  Recent measurements in current-driven spin valves demonstrate magnetization
fluctuations that deviate from semiclassical predictions. We posit that the
origin of this deviation is spin shot noise. On this basis, our theory predicts
that magnetization fluctuations asymmetrically increase in biased junctions
irrespective of the current direction. At low temperatures, the fluctuations
are proportional to the bias, but at different rates for opposite current
directions. Quantum effects control fluctuations even at higher temperatures.
Our results are in semiquantitative agreement with recent experiments and are
in contradiction to semiclassical theories of spin-transfer torque.
