Abstract
The "softening" process of the first kind of dumbbell domains (IDs) with the increase of temperature was experimentally revealed as follows: IDs first convert to ordinary hard bubbles (OHBs) through a partial loss of the vertical Bloch lines (VBLs) in their walls and then these OHBs convert to soft (normal) bubbles (SBs) through a further loss of VBLs. As a result, the critical temperature for the break down of VBL chains of IDs is apparently higher than OHBs.