What it shows:
A thin wafer of Ferromagnetic Garnet reveals its magnetic domain alignment as light and dark serpentine patterns when viewed between crossed Polarizers. These domains can be flipped by an external magnetic field, changing the pattern structure.
How it works:
The magnetic bubble apparatus consists 1 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" (figure 1b.).
Figure 1. Magnetic bubble domains subject to a small (a.) and large (b.) external magnetic field.
Setting it up:
The crystal is about 2mm square inside a 4cm diameter 1cm deep sealed container that also holds the electromagnet and polarizers. This container sits on a microscope table with. 40x-100x magnification. The electromagnet requires a variable 6V DC 1A low ripple supply. A strong permanent magnet held near the crystal can also be used for less regulated flipping (one is provided with the apparatus kit).
Comments:
The FMG crystal is grown on the top face of a second, transparent, single crystal (for this apparatus it has the formula Bi0.6Tm2.4Ga1.15Fe3.85O 12). This base crystal, also about 8μm thick, has a similar lattice size and fixes the domain orientation of the FMG as it grows. the Scientific American reference discusses magnetic bubbles as having possible computer memory applications.
References:
A. H. Bobeck, H. E. D. Scovil, Scientific American June 1971, 78
B. K. Tanner, Am. J. Phys. 48, 59 (1980)
1 Model LF.141, TEL-Atomic inc., 1223 Greenwood Ave. Jackson Mi49204 / Lennox Scientific, Ireland