  Glass-like arrest has recently been reported in various magnetic materials.
As in structural glasses, the kinetics of a first-order transformation is
arrested while retaining the higher-entropy phase as a non-ergodic state. We
show visual mesoscopic evidence of the irreversible devitrification of the
arrested antiferromagnetic-insulating phase in
$Pr_{0.5}Ca_{0.5}Mn_{0.975}Al_{0.025}O_3$ to its equilibrium
ferromagnetic-metallic phase with isothermal increase of magnetic field,
similar to its iso-field devitrification on warming. The slope of $T_g$ vs
magnetic field in half-doped manganites is shown to have a sign governed by Le
Chatelier's Principle.
