  A model of a spin valve in which electron transport between the magnetized
electrodes is due to multistep tunneling is analyzed. Motivated by recent
experiments on organic spin valves, we assume that spin memory loss in the
course of transport is due to random hyperfine fields acting on electron while
it waits for the next tunneling step. Amazingly, we identify the three-step
configurations of sites, for which the tunnel magnetoresistance (TMR) is
negative, suggesting that the resistance for antiparallel magnetizations of the
electrodes is smaller than for parallel magnetizations. We analyze the phase
volume of these configurations with respect to magnitudes and relative
orientations of the on-site hyperfine fields. The effect of sign reversal of
TMR is exclusively due to interference of the spin-flip amplitudes on each
site, it does not emerge within commonly accepted probabilistic description of
spin transport. Another feature specific to multistep inelastic tunneling is
bouncing of electron between nearest neighbors while awaiting a "hard" hop. We
demonstrate that this bouncing, being absolutely insignificant for conduction
of current, can strongly affect the spin memory loss. This effect is also of
interference origin.
