Search

The first-order phase transition and associated magnetocaloric properties of Mn0.5Fe0.5Ni0.95Cr0.05Si0.95Al0.05 have been studied by x-ray diffraction and dc magnetization measurements. The diffraction data for the sample showed that both the orthorhombic and hexagonal crystalline phases coexisted at room temperature. The temperature dependence of magnetization was measured at a constant field of 0.2 T. The first-order phase transition was observed at 325 K during heating and at 306 K during cooling, showing a thermomagnetic hysteresis of 19 K. For magnetic field change of 5 T, the entropy changes evaluated from the isothermal magnetization data peaked at 322 K during warming and at 313 K during cooling, showing a thermomagnetic hysteresis of 9 K. This difference in the magnitude of the thermomagnetic hysteresis was attributed to the virgin effect due to stress and crack formation during the first cooling from hexagonal to orthorhombic phase. Peak entropy changes of −16 J kg−1 K−1 and −42 J kg−1 K−1 were observed on heating for field changes of 2 and 5 T, respectively. The related refrigeration capacities were 74 J/kg (2 T) and 194 J/kg (5 T).

Owing to the relative abundance of its constituent elements and large magnetocaloric properties observed near room temperature, the AlFe2B2 system has attracted much attention recently. Here, we have studied the magnetic and magnetocaloric properties of Al0.85+xSi0.15Fe2B2 (x = 0.2, 0.4) prepared by drop-casting followed by annealing and acid treatment. The second order ferromagnetic phase transitions were observed near room temperature (∼298–305 K) and peak magnetic entropy changes (−ΔSM) of more than −6 J kg−1 K−1 were observed for a field change of 5 T. The results are discussed in terms of the impurity phases formed in the compounds due to excess aluminum.

We have synthesized a series of Co-doped Mn0.5Fe0.5Ni1−xCoxSi0.94Al0.06 (0.025 ≤ x ≤ 0.05) alloys by arc melting followed by a rapidly quenched vacuum suction casting technique and studied the magnetic and magnetocaloric properties of the system. All measurements were performed on the as-cast and annealed samples. X-ray diffraction data indicated that the samples exhibited a hexagonal phase at room temperature. Magnetization data showed that the annealed samples exhibited significantly larger transition temperatures than the as-cast samples. All samples showed the first-order phase transition with a thermomagnetic hysteresis of more than 20 K. The phase transitions were accompanied by isothermal entropy changes of as large as −16.4 J kg−1 K−1 and −42.4 J kg−1 K−1 for field changes of 20 and 50 kOe, respectively. Large refrigeration capacities and temperature averaged entropy changes of up to 201 J/kg and 34.05 J kg−1 K−1, respectively, were observed for a field change of 50 kOe.

We present an experimental study on the crystalline, magnetic, and magnetocaloric properties of Ni deficient (Mn-rich) (Mn0.525Fe0.5)Ni0.975Si0.95Al0.05. The study has been performed by x-ray diffraction, scanning electron microscopy, and dc magnetization measurements. X-ray diffraction measurements showed that the sample primarily exhibited the orthorhombic structure at room temperature. The coupled structural and ferromagnetic transition occurred at ∼338 K, which is significantly larger than ∼320 K observed in (Mn0.50Fe0.5)NiSi0.95Al0.05. Maximum magnetic entropy changes of ΔSM = −9.5 and 25 J kg−1K−1 for ΔH = 20 kOe and 50 kOe, respectively, have been observed in the material. Large refrigeration capacities of 60 J/kg and 160 J/kg for field changes of ΔH = 20 kOe and 50 kOe, respectively, have also been observed.

We have studied the magneto-transport properties of a series of ZrNi2-xCuxGa compounds. For all Cu concentration (x ≤ 0.5) the samples exhibited the L21 cubic structure at room temperature and demonstrated superconductivity below 2 K. A superconducting phase transition temperature of as low as ∼1 K and a critical magnetic field of 1.78 T was observed in the system. A linear extrapolation of TC to T = 0 highly indicated that a stoichiometry-dependent superconducting quantum phase transition may occur in ZrNi2-xCuxGa at xc = 0.90.

