The formation of hard bubbles (including dumbbell domains) in epitaxial garnet films by using a series of pulse bias fields was investigated in some detail. It was found that the effect of a series of pulses is not simply the adding up of that of a single pulse (the physical process of hardening of soft domain segments), but has its own regularity, there exist two other physical processes, i.e. further hardening and softening of hard domain segments. The experimental results can be reasonably interpreted in terms of the above-mentioned three processes.