Number 739, July 29, 2005
by Phil Schewe and Ben Stein
New Spintronic Speed Record
Spintronics is the science devoted to
gaining greater control over digital information processing by
exploiting electron spin along with electron
charge in microcircuits. One drawback to implementing a scheme of
magnetic-based memory cells for computers has been the relatively
slower speed of spin transistors. Hans Schumacher of the
Physikalisch-Technische Bundesanstalt, Braunscheig, Germany, has now
devised the fastest-yet magnetic version of a random access memory
(MRAM) cell, one that switches at a rate of 2 GHz, as good as or
better than the fastest non-magnetic semiconductor memories.
The
MRAM architecture is a sandwich, consisting of two magnetic layers,
with a tunneling layer in between. When the magnetic layers are
aligned (their spin orientation is the same) resistance in the cell
is low; when they are counter-aligned resistance is high. These two
conditions establish the binary 1 or 0 states. The speed of writing
or reading data to and from the cells has, for MRAMs, been limited
to cycle times of 100 MHz by magnetic excitations in the layers.
This problem has now been overcome, according to Hans Schumacher
(hans.w.schumacher@ptb.de), through a novel approach referred to as
ballistic bit addressing.
In the case of the new MRAM architecture,
the influence of magnetic excitations is eliminated through the use
of very short (500 picosecond)
current pulses for carrying out the write operation and that even a
bit whose value will remain the same undergoes a complete 360-degree
precession, whereas a change of status (say, from a 0 to a 1) will
be achieved by pivoting the magnetic status of the cell through 180
degrees.
The 2-GHz switching speed (the rate at which writing can be
accomplished) is faster than static RAM (or SRAM) memories,
currently the fastest memories, can accomplish. Furthermore, the
magnetic memories are non-volatile, which means that the status of
the memory does not disappear
if the computer is shut down.
(
Schumacher, Applied Physics Letters,
25 July 2005; and Journal of Applied Physics, to appear 1 August 2005;
general MRAM website at
www.mram-info.com)
Vibration as a Form of Artificial Gravity
French scientists have
studied how the transition from liquid to gas and back again slows
down in a weightless environment and how an artificial form of
gravity can be simulated using high-speed vibration of the sample.
This work has implications for work in space, where fluids don't
behave the way they do on the ground.
Past studies have shown that
vibrating an astronaut's legs and feet helps to prevent muscle decay
or bone decalcification. Daniel Beysens, a researcher at the
Commissariat a l'Energie Atomique (CEA, dbeysens@cea.fr) and his
colleagues study this problem at the much more basic level of
individual bubbles and droplets, and what happens to them when you
add or subtract the effects of gravity.
Movement between liquid and vapor states is aided by buoyancy:
bubbles rise and droplets fall. But without gravity these actions
cease and liquids condense only by the haphazard and slower process
of collision between droplets or bubbles. In the new experiment a
20-cubic-millimeter sample of liquid/gaseous hydrogen was levitated
in a strong magnetic field; the field grabs onto the magnetic
moments of the H2 molecules, helping to suspend them. This
essentially creates an artificial weightlessness (only about 1% of
Earth's gravity remains) and allows one to see how capillary
forces and "wetting" (the process by which a liquid layer builds up
on a surface) are dominant in a freefall environment. Then some of
the effects of gravity are artificially added back in, this time in
the form of high-speed but low-amplitude vibrations.
The vibrations
cause motion in the fluid, which induces effects that resemble
gravity. Bubbles and droplets go "up" and "down" again when the
vibration is turned on.
As far as simulating gravity, vibrations
seem to work. (
Beysens et al.,
Physical Review Letters, 15 July 2005)
Geoneutrinos Detected
Neutrinos have very little mass and interact
but rarely, but are made in large numbers inside the sun as a
byproduct of fusion reactions. They are also routinely made in
nuclear reactors and in cosmic ray showers. Terrestrial detectors
(usually located underground to reduce the confusing presence of
cosmic rays) have previously recorded these various kinds of nu’s.
Now, a new era in neutrino physics has opened up with the detection
of electron antineutrinos coming from radioactive decays inside the
Earth. The Kamioka liquid scintillator antineutrino detector
(KamLAND) in Japan has registered the presence of candidate events
of the right energy; uncertainty in the model of the Earth’s
interior makes the exact number vague, but it might be dozens of
geo-nu’s.
The neutrinos presumably come from the decays of U-238 or
Th-232. They are sensed when they enter the experimental apparatus,
where they cause a 1000-ton bath of fluid to sparkle. Scientists
believe the Earth is kept warm, and tectonic plates in motion, by a
reservoir of energy deriving from two principal sources: residual
energy from the Earth’s formation and additional energy from
subsequent radioactive decays. The rudimentary inventory of
geoneutrinos observed so far is consistent with the theory.
(Araki et al.,
Nature, 28 July 2005.)
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