Magnetism in Yttrium Intermetallics: Ab Initio Study

Abstract

This work is stimulated by the recent developments in the search of novel magnetic materials such as the simple cubic laves phase compounds of yttrium and 3d transition metals with ferromagnetic or exchange enhanced Pauli paramagnetic properties. The ground state electronic structure of the intermetallics was calculated by the full potential linearized augmented plane wave method (FP-LAPW) based on the density functional theory. The suitability of the various approximations LDA and GGA with exchange-correlations (XCs) potentials such as GGA-PBEsol, GGA-WC, etc. on the electronic structure was also investigated. From the calculated band structure and angular-momentum projected density of states (DOS), strong d–d hybridization of electrons at the Fermi level was observed. The Fe-Y and Co-Y coupling is stronger for minority component of Fe/Co d states which leads to formation of negative magnetic moment on Y sites. The magnetic moments were found to be considerably larger than the experimental moments although there is good agreement with methods based on DFT. The Y-TM compounds have mixed chemical bonding character which we have characterized by calculating the Bader charge distributions, electron localization function, as well as the magnetization spin density. High magnetization density is localized on the Fe atoms which imparts ferromagnetism to the more ionic YFe2 system, whereas the shift of electron density and magnetization density towards inter atomic regions imparts exchange enhanced paramagnetic properties in more covalent YCo2 system. In order to study the influence of d-electrons, the temperature-dependent electrical resistivity and thermopower were obtained from the band energies. Magnetism was investigated in terms of enhanced magnetic susceptibility and Sommerfeld term γ obtained from low temperature-specific heats. The dynamical stability of YX2 was investigated by calculating the vibrational phonon modes and phonon density of states. The lowermost frequency modes arise from vibrations of Y–Fe bonds which exhibit structural instability which have been interpreted in terms of electron-phonon coupling. The coupling of magnetic moments with the lattice in YFe2 verifies the magneto-elastic interactions found experimentally.