| In recent years magnetic semiconductors have attracted a great deal of attention because it is thought that they can be used to generate spin polarized current in spintronics, an emerging field where the spin degree of freedom of charge carriers is utilized in microelectronic devices. In this dissertation work, we investigate the magnetic, thermodynamic, and transport properties of the magnetic semiconductor Fe1-xCo xS2 for x less than 0.14, the doping range where Insulator-to-Metal and paramagnetic-to-ferromagnetic transitions occur. We discovered that the Kondo effect is an important ingredient in the paramagnetic region of magnetic semiconductor Fe1- xCoxS2. Disorder that comes with doping, coupled with the Kondo effect and the RKKY interaction between the local moments, leads to the observed features of spin clusters and the Griffiths phase. This inhomogeneous magnetic state can be used to explain the resulting physical properties, including the NFL behavior as evidenced by the increase of C/T at very low temperature in proximity to the zero-T critical point.; A second system, LaSb2, is found to have a very inhomogeneous superconducting transition at low temperatures. We have discovered that the application of pressure induces a much more homogeneous superconducting ground state in this highly layered compound. |
