Magneto-Optical Variable Memory Based Upon the Properties of a Transparent Ferrimagnetic Garnet at Its Compensation Temperature

Abstract

The requirements to be met by a changeable memory medium for flying spot variable memory system are discussed. Although simple reversible photochromic materials satisfy most of these requirements, their lack of a writing threshold limits their applicability. The application of a transparent ferrimagnetic garnet near its compensation temperature is discussed. At the compensation temperature the net magnetization of the garnet is zero so that the remanent magnetization of the sublattices normal to the surface of a thin crystal is not subject to demagnetization fields. The magnetization of the iron sublattice of the garnet produces a large magneto‐optical rotation of the polarization of a transmitted light beam in opposite senses for the two directions of the remanent magnetization, which can, therefore, be interrogated non‐destructively. In passing through the compensation temperature, the coercive field for a magnetization reversal passes through a sharp maximum. Thus, the direction of remanent magnetization can be changed by applying a magnetic field in coincidence with a high‐intensity pulsed light beam which produces a transient temperature excursion of ∼3°C from the compensation temperature. This provides a writing‐threshold making it possible to change the remanent magnetization of a selected crystal in an array without disturbing the magnetization of the remainder. Practical aspects of this approach are discussed.