Tailoring Magnetic and Electronic Phenomena in Fe1-xSnxBi2Se4 and FeSb2-xBixSe4 Ferromagnetic Semiconductors

Abstract

Spintronics is a growing field that can significantly impact the future of microelectronics. This is due to the added functionality, which promises to increase the speed and power efficiency of future devices. The core of this functionality arises from magnetism. Spintronic devices not only use the charge of an electron to modulate current, but it also uses the spin of electrons to help shuttle and gate currents within a device. A practical issue, which must be addressed, is the material platform. A material will need to shuttle polarized carriers throughout a device, and in order to maintain the spin-polarized current throughout the device, a magnetic material is required. A semiconductor is necessary if the next generation of spin polarized diodes and spin field effect transistors are going to become a reality. A hiccup towards progress, however, is that most semiconductors are not magnetic. Most magnetic materials are either insulating or metallic, neither of which allows tunable control over resistivity. The dilute magnetic semiconductor (DMS) is the result of a top down approach taken to creating a magnetic semiconductor. The idea being, a non-magnetic semiconductor is doped with magnetic elements to introduce a magnetic interaction such that the material becomes ferromagnetic. This approach has been fruitful creating a number of candidate materials. One issue, however, is that this approach tends to create DMS’s with interdependent magnetic ion and carrier concentrations. The magnetic dopant also acts as an electronic dopant. This creates a problem since we want to understand the full impact of both the magnetic ion and carrier concentration. When you increase magnetic ion concentration, the carrier concentration is also changed. This complicates things since the Curie Temperature (Tc) is dependent upon both of these material properties. Since changing one affects the other, elucidating the roles they each play becomes more difficult. If these dynamics are to be well understood, the codependence must be decoupled.

To tackle this issue, we developed a different type of magnetic semiconductor with the general formula MPn2Se4 (M = Mn, Fe ; Pn = Sb,Bi). This new magnetic semiconductor separates the magnetic ion centers and electronic doping sites spatially. This allows significantly more control over the magnetic ordering and carrier concentrations. As a result, we have decided to probe the effect of reducing the magnetic ion concentration while keeping the carrier concentration relatively constant in Fe1-xSnxBi2Se4. We observe a drastic reduction in Curie Temperature, ΔTC ~ 125 K as a result in increasing the average distance between Fe2+ in the crystal structure.

The second project is involves the creation of a solid solution between a p-type magnetic semiconductor and an n-type magnetic semiconductor to determine the origin of magnetism in FeSb2-xBixSe4 and related phases. This experiment is expected to deepen our understanding of the role itinerant carriers play in magnetic ordering within this magnetic semiconductor. Single crystal diffraction is used to probe the atomic distribution of the various elements within the crystal. This will is correlated with SQUID measurements and Seebeck measurements to determine the point of bipolar crossover. This has allowed us to resolve the different magnetic contributions and the role carriers have in stabilizing and enhancing magnetic phases.

Overall, this thesis contributes new templated material systems, which are used to provide new insight into the design of magnetic semiconducting systems.