Magnetic  bubble  memory  has  enjoyed a short  but  extremely

          controversial history.  The  theoretical  groundwork  for  bubble

          memory was developed in the late 1960's by  a distinguished group

          of Bell  Laboratory engineers and scientists.  The electronic and

          computer industries were enthusiastically  optimistic  about  the

          future  of  this   technolgy.    Industry   analysts   forecasted

          wide-spread use of bubble memories in  computers  of  all  sizes.

          They  predicted  the  development   of  a  muilti-billion  dollar

          industry  by  the  1980's.[1]  However, bubble memory  has  never

          reached   its   high  expectations.    Production   difficulties,

          marketing problems, and  the  rapid  decline  in  the  prices  of

          competing  technologies have plagued bubble memory.  Even  though

          bubble memory has failed to  realize  its  forecasted  potential,

          this technology has had an impact in the  computer industry.  The

          unique attributes of bubble  memory  have  allowed  it  to  claim

          specialized niches in the computer memory market.


             Magnetic bubble memories owe  their  existence  to the unusual

          properties of specially prepared garnet films.  In the absence of

          a magnetic field, garnet film is composed of oppositely polarized

          regions.  The film is  divided  into  minute  areas  of  upwardly

          directed and downwardly directed magnetization.  These regions of

          opposite polarity are  evenly  interspersed  throughout the film.

          When a weak  magnetic  field  is  applied normal to the film, the

          magnetized areas parallel to the applied field  will  grow  while

          the other magnetized areas will shrink.  The magnetization of the

          film will eventually reach a point that it will consist of small,



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          circular, oppositely charged  areas  surrounded by areas that are

          magnetized parallel to the applied field.  When  viewed  under  a

          microscope,  these  small, circular areas resemble  tiny  bubbles

          floating in water.  The tiny magnetized bubbles act  as  minature

          storage areas where information can be placed  (hence,  the  name

          bubble  memory).   If  the  applied magnetic field  is  increased

          beyond a certain point, the bubbles will collapse  and the garnet

          film  will  be  uniformily  magnetized  parallel  to the  applied

          field.  However, when the applied field is lowered  the film will

          revert   to  its  original  structure  containing  new   magnetic

          bubbles.[2]


             There are three important attributes of garnet film's magnetic

          properties that should be  discussed.   First,  under a specified

          range of  applied  field  garnet  film  will  maintain  a  stable

          environment  of magnetic bubbles.  In the stablized  environment,

          each  magnetic  bubble  will  retain  its  structural  integrity.

          Therefore,  the  bubbles  can  act  as  reliable  storage  areas.

          Second,  by  slightly adjusting the applied  field  the  magnetic

          bubbles can be distorted.  Lowering the applied field will  cause

          the bubbles to expand, while increasing the magnetic  field  will

          cause the bubbles to contract.  This type of manipulation  allows

          the bubbles to be read and replicated  without  destroying  them.

          Third, the bubbles can be destroyed by  increasing  the  magnetic

          field beyond a threshold point.  This feature allows the  storage

          area (namely, the bubble) to  be  cleared for further use.  These

          three  attributes  account for the ability of garnet film  to  be



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          used in a storage memory device.


             There  are  four important functions that  a  computer  memory

          device should perform: 1) storage of information, 2) retrieval of

          information, 3)  writing new information, and 4) erasing unneeded

          information.  The  bubble  memory  device  (consisting  of garnet

          chips and the magnetic parts necessary to create the proper field

          about  th