  We analyze the statistical distribution of switching durations in
spin-transfer switching induced by current steps, and discuss biasing
strategies to enhance the reproducibility of switching durations. We use a
macrospin approximation and model the effect of finite temperature as a
Boltzmann distribution of initial magnetization states (adiabatic limit). We
compare three model spin-valves: a spin-valve with a free layer whose easy axis
is parallel to the pinned layer magnetization (standard geometry), a pinned
layer with magnetization tilted with respect to the free layer easy axis
(pinned layer biasing), and a free layer whose magnetization is pulled away
from easy axis by a hard axis bias (free layer biasing). In the conventional
geometry, the switching durations follow a broad regular distribution, with an
extended long tail comprising very long switching events. For the two biasing
strategies, the switching durations follow a multiply-stepped distribution,
reflecting the precessional nature of the switching, and the statistical number
of precession cycles needed for reversal. We derive analytical criteria to
avoid switching events lasting much longer than the average switching duration,
in order to achieve the highest reproducibilities. Depending on the current
amplitude and the biasing strength, the width of the switching time
distribution can be substantially reduced, the best reproducibility being
achieved for free layer biasing at overdrive current of a few times unity.
