Magnetic behaviour in Dy1-xMmxCO2 compounds
TL;DR: It is found that the Mm substitution can only lead to a considerable reduction in the T(C), saturation magnetization, and Co moment, and the variation in induced moments of the Co sublattice is estimated.
Abstract: The magnetic behaviour in Dy1 − xMmxCo2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5; Mm = mischmetal) compounds is reported using temperature and field dependence of magnetization (M–T and M–H respectively) measurements. A strong composition dependent irreversibility is observed in both the M–T and M–H scans below the magnetic ordering temperature (TC). A clear change of the first-order magnetic transition of DyCo2 to a second-order one in Dy0.5Mm0.5Co2 is evidenced by M–T and a series of Arrott (M2 versus H/M) plots, obtained from the M–H isotherms around TC. The variation in induced moments of the Co sublattice is estimated. It is found that the Mm substitution can only lead to a considerable reduction in the TC, saturation magnetization, and Co moment. The observed behaviour of M–T and M–H plots with increasing Mm content is discussed in detail.
References
More filters
TL;DR: In this article, the effect of the chemical nature and stoichiometry of specific alloy families (AB5, A2B, AB/AB2 and AB2) on the hydride stability, hydrogen storage capacity and kinetics of hydrogen sorption-desorption in the solid phase/gas and solid phase-electrolyte solution systems is discussed.
Abstract: Metal hydride electrodes are of particular interest owing to their potential and practical application in batteries. A large number of hydrogen storage materials has been characterized so far. This paper deals with the effect of the chemical nature and stoichiometry of specific alloy families (AB5, A2B, AB/AB2 and AB2) on the hydride stability, hydrogen storage capacity and kinetics of hydrogen sorption-desorption in the solid phase/gas and solid phase/electrolyte solution systems. Special attention has been paid towards the electrochemical properties of metal hydrides in terms of their performance in Ni-MH rechargeable alkaline cells.
265 citations
TL;DR: In this paper, it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e).
Abstract: The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.
195 citations
TL;DR: In this paper, a comparative study of the magnetic and magnetocaloric properties of RCo 2-based compounds under various substitutions and applied pressure is presented, and it is found that the spin fluctuations arising from the magnetovolume effect reduce the strength of IEM, which subsequently leads to a reduction in the MCE.
Abstract: By virtue of the itinerant electron metamagnetism (IEM), the RCo 2 compounds with R=Er, Ho and Dy are found to show first-order magnetic transition at their ordering temperatures. The inherent instability of Co sublattice magnetism is responsible for the occurrence of IEM, which leads to interesting magnetic and related physical properties. The systematic studies of the variations in the magnetic and magnetocaloric properties of the RCo 2 -based compounds show that the magnetovolume effect plays a decisive role in determining the nature of magnetic transitions and hence the magnetocaloric effect (MCE) in these compounds. It is found that the spin fluctuations arising from the magnetovolume effect reduce the strength of IEM, which subsequently lead to a reduction in the MCE. Most of the substitutions at the Co site are found to result in a positive magnetovolume effect, leading to an initial increase in the ordering temperature. Application of pressure, on the other hand, causes a reduction in the ordering temperature due to the negative magnetovolume effect. A comparative study of the magnetic and magnetocaloric properties of RCo 2 compounds under various substitutions and applied pressure is presented. Analysis of the magnetization data using the Landau theory of magnetic phase transitions has shown that there is a strong correlation between the Landau coefficients and the MCE. The variations seen in the order of magnetic transition and the MCE values seem to support the recent model proposed by Khmelevskyi and Mohn for the occurrence of IEM in RCo 2 compounds. Metastable nature of the transition metal sublattice in RCo 2 -based compounds and its role in determining the magnetic and magnetocaloric properties is explained.
178 citations
TL;DR: In this article, the authors performed saturation magnetization and neutron diffraction measurements on cubic Laves phase compounds RCo2 in which R is Nd, Tb, Ho, and Er.
Abstract: Saturation magnetization and neutron diffraction measurements have been performed on cubic Laves phase compounds RCo2 in which R is Nd, Tb, Ho, and Er. Neutron powder patterns obtained at room temperature and at 15°K allowed the determination of the magnetic structures. The low‐temperature patterns are of the ferromagnetic or ferrimagnetic type, with large magnetic intensities superimposed on the nuclear peaks. For the compounds of Tb, Ho, and Er, the rare‐earth atoms show nearly the full moment expected for the free tripositive ion and the cobalt moment is about one Bohr magneton. The rare‐earth moments are coupled parallel to each other, but antiparallel to all the cobalt moments. For NdCo2, the observed Nd moment of 2.6±0.2 Bohr magnetons is smaller than the free ion value of 3.27 Bohr magnetons, and it is coupled parallel to the cobalt moment of 0.8±0.2 Bohr magnetons. In the Nd ion, the spin is opposite to the moment (J = L−S), while for the heavier rare earths the spin is parallel to the total momen...
134 citations
TL;DR: In this article, the first-order magnetic phase transition observed in the Er-Co sublattice is related to the itinerant electron metamagnetism of the d subsystem (Co sub-lattices) driven by the onset of magnetic ordering within the Er sub-slope, which is referred either to a weakening of the effective molecular field acting on the Co sites owing to the yttrium subsitution or to a pressure-driven increase of the critical field necessary to induce a magnetic moment on the co sites.
Abstract: The thermodynamic and transport properties of the ${\mathrm{Er}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Co}}_{2}$ system were studied in the concentration range $0.0l~xl~1.0.$ In this system, the first-order magnetic phase transition observed in ${\mathrm{ErCo}}_{2}$ at ${T}_{\mathrm{C}}=32$ K is related to the itinerant electron metamagnetism of the d subsystem (Co sublattice) driven by the onset of magnetic ordering within the Er sublattice. By employing magnetic, specific heat, thermal expansion, and resistivity measurements we show that in a limited concentration range ${x}_{{\mathrm{cr}}^{\ensuremath{'}}}lxl{x}_{\mathrm{cr}}$ and pressure ${P}_{{\mathrm{cr}}^{\ensuremath{'}}}lPl{P}_{\mathrm{cr}}$ the itinerant Co sublattice orders magnetically at ${T}_{\mathrm{C}}^{\mathrm{Co}},$ which is lower than ${T}_{\mathrm{C}}^{R}$ of the Er sublattice. This is referred either to a weakening of the effective molecular field acting on the Co sites owing to the yttrium subsititution or to a pressure-driven increase of the critical field necessary to induce a magnetic moment on the Co sites. On further increasing the yttrium concentration or the pressure only the Er sublattice exhibits long-range order. The theoretical calculations within the molecular field approximation are in agreement with the experimental magnetic x-$T$ phase diagram of the ${\mathrm{Er}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Co}}_{2}$ system and confirm the effect of a separate ordering of the magnetic sublattices with reasonable parameters used for the intrasublattice Er-Er and intersublattice Er-Co exchange interactions. A field-induced collapse of the Co moment, inverse itinerant electron metamagnetism, is well observable by magnetoresistance measurements at appropriate values of concentration and external pressure. The existence of itinerant electron metamagnetism in the Co sublattice is found to be limited in temperature by ${T}_{0},$ a characteristic temperature which is sensitive to substitution and pressure.
57 citations