Electronic structure, magnetic properties and magnetocaloric effect of GdCo2-xNix

Abstract

The electronic structure, magnetic properties, and magnetocaloric effect of GdCo2-xNix with x = 0.2, 0.25, and 0.3 were investigated. X-ray diffraction measurements showed that all studied samples are single phase with the cubic MgCu2 - Laves type structure. In agreement with experiments, the band structure calculations explain the following issues: the antiferromagnetic ground state of the system, the coupling between Gd and the transition metals (TM) mediated by the 5d electrons of Gd hybridized with the 3d electrons of the TM (Co, Ni), the reduction of the spin magnetic moment of Co when substituting with Ni due to Co-Ni hybridization. XPS measurements showed no significant shift in the binding energy of the investigated Co core levels with the change of Ni concentration. For all investigated samples the exchange splitting estimated to about 3.2 eV between the high spin final state and the low spin final state of the Co 3 s core level gives direct evidence of the local magnetic moments on Co sites. The measured increase of the saturation magnetizations when substituting cobalt by nickel confirms the ferrimagnetic-type ordering. The decrease of the Co magnetic moments is related to Co-Ni hybridization and filling of bands with the extra-electrons provided by Ni with respect to Co. Moreover, the Arrott plots, the temperature dependence of Landau coefficients and the shapes of magnetic entropy changes confirm the presence of a second order magnetic phase transition. The RCP(∆S)/∆B values decreases slowly when the Ni concentration increases and could be attributed to the narrowing of the │ΔSm│peaks.