  Motivated by observation of very high tunnel magnetoresistance (TMR) in
Fe-MgO-Fe magnetic tunnel junction devices, we propose a theoretical model for
these devices based on a single-band tight-binding approximation. An effort is
made to capture the band dispersions over the two dimensional transverse
Brillouin zone. In the transport direction, spin dependent Hamiltonian is
prescribed for Delta_1 and Delta_5 bands. Non-equilibrium Green's function
formalism is then used to calculate transport. Features like voltage dependence
of TMR are captured quantitatively within this simple model and the trends
match well with the ones predicted by ab-initio methods and experiments.
