Roger L. Aagard

Optical mass memory experiments on thin films of MnBi

Experimental studies of laser Curie-point writing, erasure, and magnetooptical read-out on thin films of MnBi have been performed. These films have a thickness of around 800 A, an optical density of 1.6, and a Faraday rotation of 5 degrees (0 to full saturation) at a 6328-A wavelength. Their easy direction of magnetization is normal to the film plane; coercive force H c is between 800 and 1100 Oe; and Curie temperature T c is 360°C. These properties make them well suited for optical mass memory applications. This has been demonstrated with a scanning optical system designed to simulate the functions of a computer memory. A 50-mW He-Ne laser beam is modulated with a Kerr cell, deflected with a galvanometer, and focused onto the film with a microscope objective. Memory bits of 1-μm diameter were written, read, and erased through 105cycles with no degradation of its memory characteristics. The experiments were carried out at room temperature in air.

Experimental evaluation of an MnBi optical memory system

An experimental optical memory system is described for evaluating the optical memory characteristics of MnBi films. The preparation procedure for obtaining large uniform films is discussed. These films require about 10 mW of laser power to write 1-μm bits and a coincident field of 600 Oe to erase them. A packing density of 1.5 × 108bit/in2with a typical signal-to-noise ratio of 10 is readily achieved. Large area films have been sample tested and found to be of usable quality over 99.9 percent of the area. Test bits have been repeatedly cycled to the Curie point more than 106times retaining an adequate signal-to-noise ratio.

Observations of repeated Curie-point writing effects on MnBi films

Curie-point writing on MnBi films with a focused laser beam provides a means of substantially improving the packing density of magnetic recording. The MnBi film, however, has two crystallographic phases with different Curie temperatures. This causes the laser power requirement for writing to become a function of the operating history of the data bit location. Fortunately, the normal room-temperature phase has a higher laser power requirement than the quenched or mixed phase compositions. Therefore, the initial conditions for writing are suffieient for any subsequent condition. The time constants for these trasformations and the resulting optical memory characteristics have been observed and measured. Long-term operational lifetime has also been examined. The results show that when the laser writing power is 10% above writing threshold, the readout signal remains constant indefinitely; whereas when the writing laser power is more than 20% above the threshold value, continual degradation occurs. These experiments further confirm MnBi as a practical magneto-optic storage medium.

Signal-to-noise ratio for magnetooptic readout from MnBi films

The signal-to-noise ratio in an optical memory has been analyzed and examined experimentally. A solid-state detector can be used with MnBi as the storage media, but for media with smaller magnetooptic rotation, a photomultiplier is considered necessary. Analyzer orientation and light beam intensity have been included in the analysis. Results with experimental apparatus employing a crystal beam splitter at 45° angle and solid-state detectors compares well with analysis.