F. B. Meng

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 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.

Magnetic-field-induced transformation in FeMnGa alloys

A kind of ferromagnetic shape memory alloy with off-stoichiometric composition of Heusler alloy Fe2MnGa has been synthesized. By optimizing composition, the martensitic transformation has been modified to occur at about 163 K accompanying spontaneous magnetization, which enables a magnetic field-induced structural transition from a paramagnetic parent phase to a ferromagnetic martensite with high magnetization of 93.8 emu/g. The material performs a quite large lattice distortion through the transformation, (c−a)/c=33.5%, causing a shape memory strain upto 3.6%. Such large lattice distortions strongly influence the electron structures, and thus some special physical behavior related to the transport and conductive properties is investigated.

Giant exchange bias based on magnetic transition in gamma-Fe2MnGa melt-spun ribbons

Giant exchange bias with a shift up to 3.86 kOe has been observed in gamma-Fe2MnGa alloy in which a ferromagnetic to antiferromagnetic transition takes place at 250 K. The transition can be suppressed to lower temperatures by higher magnetic fields at a shift rate of 10.3 K/kOe. Exchange bias and enhanced coercivity occur simultaneously, revealing an exchange coupling between the coexisting antiferromagnetic and ferromagnetic phases. Meanwhile, the internal exchange coupling inside the antiferromagnetic clusters dynamically ensures their unidirectional anisotropy during their size changing following the external magnetic field

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.

Superelasticity of CoNiGa : Fe single crystals

We have fabricated CoNiFeGa single crystals with excellent superelasticity. The superelastic strains of 4% and 6.7% in compression have been obtained along the [001] and [110] directions, respectively. These single crystals show strong anisotropy in strains, superelastic parameters, and even transformation path related to the different crystalline directions. A large superelastic strain up to 11% has been obtained in tension test. The perfect superelasticities have also displayed in bending and torsion tests.

Tuning exchange bias by thermal fluctuation in Fe52Mn23Ga25 melt-spun ribbons

In Fe52Mn23Ga25 ribbons, the exchange bias becomes very sensitive to the proportions and sizes of the ferromagnetic (FM) and antiferromagnetic (AFM) phases. With high cooling fields, the AFM clusters with small sizes are embedded in the FM matrix which is kinetically arrested at low temperatures, resulting in a small exchange bias. The arrested FM phase can be dearrested by thermal fluctuation, and simultaneously, the proportion and sizes of AFM clusters increase with a stable unidirectional anisotropy. With different thermal fluctuations, a continuous tuning of exchange bias field from 0.202 kOe to 2.11 kOe is realized.

Ferromagnetic exchange interaction between Co and Mn in the Heusler alloy CuCoMnAl

The ferromagnetic exchange interaction between Co and Mn in Heusler alloys has been phenomenologically investigated by analyzing the composition dependence of the magnetic moment and the Curie temperature in a series of quaternary CuCoMnAl alloys. The curves of the composition dependence of the magnetic moment show an interesting valleylike profile and their minima are positioned at different Co contents for different Mn concentrations. The ferromagnetic Co–Mn exchange interaction is a short-range effect which is only effective at the nearest-neighbor distance. At this distance, the exchange interaction can be further enhanced by a Mn-rich composition, but it might be destroyed by the lattice distortion due to the martensitic transformation.

Magnetic and structural properties of Co0.8MnxFe2.2−xO4 (x=0.2, 0.4, 0.6, 0.8) polycrystalline powders synthesized by sol-gel process

Co0.8MnxFe2.2−xO4 (x=0.2, 0.4, 0.6, and 0.8) powders were prepared by a sol-gel process and annealed at different temperatures (Ta=600, 800, 1000, and 1200u2009°C). X-ray diffractometer results reveal that all the samples are inverse-type spinel with cubic structure. The magnetic properties were measured by vibrating sample magnetometer and the results indicate that the saturated magnetization (Ms) and coercivity (Hc) decrease with increasing Mn content x. The decrease in Ms is related to the magnetic moment of Mn3+ is lower than that of Fe3+. The decrease in Hc is ascribed to the content decrease in Co2+ on B sites. Increasing grain sizes at higher annealing temperature (Ta) also have an intensive influence on Hc.

The effect of internal and external stress on two-way shape-memory behaviour in Co49Ni21.6Ga29.4 single crystals

The effect of the internal stress on the two-way shape memory in Co49Ni21.6Ga29.4 single crystals has been investigated. We found that the internal stress generated natively by the solidifying process works as a tensile force along the growth direction. Applying different compressive pre-stresses along the [0 0 1] direction, the shape-memory strain can be continuously changed from +1.0% to −2.3%. In the [1 1 0] direction, the strain monotonically increases from −2.0% to −4.0% due to a strong detwinning produced by the consistent effect of the external and internal stresses.