  A Feynman-Kac type formula of relativistic Schr\"odinger operators with
unbounded vector potential and spin 1/2 is given in terms of a three-component
process consisting of Brownian motion, a Poisson process and a subordinator.
This formula is obtained for unbounded magnetic fields and magnetic field with
zeros. From this formula an energy comparison inequality is derived. Spatial
decay of bound states is established separately for growing and decaying
potentials by using martingale methods.
