free samples. By applying a bias field of up to 2 T, the shape memory strain can be continuously controlled from negative 2.3% to positive 2.2% giving it a total strain of 4.5%. The magnetic properties of CoNiGa show that it is a good shape memory material working at relatively high temperature of up to 450 K, and has a lower magnetic anisotropy than NiMnGa.

Giant magnetostriction on Fe85Ga15 stacked ribbon samples

Large magnetostrictions of −1300 and +1100 ppm related in the different directions have been obtained in our stacked Fe85Ga15 ribbon samples. In the case of non-180° domain magnetization in the high anisotropic samples, the magnetostrictions are mainly attributed to the existence of Ga clusters which preferentially orient with the ribbon normal due to the ribbon grain texturing. Forming the modified DO3 structure, the Ga–Ga atom pairs distribute in the matrix and cause the x-ray diffraction peak split in melt-spun ribbons. As a special micromorphology, Ga clusters highly condensed in some nanoscale dots have also been experimentally observed.

Large negative magnetoresistance in quaternary Heusler alloy Ni50Mn8Fe17Ga25 melt-spun ribbons

Quaternary Heusler alloy Ni50Mn8Fe17Ga25 ribbons have been prepared by the melt-spun method. The ribbons exhibit large negative magnetoresistance (MR){=[R(H)−R(0)]∕R(0)} over a wide temperature region, particularly in the region during the martensitic phase transformation. The MR decreases significantly after annealing. The large MR is isotropic and is mainly attributed to the local magnetic disorders, magnetic clusters, and heterogeneity. The maximum MR at martensitic transformation may be due to the redistribution of electrons and the increase of phase boundary scattering. This feature adds a useful functionality to the already interesting Heusler alloys.

Half-metallic ferromagnetism in hypothetical semi-Heusler alloys NiVM (M=P, As, Sb, S, Se, and Te)

A theoretical study based on first-principle band-structure calculations is carried out for the hypothetical semi-Heusler alloys NiVM (M=P, As, Sb, S, Se, and Te). For all compounds it is found that the ferromagnetic state is more favorable than the paramagnetic state. NiVAs is predicted to be a half-metallic ferromagnet with a small half-metallic gap of 0.07 eV and an integer magnetic moment of 2 μB, and NiVP and NiVSb are so-called nearly half-metallic ferromagnets. Furthermore, we find a clear indication that the substitution on the sp atoms influences the hybridization between Ni and V atoms, and can even destroy the half metallicity.

The structural change and superconductivity in MgCxNi3 (x=0.5–1.55) and MgxCyNi3 (x=0.75–1.55, y=0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg–C–Ni ternary system

The structural change and superconductivity in the nominal composition MgCxNi3 (x = 0.5-1.55) and MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) were investigated and the phase diagram of Ni-rich region in Mg-C-Ni ternary system was determined. It was found that in MgCxNi3 system there are two phases: alpha-MgCxNi3 and beta-MgCNi3. alpha-MgCxNi3 phase exists in the MgCxNi3 samples with x 1.0 prepared at the temperature of 900985 degreesC and in the samples with x < 1.0 prepared at the temperature higher than 965 degreesC. It has a large and constant lattice parameter of 0.3810 run. The beta-MgCNi3 phase is superconducting. The investigation results indicate that a homogeneous region with changeable content of Mg does not exist in MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) systems

Large magnetostriction in Fe100−xAlx(15⩽x⩽30) melt-spun ribbons

Magnetostriction of Fe100−xAlx(15⩽x⩽30) al1oys has been largely improved by using melt-spun method. The large magnetostriction up to −700ppm obtained in Fe81Al19 sample is about 5 times as large as that in conventional bulk samples of Fe–Al composition. It has been ascribed to the high concentration of Al–Al atom pairs created by melting-spinning method and their strongly preferential orientation in [100] textured ribbon plane. The remarkable anisotropic magnetostriction reflects the magnetoelastic competition occurring in those strong textured and thin ribbon samples. The composition dependence of the magnetostriction in ribbon samples has been found to be consistent with bulk materials.

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.

Magnetically controlled high damping in ferromagnetic Ni52Mn24Ga24 single crystal

An unusually high damping peak of 20% has been observed in Ni52Mn24Ga24 single crystals accompanying with martensitic transformation. The high damping capacity can be retained in the low-temperature martensite phase when a bias field of 0.4T was applied along the measurement direction. The magnetically controlled high damping and adjustable martensitic-austenitic transformation temperature indicate a great potential of using Ni–Mn–Ga alloys as high damping or energy absorbing materials in a wide temperature range.