Biomedical Engineering Reference
In-Depth Information
scan performed on the sample at a temperature of 12 K. Two effects are noted.
First is the shift of the peaks. The second is the sharpening of the peaks due
to the decrease of the phonon energy broadening.
3.12 DesignExample:OpticallyAddressed
High-Speed,Nonvolatile,Radiation-
HardenedDigitalMagneticMemory
Military and space applications require a number of unique memory char-
acteristics. These include [47] nonvolatility, maintainability, security, low
power and weight, small physical size, radiation hardness, and the ability to
operate in severe environments (temperature, humidity, shock and vibration,
and electromagnetic fields). The need for radiation hardness is of special
interest where the use of various directed energy weapons (DEW) against
satellite assets is anticipated. The goal of DEW is to destroy at least part of
the threat nuclear warheads in the exoatmosphere. This scenario will expose
the on-board electronics and associated computer memory to many types of
high-level radiations.
Considering these requirements, along with the goals of further improved
life cycle cost, programmability, speed, volume, and power consumption of
present devices—the crosstie memory system discussed herein represents
an excellent choice for nonvolatile memory in a radiation environment. The
design discussion increases the read speed of this memory by more than an
order of magnitude, providing the possibility for multiple applications.
3.12.1 History of the Magnetic Crosstie Memory
The crosstie magnetic effect in thin film magnetic material was discovered
in 1958 by E.E. Huber. The magnetic thin film exists in two possible mag-
netic states: the Neel wall state which represents the fundamental magnetic
domain wall; and the crosstie/Bloch line pair on the Neel wall, which rep-
resents the second magnetic domain state. The crosstie wall constitutes a
transition form between Bloch walls of very thick films and the Neel walls of
very thin films. As the film thickness is decreased, the Neel wall forms seg-
ments of opposite polarity, which reduces the magnetostatic energy in the
wall. These segments are separated by perpendicular magnetization circles
called Bloch lines, which minimize energy by forming spiked walls or cross-
ties on alternate Bloch line locations. The crosstie Bloch line pairs represent
digital information stored in the computer memory.
Work began in the early 1970s to develop a computer memory utilizing
serrated strips of thin film 80-20 Ni-Fe permalloy data tracks that stored
information in a series of crosstie Bloch line patterns on a Neel wall. The
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