Pnina Ari-Gur

Magnetocaloric and thermomagnetic properties of Ni2.18Mn0.82Ga Heusler alloy in high magnetic fields up to 140 kOe

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H = 140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H = 140 kOe were found to be ΔT = 8.4 K at initial temperature 340 K and ΔQ = 4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the ...

Crystalline structure and magnetic behavior of the Ni41Mn39In12Co8 alloy demonstrating giant magnetocaloric effect

Magnetic cooling is a green cooling technology, which is more energy efficient than existing fluid-compression cooling machines. Ni41Mn39In12Co8 alloy, which demonstrates promising magnetocaloric performances, was investigated using neutron diffraction and thermomagnetic measurements. The austenite structure is cubic L-21 (Fm (3) over barm), while that of the martensite is a mix of 8 and 6 M modulated monoclinic structures (P 12/m 1). The austenitic site occupancy refinements reveal that all substituting Co atoms occupy Ni-sites. Most Mn atoms (65%) are in the Mn-sites and the rest go to In-sites (about 35%) and Ni-sites (less than 5%). This disorder of the magnetic atoms (Mn, Ni and Co) in the austenitic phase remains unchanged during the martensitic transition. The distortions of the interatomic distances due to the modulation of the martensitic structures further enhance the disorder in the magnetic interactions. Thermomagnetic measurements indicate that the austenitic phase is ferromagnetic. Cooling to below 250 K, where the alloy loses its ferromagnetic nature, and down to 50 K, the lack of any antiferromagnetic Bragg peaks suggests no antiferromagnetic ordering in the martensitic phase. At very low temperatures in the martensitic phase, spin glass magnetic nature is identified by magnetic measurements, and the spin-glass transition temperature is similar to 19 K.

Effects of Cobalt on the Crystalline Structures of the Ni-Mn-In Giant Magnetocaloric Heusler Alloys

The giant inverse magnetocaloric effect driven by a merged magneto-structural transformations in Ni-Mn-In-Co Heusler alloys, makes them highly promising as solid state refrigerants near room temperature. Knowledge of the crystallographic behavior of these alloys at a broad temperature range is critical to the understanding of the giant magnetocaloric effect. In this study, three Ni-Mn-In-Co alloys were investigated by neutron and synchrotron diffraction techniques. The chemical compositions of the alloys, determined by the Rutherford Backscattering Spectrometry (RBS) technique, were Ni41Mn39In12Co8, Ni48Mn34In12Co6 and Ni52Mn25In16Co7. The austenitic (A) phase of all three alloys was cubic L21 (Fm3m). Martensitic (M) phase of the Ni41Mn39In12Co8 alloy was a mix of 8M and 6M modulated monoclinic structures, while the other two alloys had a M composed of a mix of 7M and 5M modulated monoclinic structures. All modulated structures belong to the P 1 2/m 1 space group. Site occupancy refinements of the A phases of all three alloys, revealed that almost all the Co atoms (~97%) occupy the regular Ni (8c) sites. In the studied temperature range (50–250 K) of the M phase of the Ni41Mn39In12Co8 alloy has very low magnetization. Also, no antiferromagnetic ordring was observed in the neutron diffraction refinement of the M phase. Therefore by eliminating the possibilities of ferromagnetism and antiferromagnetism, it is concluded that the M phase of the Ni41Mn39In12Co8 alloy is spin glass.

Neutron Diffraction Study of a Non-Stoichiometric Ni-Mn-Ga MSM Alloy

Abstract. The structure and chemical order of a Heusler alloy of non-stoichiometric composition Ni-Mn-Ga were studied using constant-wavelength (1.538 Å) neutron diffraction at 363K and the diffraction pattern was refined using the FullProf software. At this temperature the structure is austenite (cubic) with Fm space group and lattice constant of a = 5.83913(4) [Å]. The chemical order is of critical importance in these alloys, as Mn becomes antiferromagnetic when the atoms are closer than the radius of the 3d shell. In the studied alloy the refinement of the site occupancy showed that the 4b (Ga site) contained as much as 22% Mn; that significantly alters the distances between the Mn atoms in the crystal and, as a result, also the exchange energy between some of the Mn atoms. Based on the refinement, the composition was determined to be Ni1.91Mn1.29Ga0.8

Effect of Heat-Treatment on the Phases of Ni-Mn-Ga Magnetic Shape Memory Alloys

The Heusler alloys Ni50Mn25+xGa25-x display magnetic shape memory effect (MSM) with very fast and large reversible strain under magnetic fields. This large strain and the speed of reaction make MSM alloys attractive as smart materials. Our crystallographic investigation of these alloys, focused on non-stoichiometric composition with excess of manganese. Using neutron diffraction, we revealed the necessary processing parameters to achieve and preserve the homogeneous metastable one-phase martensitic structure that is needed for an MSM effect at room temperature.

Phase mapping of multi-component oxides derived from sol-gel precursors

A sol-gel coprecipitation technique was used to prepare a wide range of compositions in two systems: MgO-TiO2 and Al2O3-ZrO2. Nanocrystalline, metastable phases were found in both systems after firing in temperatures up to 900oC for several hours. Above 1000oC, the metastable phases gradually transformed to the structures expected by the equilibrium phase diagrams. In the MgO-TiO2 system, a single phase with structure similar to the H.T. inverse spinel (qandilite) was formed from xerogels 500oC below the eutectoid point. Furthermore, this phase tolerated a wide range of compositions, ranging from Mg/Ti = 1.2 to 2.0. In the Ti-rich side, orthorhombic (karrooite–like) solid solutions were found from xerogels at compositions ranging from Mg/Ti = 0.5 to 0.63. The stability of these phases was tested up to 1500oC for a period of 3 h. It was found that at maximum deviation from stoichiometry, the spinel–like structure is stable up to 700oC and the karrooite-like non-stoichiometric phases are stable up to 600oC. Annealed xerogels prepared by sol-gel coprecipitation from the alcoholates of Zr and Al showed a wide solubility range for alumina in nanocrystalline zirconia, up to about 26 mol-%, accompanied by stabilization of the high-temperature tetragonal and cubic zirconia phases. The phases found in this research do not exist in the equilibrium phase diagrams at the same compositions and temperatures.