  Thermally-induced transitions between bistable magnetic states of magnetic
tunnel junctions (MTJ) are of interest for generating random bitstreams and for
applications in stochastic computing. An applied field transverse to the easy
axis of a perpendicularly magnetized MTJ (pMTJ) can lower the energy barrier
($E_b$) to these transitions leading to faster fluctuations. In this study, we
present analytical and numerical calculations of $E_b$ considering both
coherent (macrospin) reversal and non-uniform wall-mediated magnetization
reversal for a selection of nanodisk diameters and applied fields. Non-uniform
reversal processes dominate for larger diameters, and our numerical
calculations of $E_b$ using the String method show that the transition state
has a sigmoidal magnetization profile. The latter can be described with an
analytical expression that depends on only one spatial dimension, parallel to
the applied field, which is also the preferred direction of profile motion
during reversal. Our results provide nanodisk energy barriers as a function of
the transverse field, nanodisk diameter, and material characteristics, which
are useful for designing stochastic bitstreams.
