  Using a quantum theory including spin-splitting effect in diluted magnetic
semiconductors, we study the dependence of tunneling magnetoresistance (TMR) on
barrier thickness, temperature and applied voltage in GaMnAs/GaAs/GaMnAs
heterostructures. TMR ratios more than 65% are obtained at zero temperature,
when one GaAs monolayer ($\approx$ 0.565 nm) is used as a tunnel barrier. It is
also shown that the TMR ratio decreases rapidly with increasing the barrier
thickness and applied voltage, however at high voltages and low thicknesses,
the TMR first increases and then decreases. Our model calculations well explain
the main features of the recent experimental observations.
