Magnetic Bubble

a portion of computer memory which consists of a small region in a material such as garnet (a silicate mineral), which is magnetized in one direction. Slices of this material placed on a SUBSTRATE (base material) produce a magnetic chip that under a magnetic field produces magnetic bubbles. Information can be stored on the chip, which may contain up to one million bubbles in 20 square millimetres, in BINARY form, through the presence or absence of a bubble in a specific location.

Taken from Dictionary of Science

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The magnetic bubble apparatus consists of a thin (8-12μm) single crystal film of Ferromagnetic Garnet (FMG) sandwiched between a pair of crossed Polaroids. The FMG crystals are magnetically anisotropic, that is, they have a strong tendency to orient themselves in fixed directions under the influence of an external magnetic field. The preferred or "easy" axis of orientation is perpendicular to (in or out of) the crystal surface. With no external magnetic field, the domains in the crystal orient up or down in roughly equal amounts. Polarized light passing through the crystal will have its plane of polarization rotated by due to interaction with the magnetic field of the domains (an effect called Faraday rotation). For the 'up' domains, the light will be crossed with respect to the exiting Polaroid therefore appearing dark, and for 'down' domains uncrossed (or vice versa) so appearing bright. The domains appear as serpentine patterns of alternating bright and dark (figure 1a.). Application of an external magnetic field (provided by a built-in electromagnet) flips the domains to one preferred orientation. As the field is increased, the serpentine patterns gradually disappear until you are left with one or two strings and isolated "bubbles".

Taken from fas.harvard.edu