PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 739  July 28, 2005  by Phillip F. Schewe, 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{at}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, August 1; 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 astronauts' legs
and feet help to prevent muscle decay or bone decalcification. Daniel
Beysens, a researcher at the Commissariat a l'Energie Atomique (CEA,
dbeysens{at}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 this 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.,
Physi
cal 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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