The spin Hall effect (SHE) has been shown to be strong in several heavy, nonmagnetic metals such as Pt, beta-phase Ta and beta-phase W. Owing to its relatively low electrical resistivity, Pt is a more favorable choice for the channel of three-terminal magnetic tunnel junction (3T-MTJ) devices for power efficient magnetoresistive memory applications. In this dissertation, I will be discussing our efforts in understanding the dominant SHE mechanism in thin and nonuniform Pt films. Our results support the dominant role of the intrinsic SHE in Pt which implies that the spin Hall angle of Pt scales linearly with, and thus can be further enhanced by increasing, its resistivity. We demonstrate that by doping Pt with Hf impurities, the spin torque efficiency of PtHf/Co bilayer can be more than 2 times larger that of pure Pt due to 3 times higher resistivity. The second half of the dissertation describes the development of 3T-MTJ devices using a Pt channel. We demonstrate that a thin Hf spacer between the Pt channel and the MTJ results in 2 times lower magnetic damping without any significant detrimental effect on the interfacial spin transparency. Moreover, the 3T-MTJ devices with PtHf channel and a Hf spacer exhibit low power and nanosecond fast switching, promising for power efficient memory applications. We also demonstrate that the performace of Pt-based 3T-MTJ devices is robust at cryogenic temperatures, thus making this structure attractive for cryogenic applications.