Rare earth iron garnets were used in experiments to determine the feasibility of optically pumping a magnetic material to effect the switching process. It was found that rare earth garnets are limited by an absorption edge, only terbium and dysprosium offer a possibility of pumping at energies below the conduction band edge

Grismore, F. L.

Rhodes, J. E.

English

NASA Technical Reports Server

March 1969	 Brief 69-10060 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151. 
Optically Exciting a Magnetic Memory: A Feasibility Study 
A study has been made to determine the feasibility 
of modifying the magnetic moment of a magnetically 
ordered material by optical pumping for application to 
digital memories. 
The potential of optically addressed (often referred 
to as beam addressable) digital memories has been 
considered by a number of workers and is presently an 
area of much research. The goal to which these efforts 
are directed is the achievement of high-density bulk 
memory with storage capacity of approximately 108 
bits. This density can be achieved in reasonable size, 
for example a planar area of 10 x 10 cm, with bit res-
olution of 10 microns. Such a bit size is about an order 
of magnitude larger than diffraction-limited resolution 
of visible light. Thus, optical approaches appear attrac-
tive. In addition, the interaction of polarized light with 
magnetic materials exhibiting the magneto-Kerr effect 
and Faraday effect provides a means for detecting the 
magnetization state of a local region via reflection or 
transmission providing communication with the 
"store" without interconnecting wires. 
The limiting operation, however, in all presently pro-
posed memory schemes is the write process. This has 
generally been accomplished by applying heat to the 
lattice in order to effect a change in coercivity of a local 
spot. The heat source may be a laser or electron beam; 
both have been proposed and experimentally studied. 
The use of temperature changes to modify the mag-
netic state of a memory medium introduces speed per-
formance limitations. In addition to the relatively long 
thermal time constants, another problem associated 
with thermal excitation is that of thermal creep. In 
order to prevent adjacent bit disturbing effects it ap 
pears necessary to provide thermal barriers between 
storage element locations. This results in a reduction
of achievable storage density and increases the fabri-
cation process complexity. To overcome these limita-
tions associated with thermal excitation, the study 
sought to determine the feasibility of optically pump-
ing a magnetic material to effect the switching proc-
ess. This approach provides the potential of achieving 
excitation and decay times in the sub microsecond 
range without the problem associated with thermal 
creep. The experimental work to date has been direct-
ed toward the rare earth iron garnets. These materials 
show a rapid change in coercive force versus tempera-
ture at the antiferromagnetic compensation tempera-
ture. This effect is attributed to the fact that slight 
variations in temperature cause a net magnetic mo-
ment to exist. A technique is therefore being sought 
for holding the lattice temperature constant while the 
magnetic moment is changed by optical excitation. 
It is apparent from the study to date that the gado-
linium iron garnet crystals grown by the present 
molten flux technique will not exhibit the desired char-
acteristics. The conduction band causing the large 
competing absorption is probably a characteristic of 
the iron—oxygen combination rather than impurity-
induced interband gap levels. As such, the rare earth 
garnets are all limited by this absorption edge. Only 
terbium and dysprosium offer a possibility of pump-
ing at energies below the conduction band edge. Anti-
ferromagnets overcome the problem of lattice pho-
nons creating uncompensation, but little is known of 
the optical properties of the potentially useful ma-
terials. 
The concept of uncompensating an antiferromag-
netically ordered system is of significant interest both 
theoretically and practically. The significance of 
achieving 10 8 bit high-speed random access computer 
(continued overleal) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000060 2020-03-16T17:49:23+00:00Z
memories with passive element reliability should not	 Patent status: 
be understated. Optical pumping is considered a pow- 	 No patent action is contemplated by NASA. 
erful approach towards achieving that goal. 	 Source: F. L. Grismore and J. E. Rhodes 
Note:	 of Georgia Institute of Technology 
Documentation is available from:	 under contract to 
Clearinghouse for Federal Scientific	 Marshall Space Flight Center 
and Technical Information	 (MFS 14854)
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10060
.-
Brief 69-10060	 Category 02


Optically exciting a magnetic memory - A feasibility study

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