Giant fluctuations of local magnetoresistance of organic spin valves

•Conventional Julliere's formula for the tunnel magnetoresistance (TMR) is generalized.•Two spin rotations on the way between the electrodes can lead to negative TMR.•Local TMR of a spin valve is widely distributed with sizable portion of negative values.•The origin of negative TMR is the interference between the spin-flip amplitudes.

We generalize the seminal Julliere formula for the tunnel magnetoresistance (TMR) of a spin valve to include the spin memory loss of an electron in course of travel between the electrodes. This generalized version applies locally and for arbitrary mechanism of the spin dephasing. On the basis of the generalized formula we demonstrate that the distribution of TMR along the surface of magnetized electrodes is very broad and includes the sign reversals.