Showing papers by "Laura H. Lewis published in 2006"

Magnetism and the defect state in the magnetocaloric antiperovskite Mn3GaC1−δ

Abstract: Magnetic and spectroscopic techniques were used to study the intermetallic antiperovskite Mn3GaC. An antiferromagnetic–ferromagnetic magnetostructural transition at 160 K underlies a remarkable magnetocaloric effect; these phenomena are suppressed in the substoichiometric composition Mn3GaC1−δ. X-ray absorption spectroscopy (XAS) data reported for three compositions Mn3GaC1−δ, δ = 0, 0.10, 0.22, are the basis for drawing inferences concerning the mechanism controlling magnetic order as a function of carbon stoichiometry. While the temperature dependence of the Mn3GaC carbon K edge reveals no observable change across the first-order magnetic transition, a clear splitting of the carbon absorption bands is observed that increases with increasing carbon deficiency. The room temperature Mn and Ga K edges indicate no significant variation with C content. FEFF 8.2 code calculations are in good qualitative agreement with data for the stoichiometric sample, but do not predict the changes in XAS observed in C-deficient samples. These results and the Goodenough–Anderson–Kanamori rules are the basis for a phenomenological model that attributes the carbon content dependence of the low temperature transition to the promotion of weak near-neighbour 90° Mn–Mn pairs in the carbon-deficient compound over the stronger 180° Mn–C–Mn interaction, locking in dominant ferromagnetism at low temperatures.

Magnetism and metastability of melt-spun Pd40(Fe,Ni)40P20 metallic glass

Abstract: The magnetic character of Pd40FexNi40−xP20, a metallic glass where x=175, is experimentally studied over a temperature range of 10–375K Hysteresis and temperature-dependent magnetization measurements derived from superconducting quantum interference device magnetometry on the amorphous alloy provide insight into the role of applied magnetic field on magnetic phase formation during the nanocrystallization process Specifically, the microstructure of the melt-spun form of Pd40(Fe,Ni)40P20 is discussed in correlation to the observed superparamagnetic and spin-glass behavior Despite the extremely high quenching rate used to synthesize the alloy, this composition shows evidence of chemical segregation on the nanoscale in the as-solidified state

Magnetostructural transitions and adiabatic temperature variation in polycrystal and single crystal Ni2MnGa alloys

Abstract: The magnetocaloric response of off-stoichiometric Ni2MnGa polycrystal and single-crystal samples with a Curie temperature coincident with the martensitic transition temperature was investigated. The direct measurement of temperature change (ΔTad) during an adiabatic transformation under a maximum field variation ΔH=5.6×106A∕m (7T) was performed. The polycrystalline sample shows a maximum ΔTad≈1.5K at a temperature close to the structural transition. The value of ΔTad and the 20K span of the ΔTad peak encourage further studies of the effect of microstructure on the magnetocaloric response of Ni2MnGa.

Phase-separated alloys for bulk exchange-biased permanent magnets

Abstract: Explorations in the metallurgical synthesis from the melt of bulk permanent magnets with coercivity conferred by the exchange bias mechanism were carried out in a two-phase materials system composed of ferromagnetic FeCo and antiferromagnetic, nominally equiatomic AuMn. Rapid solidification synthesis of composite alloys of nominal composition (Fe65Co35)100−x(AuMn)x (x=5, 10, and 15) was carried out. The ribbons possess AuMn and Au2Mn spherical phases in a bcc (Fe,Co) matrix with a bimodal size distribution of nanoscaled and micron sizes. Magnetization measurements of the composite confirm an exchange bias effect correlated with a coercivity increase over that of the (Fe,Co) melt-spun base alloy. While the exchange bias effect is small, the enhanced coercivity and shifted hysteresis loop observed in a single sample create a strong argument for coercivity enhancement conferred by the exchange bias interaction between antiferromagnetic and ferromagnetic phases in a bulk melt-spun nanocomposite material.

Magnetic and transport properties of MnBi∕Bi nanocomposites

Abstract: The magnetic and transport properties of a nanostructured Mn-Bi eutectic composition (∼Mn5Bi95) produced by melt spinning and low-temperature/short time vacuum annealing were studied. A hysteretic magnetostructural transformation from low-temperature phase to high-temperature phase MnBi is confirmed at 520K. The fact that the transition temperature is lower than that reported for bulk MnBi (633K), is tentatively attributed to interfacial strain between MnBi and the Bi matrix. A positive temperature coefficient of coercivity is confirmed in the nanocomposites, with a maximum coercivity value of 36kOe at 500K. Magnetic field annealing the as-spun composites (525K at 10kOe) produces nanoparticle alignment. Annealed MnBi∕Bi composites have a very large ordinary magnetoresistance (MR) ratio normal to the ribbon at 5T, 275% at room temperature and 10 000% at 5K.

Magnetic ordering in Pd∕Mn oxide nanocomposites

Abstract: An increase in the magnitude of resistivity and magnetic susceptibility (10×) with increase of oxygen in the assist beam during reactive ion-beam assisted deposition of Pd94Mn6 nanocomposite films is reported. These results imply that a portion of the oxygen is implanted in the as-deposited films within the Pd matrix, forming a metastable (PdMnO) solid solution that creates an increase in the effective electronic density of states at the Pd Fermi level and fosters an electronic state that approaches the cooperative magnetism. Vacuum annealing of films is hypothesized to foster the formation of Mn oxides that exhibit antiferromagnetism in the temperature range 50