Magnetic films and optics in computer memories

The thermal cycle-time and read bandwidth are calculated for a magnetic film memory which is written by heating from focussed light and read by the same beam at reduced power. The heat flow model approximates the memory bit as a planar disc source and the cycle time
T
is determined by the condition that, for repetitive writing, the ratio of transient to dc temperature must be >1. Then it is found that the bit radius
a
must be
< \sqrt{kT}
= thermal diffusion length in time
T
(
k
= thermal diffussivity of substrate); for
T < 1\mu
s and an Ag substrate this gives
a \sim 5 \mu
. For a magneto-optical reflection coefficient
k \sim 10^{-3}
the shot-noise limited bandwidth is then found to be ∼100 MHz for a temperature rise of 10 degrees in the memory bit. Surface noise due to imperfections will limit the bandwidth to less than the shot-noise limit unless some form of signal processing can be used to extract the magnetic information. For incident light polarized at, say, 45° to the plane of incidence, phase modulation of one of the components will generate a magneto-optical signal with a phase of 0 or π relative to the modulation, depending on the magnetic state of the memory bit. Additional topics which are considered include a review of magneto-optical enhancement using multilayer dielectric films, a discussion of possible memory materials, and a proposal for a content-addressed memory which is interrogated magneto-optically.