Highly spin-polarized cobalt iron disulfide thin films for spintronic studies

Highly spin polarized materials offer many advantages for future spintronic devices. Recently, Co1-xFexS2, an isostructural alloy of CoS2 and FeS2, has been shown to exhibit a high, composition tunable, spin polarization (-56% < P < +85%) in bulk polycrystals while circumventing many of the traditional problems with half-metallic ferromagnets. These unique characteristics make Co1-xFexS2 an interesting system for fundamental spintronic device studies, although exploitation of these unique properties clearly requires development of a reliable thin film synthesis route. In this work we demonstrate that ex situ sulfidation of Co1-x Fex thin films provide such a synthesis method. Detailed structural characterization demonstrates that we can obtain single-phase, stoichiometric polycrystalline thin films, while magnetometry and transport measurements confirm bulk-like properties. Moreover, these films are found to exhibit an unexpected low-field hysteretic magnetoresistance, which is proven to be due to spin-dependent intergranular tunneling. The significance of this observation is that it provides a simple probe of P in such thin film samples, which we found to be as high as 90%.;In order to understand how Co1-xFexS2 films behave at thicknesses useful for spintronic device studies, we studied the t dependent evolution of the structural, magnetic, and electronic properties of CoS2 films prepared by ex situ sulfidation. Interestingly, we find increased grain boundary resistance and grain boundary widening with decreased film thickness, which we associate with an excess S accumulation at CoS2 grain boundaries. This S accumulation ultimately leads to significant changes in film properties including the observation of a metal to insulator transition, decreased magnetization, and enhanced spin-dependent intergranular tunneling. We show that insulating CoS2 films exhibit granular metal behavior where a Coulomb energy penalty associated with charging of individual grains becomes visible in the electrical transport. Remarkably, we find that even in the limit of insulating behavior films are still ferromagnetic, and exhibit a tenfold increase in spin-dependent intergranular tunneling when compared to thicker, metallic films. We also demonstrate, with two magnetoresistance models, the ability to extract the temperature dependence of the spin polarization in such granular metal CoS2 films, a property that is extremely difficult to obtain via other techniques.;In the final part of this work I discuss our recent progress in synthesizing Co1-xFexS2 films on GaAs(001) substrates for tunable semiconductor spin injection, where the reactivity of GaAs with Co, Fe, and S makes for significant materials challenges. I will show through synthesis optimization that CoS2 films can be synthesized on GaAs substrates, albeit with excess S concentrations. While the presence of excess S is problematic, I will demonstrate how a 2 nm thick Al2O 3 tunnel barrier can prevent S diffusion into underlying layers during ex situ sulfidation. Not only does this surprising discovery offer a simple way to prevent unwanted S diffusion into GaAs, it also provides a simple way to overcome the conductivity mismatch problem when mating Co1-xFexS2 with GaAs.