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A model of a spin valve in which electron transport between the magnetizedelectrodes is due to multistep tunneling is analyzed. Motivated by recentexperiments on organic spin valves, we assume that spin memory loss in thecourse of transport is due to random hyperfine fields acting on electron whileit waits for the next tunneling step. Amazingly, we identify the three-stepconfigurations of sites, for which the tunnel magnetoresistance (TMR) isnegative, suggesting that the resistance for antiparallel magnetizations of theelectrodes is smaller than for parallel magnetizations. We analyze the phasevolume of these configurations with respect to magnitudes and relativeorientations of the on-site hyperfine fields. The effect of sign reversal ofTMR is exclusively due to interference of the spin-flip amplitudes on eachsite, it does not emerge within commonly accepted probabilistic description ofspin transport. Another feature specific to multistep inelastic tunneling isbouncing of electron between nearest neighbors while awaiting a "hard" hop. Wedemonstrate that this bouncing, being absolutely insignificant for conductionof current, can strongly affect the spin memory loss. This effect is also ofinterference origin.

Tunnel magnetoresistance in organic spin valves in the regime of multi-step tunneling