  Temperature plays an important role in spin torque switching of magnetic
tunnel junctions causing magnetization fluctuations that decrease the switching
voltage but also introduce switching errors. Here we present a systematic study
of the temperature dependence of the spin torque switching probability of
state-of-the-art perpendicular magnetic tunnel junction nanopillars (40 to 60
nm in diameter) from room temperature down to 4 K, sampling up to a million
switching events. The junction temperature at the switching voltage---obtained
from the thermally assisted spin torque switching model---saturates at
temperatures below about 75 K, showing that junction heating is significant
below this temperature and that spin torque switching remains highly stochastic
down to 4 K. A model of heat flow in a nanopillar junction shows this effect is
associated with the reduced thermal conductivity and heat capacity of the
metals in the junction.
