  Engineering phase change materials (PCM) to realize superior data storage
devices requires a detailed understanding of crystallization kinetics and its
temperature dependence. The temperature dependence of crystallization differs
distinctly between crystallizing from the glassy phase and the undercooled
liquid (UCL). Hence, knowing the phase from which crystallization occurs is
necessary for predicting the switching ability. Here, we measure the glassy
dynamics and crystallization kinetics using calorimetry for heating rates
spanning over six orders of magnitude. Our results show that the prominent PCM
(Ag,In)-doped Sb2Te (AIST) exhibits a change from crystallizing from the glassy
phase to crystallizing from the UCL at a critical heating rate of 5,000 K/s.
Above the glass transition, the activation energy of crystallization changes
drastically enabling rapid crystallization at elevated temperatures.
