We report the temperature and filed dependence of magnetic behaviour of the Ho1-xMmxCo2 (x = 0, 0.1, 0.2, 0.3 and 0.4; Mm = mischmetal) alloys and their crystalline hydrides. The change of magnetic transition temperature (TC), order of magnetic transition and total magnetization of the alloys with increasing Mm content is studied. The alloys exhibit strong composition and field dependence of thermal hysteresis and spin-reorientation below TC. The effect of hydrogen absorption on the magnetic behaviour of alloys is studied, and the results are correlated to the unit cell volume expansion and decrease in the conduction electron density. © 2010 Elsevier B.V. All rights reserved.

Sundara Ramaprabhu

Department Of Physics

Conduction electrons

Effect of hydrogen

Field dependence

Hydrogen absorption

LAVES-PHASE

Magnetic behaviour

magnetic phase transitions

Magnetic transition temperature

Magnetic transitions

Misch metal

SPIN-REORIENTATION

Thermal hysteresis

TOTAL MAGNETIZATION

UNIT-CELL VOLUME

Absorption

Cerium alloys

Holmium

Hydrogen

Magnetic materials

Magnetization

Nanocrystalline alloys

Phase Transitions

Gas adsorption

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IIT Madras

Journal of Alloys and Compounds

The influence of Mm (Mm=mischmetal) substitution and hydrogen absorption on the magnetic properties of Ho1-xMmxCo2 (x=0, 0.1, 0.2, 0.3 and 0.4) alloys have been determined through the temperature dependence of ac susceptibility and thermopower measurements. The changes in magnetic-ordering temperature of Ho1-xMmxCo2 alloys have been explained based on the dilution of the magnetic ions and weakening of 4f-3d exchange interactions. The gradual disappearance of the magnetic transition temperature upon increasing hydrogen concentration (y) has been interpreted by the lattice expansion and charge transfer between absorbed hydrogen and 3d-band of Ho1-xMmxCo2. © 2008.

Journal of Magnetism and Magnetic Materials

Magnetic properties of Ho1-xMmxCo2 (x=0, 0.1, 0.2, 0.3 and 0.4) alloys and their hydrides

The effect of hydrogen absorption-desorption on the structural properties of Laves phase Dy1-xMmxCo2 (x = 0.1, 0.3 and 0.5; Mm = mischmetal, a natural mixture of the light rare earth metals containing 50 wt% Ce, 35 wt% La, 8 wt% Pr, 5 wt% Nd and 1.5 wt% of other rare earth elements and 0.5 wt% Fe) alloys has been investigated by means of hydrogen absorption-desorption pressure-composition (PC) isotherms, kinetics of hydrogen absorption and powder x-ray diffraction (XRD). The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.001-1 bar and temperature range 50-200 °C using Sieverts-type apparatus. The experimental results of the kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions have been identified from the different slope regions of the PC isotherms and first-order type kinetic plots. The dependence of the reaction rate parameter upon hydriding pressure and temperature in the (α+β)-phase region has been discussed. The effect of hydrogenation pressure, temperature and Mm concentration on the hydrogen-induced transformation from crystalline Dy1-xMm xCo2-H to amorphous Dy1-xMmxCo 2-H and decomposition into crystalline (Dy, Mm)H2 and Co have been discussed in detail. Further, the effect of dehydrogenation on the recovery of the crystalline Laves phase structure of Dy1-xMm xCo2 from its decomposed state is presented. This hydrogenation-disproportionation-desorption-recombination (HDDR) process can be conveniently used in powder metallurgy. © 2008 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Influence of hydrogen absorption-desorption on structural properties of Dy1-xMmxCo2 alloys

The structural and electrical transport properties of Ho1-x Mmx Co2 (x=0, 0.1, 0.2, 0.3, and 0.4 and Mm=mischmetal) alloys and their hydrides in the hydrogen concentration (y) range of 0≤y≤3.6 have been determined through the powder x-ray diffraction (XRD) and temperature dependence of electrical resistivity [ρ (T)] measurements. At room temperature, these compounds crystallize in Mg Cu2 -type (C15) structure. The crystalline nature and lattice expansion of hydrogenated alloys have been studied using the hydrogen concentration dependence of XRD peak analysis indicating the coexistence of two hydride phases at intermediate hydrogen concentrations. The temperature dependence of the electrical resistivity of alloys has been discussed based on the conduction electron scattering and spin fluctuation scattering mechanisms. The changes in magnetic ordering and transition temperature upon Mm substitution and at different concentrations of hydrogen loadings have been discussed. Furthermore, the transformation from metalliclike conductivity to thermally activated conduction mechanism and different conduction regimes of temperature dependent resistivity upon increasing H concentration have been well described by Kondo-type and electron-electron scattering effects. © 2007 American Institute of Physics.

Fulltext

Journal of Applied Physics

Influence of hydrogen absorption on structural and electrical transport properties of Ho1-xMmxCo2 alloys

Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1-xMmxCo2 (x ≤ 0, 0.1, 0.2, 0.3 and 0.4, Mm ≤ mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50-200 °C and pressure range 0.001-1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho 1-xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1-xMmxCo 2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1-xMm xCo2-H to amorphous Ho1-xMmxCo 2-H and recrystallization into elemental hydrides have been studied and discussed. © 2007 IOP Publishing Ltd.

Journal of Physics D: Applied Physics

Thermodynamic and hydrogen-induced structural properties of Ho 1-xMmxCo2-hydrides

The hydrogen absorption-desorption pressure-composition (PC) isotherms and kinetics of Ho1 - x Mmx Co2 (x = 0, 0.1, 0.2, 0.3 and 0.4, Mm = mischmetal) have been carried out in the temperature and pressure ranges 50-200 {ring operator} C and 0.001-1 bar using Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydrides stability has been discussed. The kinetics of hydrogen absorption at near equilibrium plateau region have been analyzed by a rate function. The reaction rate constants in the two-phase coexistence region and diffusion coefficient in the hydride phase region were calculated and its pressure and temperature dependence have been discussed. The hydrogen desorption from interstitial sites of Ho1 - x Mmx Co2 H has been performed using differential scanning calorimetry (DSC) and thermogravimetry (TG) analysis. The results are explored to identify the preferential occupation of different interstitial sites of hydrogen atoms and complete desorption temperature. The observed phase transformations as a function of hydrogen concentration and temperature have been discussed. © 2006 International Association for Hydrogen Energy.

International Journal of Hydrogen Energy

Hydrogen absorption and desorption properties of Ho1 - x Mmx Co2 alloys

Magnetic behaviour of hydrogenated Ho1-xMmxCo2 (x = 0-0.4) alloys

Journal	Journal of Alloys and Compounds
ISSN	09258388
Open Access	No