| The purpose of this dissertation is to investigate the magnetic and transport properties of the dilution series connecting the mono-silicides FeSi, CoSi, and MnSi. We have focused on these materials since they allow the exploration of carrier doping of an unusual insulator, FeSi, whose properties are dominated by strong Coulomb interactions. These monosilicides all have the same cubic B-20 crystal structure making them ideal for an exploration of Fe1--x MnxSi and Fe1--yCoySi for all x and y between 0 and 1. The carrier sip and densities, as well as the proximity to magnetic phases can be controlled by the level of chemical substitution across this series. We have investigated the transport and magnetic behavior of this system, centered on the Kondo insulator FeSi, by carrying out magnetization, Hall effect, resistivity and magnetoresistence measurements. We have discovered that MnSi and Fe1--yCo ySi, which both are itinerant helimagnetic compounds, differ in that the Co doped FeSi is nearly spin polarized and has a novel temperature and field dependent conductivity. We discovered that the MR in Fe1--y CoySi is due to quantum interference effects which are substantial in this compound up to 100 K. Fe1--yCoySi, (0 < y ≤ 0.3) is a strongly scattering low charge density metal in which we have also discovered an extraordinarily large anomalous Hall effect. In contrast the hole doped insulator, Fe1--xMnxSi, remains paramagnetic up to x < 0.9 with a large quasiparticle mass and a conductivity (sigma) that is dominated by electron-electron (e-e) interaction effects. At low temperatures the hole carriers are localized beyond that due to the usual square-root singularity associated with quantum interference effects. In fact, the (sigma) and susceptibility are comparable to the diluted magnetic semiconductors, such as the Mn doped II-VI compounds. |
