The Free On-line Dictionary of Computing (30 December 2018):

bubble memory

   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.

   Bubble memory is a kind of non-volatile storage but
   EEPROM, Flash Erasable Programmable Read-Only Memory and
   ferroelectric technologies, which are also non-volatile, are
   faster.

   ["Great Microprocessors of the Past and Present", V 4.0.0,
   John Bayko , Appendix C]

   (1995-02-03)