Composition controlled spin polarization inCo1−xFexS2: Electronic, magnetic, and thermodynamic properties

Abstract

We recently demonstrated that alloy composition can be used to fine tune the position of the Fermi level in ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{S}}_{2}$ alloys leading to composition-controlled spin polarization and the ability to engineer high conduction electron spin polarizations of up to 85%. We present here a comprehensive experimental investigation of the structure, stoichiometry, magnetic, magnetotransport, and thermodynamic properties of bulk polycrystalline solid solutions of ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{S}}_{2}$. These data are supplemented with direct measurements of the spin polarization at the Fermi level by point contact Andr\'eev reflection (PCAR) and first principles electronic structure calculations. The compositions studied are in the range $0.0<x<0.30$, the most relevant part of the phase space in terms of composition control over high spin polarization. By measuring the Fe doping dependence of the saturation magnetization, high field magnetoresistance, and anisotropic magnetoresistance, and combining them with PCAR, we are able to show that Fe doping first leads to a crossover from minority to majority spin polarization, followed by attainment of a highly spin polarized state for $x>0.07$. The experimentally determined spin polarization can be tuned by alloy composition between $\ensuremath{-}57%(x=0)$ and $+85%(x=0.15)$. The evolution of the magnetic, transport, and thermodynamic properties with increasing Fe doping is discussed in terms of the composition dependence of the conduction electron spin polarization and the spin-dependent band structure.