Strain-induced effects on the magnetic and electronic properties of epitaxial Fe$_{1-x}$Co$_{x}$Si thin films

TL;DR

This study explores how Co doping and strain affect the structural, electronic, and magnetic properties of epitaxial FeCoSi thin films, revealing tunable conductivity, magnetic ordering, and spin polarization.

Contribution

It provides detailed insights into the strain-induced lattice deformation and the doping-dependent transition from semiconducting to metallic behavior in FeCoSi thin films.

Findings

Films are phase-pure and defect-free with ~1 nm roughness.

Resistivity changes from semiconducting to metallic with Co doping.

Films exhibit positive magnetoresistance and large anomalous Hall effect.

Abstract

We have investigated the Co-doping dependence of the structural, transport, and magnetic properties of \epsilon-FeCoSi 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 \epsilon-FeCoSi 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-like for x \gtrsim 0.3. The films exhibit the positive linear magnetoresistance that is characteristic of \epsilon-FeCoSi below their magnetic ordering temperatures $T_\mathrm{ord}$, as well as the huge anomalous Hall effect of order several \mu\Omega cm. 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 \mu_{B}/ Co atom for x \lesssim 0.25. When taken in combination with the carrier density derived from the ordinary Hall effect, this signifies a highly spin-polarised electron gas in the low x, semiconducting regime.