  We have investigated the magnetic phase diagram of a fully oxygenated
detwinned YBa2Cu3O7 single crystal by means of a combination of a
low-temperature thermal analysis (DTA) technique with a simultaneous
application of a small oscillating magnetic field. We observed a dip-shaped
feature in magneto-caloric data deep inside the vortex-solid region of
YBa2Cu3O7, which was activated by the oscillating magnetic field. The feature
can be explained by a self-heating effect due to vortex motion. However, the
origin of this new feature is unknown.
