  We present experimental results and macrospin simulations of the switching
characteristics of orthogonal spin-transfer devices incorporating an
out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve
device at cryogenic temperatures. At $T=4$ K we demonstrate: high speed
deterministic switching at short pulse lengths - down to 100 ps - with
sufficient measurement statistics to establish a switching error rate of
$10^{-5}$; coherent precessional switching at longer times; and ensemble
decoherence effects at even longer times. Finite temperature macrospin models
capture the precessional switching well but fail to fully reproduce all the
decoherence and switching error behavior.
