Highly Spin-Polarised Chiral Transition Metal Silicide Epilayers

We have investigated the Co-doping dependence of the structural, transport, and magnetic properties of �-Fe1−xCoxSi epilayers grown by molecular beam epitaxy on silicon (111) substrates. Low energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy studies have confirmed the growth of phase pure,
defect free �-Fe1−xCoxSi epitaxial films with a surface roughness of � 1 nm. These epilayers are strained due to lattice mismatch with the substrate,deforming the cubic B20 lattice so that it becomes rhombohedral. The temperature dependence of the resistivity changes as the Co concentration is increased, being semiconducting-like for low x and metallic for x less than 0.3. The films exhibit the positive linear magnetoresistance that is characteristic of �-Fe1−xCoxSi below their magnetic ordering temperatures (Tord), as well as the huge anomalous Hall effect of order several μcm. The ordering temperatures are higher than those observed in bulk, up to 77 K for x = 0.4. The saturation magnetic moment of the films varies as a function of Co doping, with a contribution of � 1 μB/ Co atom for x less than equal to 0.25. When taken in combination with the carrier density derived from the ordinary Hall effect, this signifies a highly spin-polarized electron gas in the low x, semiconducting regime.
To understand the electronic structure and evolution of magnetism in B20 system we used soft X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy in total electron yield mode (TEY) to probe the L2,3 edges of Fe and Co in Fe1−xCoxSi thin films. Branching ratios (L3/(L2 + L3)) as a function of x, suggests that the number of holes associated with Co increases from x=0.1 to x=0.5 where as that associated with Fe changes little. Variation in the occupation states of Fe and Co atoms coupled with shift in L2,3 edges (� 500 meV ) and the evolution of the L3 edge line shape indicates a modified band structure. The dichroism on Fe L3 edge (TEY) varies from 0.6 × 10−3 for x=0.1 to 1.4 × 10−3 for x=0.5 and that of Co evolves from being negligible for x=0.1 to 1.7 × 10−3 for x=0.5. Whilst the magnetism in Fe1−xCoxSi system arises from the Co doping, these asymmetry spectra clearly show that the magnetic moment is delocalised on both Co and Fe sites.