The possibility of thermomagnetic bubble memory was investigated based on recent observations that domains can be written and erased simply by modulating the laser power without changing the bias field [1,2]. We focus our attention on writing with a rapidly pulsed laser beam. Writing process on TbFeCo films was observed real-time with a video camera mounted on a polarized optical microscope.
It was found that a thermomagnetically written domain may expand and shrink reversibly at a fixed bias magnetic field when laser pulses with various lengths are irradiated. When a spot on a film was irradiated with pulses with successively shorter time durations, the domain shrank and finally collapsed after the domain size became less than 0.2 μm. The domain size can be controlled reversibly by changing the pulse widths.
When a rapidly pulsed laser beam is scanned on the film, a cylindrical domain appears at the beam spot. When the beam spot is moved at a speed of about 1 μm/s, the domain follows the beam spot without any trace left behind. Small domains in the path are swallowed and disappear. However, when the path of the beam spot crosses a large domain, a stripe domain starts to be formed. The stripe shrinks when the scanning direction is reversed. When the stripe crosses another domain, the older domain has a tendency to be dragged to the direction of the scan. These phenomena are explained by assuming contraction of a cylindrical domain during the cooling process.