TEL-300

Principle
In the search for magnetic materials which could be used for bubble memories, Bobeck discovered that ferrimagnetic garnets (FMG) could be "engineered" to produce the very small, isolated cylindrical domains (called bubbles because of the end-on shape when viewed) which were stable yet capable of being formed, moved and destroyed at very high rates.

To achieve the correct orientation of the FMG it has to be grown on the face of another single crystal of similar lattice size. In this way both the magnetic and optical properties can be controlled.

The material in this apparatus has the formula Bi0.6Tm2.4Ga1.15Fe3.85O12 and is about 8um thick and transparent.

Typical views as seen through a microscope X100 magnification

Faraday Effect
Materials are magnetic because of the anisotrophy and this in turn means there is interaction between the electrons when any electromagnetic radiation passes through. The resulting effect is to rotate the plane of vibration of the radiation — the so-called Faraday effect. Thus the plane of polarized light entering such a medium will be rotated. The amount of rotation depends on path length and the amound of interaction — optical activity.

The film of FMG is transparent and yellow in unpolarised light, but when viewed through crossed Polaroids, the pattern of the domains becomes visible in the unmagnetised remnant state or a striped (serpentine) form. To view the domain patterns a microscope of about x100 magnification is needed.

Forming Bubbles
If the FMG is maintained in a static transverse magnetizing field, bubbles can be produced by small local changes in the field.

Plotting M-H Loops
Graphs can be plotted from measurements of the magnetization M of the material resulting from a magnetizing field of strength. In general, M will follow H up to saturation but not necessarily linearly. After saturation, M remains constant with increasing H. What happens to M when H is reduced to zero and eventually reversed depends on the nature of the material.

In the unmagnetised state FMG contains equal volumes of both kinds of domains, and when an external magnetizing field H is applied, magnetization occurrs by the growth of one domain at the xpense of the others. The intensity of light transmitted through FMG is proportional to the magnetization M, so by using a photo cell a measure of M can be obatined. Since the magnetic field H is directly proportional to the current in the coil producing it, M-H loops can be plotted.

Each Magnetic Domain includes:
Magnetic bubble apparatus constructed with solid brass body containing a wire coil of 300 turns; Connector plug with hookup wire; Complete experiment instructions.

Accessories required for viewing: (not included)
Microscope (x100 magnification); Low ripple power supply - 6 VDC, 1 amp or battery; Ammeter; Potentiometer; Light dependent resistor (for plotting hysteresis loops).