We have explored the structural and superconducting properties of ZrNi2-xTMxGa (TM = Cu, Co) Heusler compounds via x-ray diffraction, scanning electron mi croscopy, electrical resistivity, dc magnetization and ac susceptibility measurements. All samples crystallized in the cubic L21 structure at room temperature. For x ≤ 0.25, all the ZrNi2-xCuxGa compounds showed superconducting properties and a decrease in TC with increasing Cu concentration. The dc magnetization data suggested type-II superconductivity for all the Cu-doped compounds. Contrary to the ZrNi2-xCuxGa compounds, no superconductivity was observed in the ZrNi2-xCoxGa compounds. Substitution of Ni by a small concentration of Co destroyed superconductivity in the Co-doped compounds. The experimental results are discussed and possible explanations are provided.

The phase transitions and associated magnetocaloric properties of the Ni2Mn0.55CoxCr0.45-xGa (0 ≤ x ≤ 0.25) Heusler alloy system have been investigated. All samples exhibit a first-order martensitic phase transition, evidenced by a sharp drop in the resistivity versus temperature data and a thermomagnetic irreversibility in the dc magnetization data of the respective samples. Large magnetic entropy changes have also been observed near the phase transitions. The martensitic transformation temperature increases as Cr is partially replaced with Co. Additionally, this substitution leads to a partial decoupling of the magnetic and structural phase transitions, dramatically suppressing any magnetic hysteresis losses. Furthermore, the change in electrical resistivity during the phase transition remains relatively constant across the system, despite major changes in the degree of structural disorder and magnetostructural phase transition coupling. Detailed experimental results and conjectures as to the origin of these behaviors have been provided.

The superconducting properties of a series of Zr1+xNi2-xGa and Zr1-xNi2+xGa compounds have been investigated by x-ray diffraction, electrical resistivity, dc magnetization, and ac susceptibility measurements. While the parent compound, ZrNi2Ga, exhibited the cubic L21 Heusler structure, multiple non-cubic structures formed in the Zr and Ni rich doped materials. For x ≤ 0.3, all Zr1-xNi2+xGa compounds demonstrated superconducting behavior, but no superconductivity was observed in the Zr1+xNi2-xGa alloys for x > 0.2. The magnetization data revealed that all materials in both Zr1+xNi2-xGa and Zr1-xNi2+xGa series exhibited type-II superconductivity. With increasing doping concentration x, the paramagnetic ordering were enhanced in both systems while the superconducting properties were found to weaken. The observations are discussed considering the structural disorders in the systems.

Here we report on the structural and magnetic properties of Ni0.2Mn3.2Ga0.6 melt-spun ribbons. The as-spun ribbons were found to exhibit mixed cubic phases that transform to non-cubic structure upon annealing. Additionally, an amorphous phase was found to co-exist in all ribbons. The SEM images show that minor grain formation occurs on the as-spun ribbons. However, the formation of extensive nano-grains was observed on the surfaces of the annealed ribbons. While the as-spun ribbons exhibit predominantly paramagnetic behavior, the ribbons annealed under various thermal conditions were found to be ferromagnetic with a Curie temperature of about 380 K. The ribbons annealed at 450 °C for 30 minutes exhibit a large coercive field of about 2500 Oe. The experimental results show that the microstructure and associated magnetic properties of the ribbons can be controlled by annealing techniques. The coercive fields and the shape of the magnetic hysteresis loops vary significantly with annealing conditions. Exchange bias effects have also been observed in the annealed ribbons.

An experimental study has been performed on a series of Ni2Mn0.4-xFexCr0.6Ga Heusler alloys. At room temperature, the alloys crystallize in either the tetragonal martensite (x < 0.1) or cubic L21 structure (x ≥ 0.1). Additionally, a Cr-Fe based face-centered cubic γ-Fe type secondary phase was found to co-exist in the samples. Magnetization and transport measurements revealed that the Curie and martensitic transition temperatures decrease as Mn is replaced with Fe. Atypical to other Ni2MnGa-derivative Heusler alloys, the transition temperatures decrease at the same rate with respect to x for x ≥ 0.1. Thus, the two transitions do not couple in to a single magnetostructural transition at any composition. Transport measurements revealed that all samples exhibit a sharp drop in resistivity during the martensitic phase transition (13 - 17 %), with the magnitude of this drop remaining relatively constant over the entire series. The possible origins of the observed experimental behavior are discussed.