A storage device built using materials such as gadolinium gallium garnet which are can be magnetised easily in only one direction. A film of these materials can be created so that it is magnetisable in an up-down direction. The magnetic fields tend to join together, some with the north pole facing up, some with the south.
When a veritcal magnetic field is imposed on this, the areas in opposite alignment to the field shrink to circles, or 'bubbles'. A bubble can be formed by reversing the field in a small spot, and can be destroyed by increasing the field.
["Great Microprocessors of the Past and Present", V 4.0.0, John Bayko <firstname.lastname@example.org>, Appendix C]