In an external magnetic field B, the spins of the electron and hole will precess in effective fields b(e) + B and b(h) + B, where b(e) and b(h) are random hyperfine fields acting on the electron and hole, respectively. For sparse "soft" pairs the magnitudes of these effective fields coincide. The dynamics of precession for these pairs acquires a slow component, which leads to a slowing down of recombination. We study the effect of soft pairs on organic magnetoresistance, where slow recombination translates into blocking of the passage of current. It appears that when b(e) and b(h) have identical Gaussian distributions the contribution of soft pairs to the current does not depend on B. Amazingly, small inequivalence in the rms values of b(e) and b(h) gives rise to a magnetic field response, and it becomes progressively stronger as the inequivalence increases. We find the expression for this response by performing the averaging over b(e), b(h) analytically. Another source of magnetic field response in the regime when current is dominated by soft pairs is inequivalence of the g factors of the pair partners. Our analytical calculation indicates that for this mechanism the response has an opposite sign.
