Materials with spin-orbit coupling (SOC) exhibiting spin-momentum locking (SML) are of great current interest in spintronics because of their ability to efficiently convert charge signals into spin signals and vice versa. This thesis develops a generalized diffusion equation with four electrochemical potentials starting from the standard Boltzmann transport equation and maps it to a transmission line model. This model applies to diverse materials with SOC including topological insulators, transition metals, narrow bandgap semiconductors, perovskite oxides, etc. and presents a new viewpoint suggesting that materials with low Fermi wave vector lead to larger spin voltages. The model has been used to make a number of predictions some of which have later received experimental confirmation up to room temperature. We also use it to propose new devices for writing and reading information to and from magnets. Specifically, we show using experimentally established phenomena that magnetic state can be read without conventional magnetoresistive devices. We analyze the proposals with SPICE compatible multi-physics framework along with a new model developed in this thesis for pure spin conduction by magnon diffusion in ferromagnetic insulators.