Electronic contribution to the enhancement of the ferromagnetic ordering temperature by Si substitution in Gd-5(SixGe1-x)(4)

Abstract

By using a combination of x-ray spectroscopic and diffraction measurements at high pressures together with density functional theory (DFT) calculations, we show that the increase in Curie temperature T-C, induced by Si substitutions in Gd-5(SixGe1-x)(4) giant magnetocaloric materials is predominately electronically driven as opposed to lattice driven. Whereas, a lattice contraction with applied pressure increases the strength of exchange (magnetic) interactions between Gd spins, leading to a modest increase in T-C at a rate of 1.2 K/angstrom(3), much larger enhancements in T-C are obtained with Si doping for the same volume reduction (13.5 K/angstrom(3)), indicating that volume (lattice) effects are secondary. Similarly, an orthorhombic [O(II)] to monoclinic (M) structural phase transition is observed to take place with applied pressure in the paramagnetic state of a Gd-5(Si0.125Ge0.875)(4) sample at room temperature at a much smaller volume than needed to drive the same structural transition with Si doping, indicating that, even in the absence of magnetic order, electronic effects with Si doping dominate the energetics of structural transformations over lattice (volume) effects. DFT calculations show that the electronic mechanism behind this effect is a stronger Si 3p-Gd 5d than Ge 4p-Gd 5d hybridization, a critical ingredient in mediating indirect exchange interactions between localized Gd 4f spins. The results highlight the strong sensitivity of the magnetic ordering temperature to the nature of p-d hybridization, opening opportunities for tailoring the magnetocaloric properties of these compounds by substituting other p and rare-earth elements at the Si/Ge and Gd sites, respectively.