Thin films of MnBi for beam-addressable-beam-alterable optical memory applications

Abstract

Laser Curie-point writing on thin magnetic films in conjunction with a magneto-optic readout technique provides a number of advantages over presently available mass memory systems. This approach, however, imposes stringent requirements on the material properties of the film memory plane. We have found that thin films of MnBi possess many unusual physical properties and are particularly suitable for this application. Films prepared on mica substrates typically have a specific Faraday rotation of 5 \times 10^{5} degrees/cm, and an absorption coefficient of 2 \times 10^{5} cm-1for 6328A wavelength, permitting a readout rate of 109 bits/sec at 1 mw laser power level using available detectors. Heating of a spot 3 microns in diameter in a 1000A thick film with a 10-mw Gaussian laser beam has been calculated using the heat flow equation. The result shows that the entire spot can be raised to above the Curie temperature (345°C) from 20°C at the end of a 1µ sec laser pulse. For a 106 bits/cm2packing density, heating at adjacent spots is negligible. Experimentally, we have been able to write a manually scanned line of 3.7µ width with a 10-mw laser on a SiO coated Mn, Bi film in air. To reduce the required writing magnetic field, films of discrete squares of 4µ sides and 12µ separation have been prepared. Calculations show that the writing magnetic field in this case can be as low as 10 Oe. Analytical and experimental results as well assystem considerations will be presented.