Q. Illustrate working of Magnetic Bubble Memories?

In many materials like garnets on applying magnetic fields certain cylindrical areas whose direction of magnetization is opposite to that of magnetic field are created. These are known as magnetic bubbles. Diameter of these bubbles is found to be in range of 1 micrometer. These bubbles can be moved at high speed by applying a parallel magnetic field to plate surface. So rotating field can be produced by an electromagnetic field and no mechanical motion is needed. 

In these devices deposition of a soft magnetic material known as Perm alloy is made as a predetermined path so making a track. Bubbles are forced to move continually in a fixed direction on these tracks. In these memories presence of a bubble signifies a 1 while absence signifies a 0 state. For writing data in a cell a bubble generator to introduce a bubble or a bubble annihilator to eliminate a bubble are needed. A bubble detector performs read operation. Magnetic bubble memories having capacities of 1M or more bits per chip have been created. Performance and cost of these memories fall between semi-conductor RAMs and magnetic disks.

These memories are non-volatile in comparison to semi-conductor RAMs. Additionally because there are no moving parts they are more reliable than magnetic disk. However these memories are hard to manufacture and hard to interface with in conventional processors. These memories at present are used in specialized applications for example like a secondary memory of air or space borne computers where very high reliability is needed.