Number 281, July 29, 1996 by Phillip F. Schewe and Ben Stein
SOLAR NEUTRINO FLUX IS NOT CORRELATED WITH SUNSPOT ACTIVITY. The Kamiokande
detector, situated a kilometer underground west of Tokyo, has been watching
the sun since January 1987. It sees not the photons that come from the
bright surface but rather the neutrinos that issue from the sun's core.
In the past decade Kamiokande has ascertained several facts: neutrinos
do come from the direction of the sun (in case there were any doubt); the
neutrinos are largely those from the decay of boron-8 (other detectors
specialize in neutrinos coming from additional nuclear reactions in the
sun); and the neutrinos show no day/night preference. Having monitored
our local star over almost a complete 11-year solar cycle, the researchers
(Yoichiro Suzuki, suzuki@icrkm4.icrr.u-tokyo.ac.jp) now take stock of their
neutrino inventory and report that the neutrino flux shows no correlation
with sunspot activity. (Y. Fukuda et al., Physical Review Letters, 26 August
1996.)
THE PHYSICS OLYMPIAD IN OSLO , like the sports Olympiad in Atlanta, asks
teenagers to race, hurdle, and vault past well-trained competitors from
around the world. In Norway the obstacles were all on paper. In one event,
for example, the participants were required to determine the size of tides
on Earth in the plane of the Moon's orbit, while not neglecting to allow
for the Earth's rotation. Teams from 55 countries came to the Olympiad,
which ended July 7. China collected the greatest number of points, with
Romania as runnerup. The U.S. was third, followed by Russia, Vietnam, Germany,
and Iran. The five American students all won medals; gold-medalist Christopher
Hirata of Deerfield, Illinois, aged 13, got an award for being the youngest
medalist. (For more information contact Dwight E. Neuenschwander, AIP,
301-209-3010.)
TUNNEL JUNCTION MAGNETORESISTANCE may lead to higher-density magnetic
storage devices. Physicists have known for some time that sandwiches of
alternating magnetic and nonmagnetic microlayers can undergo a change in
electrical resistance in the presence of an external magnetic field (arising,
say, from a tiny domain on a segment of magnetic tape). This magnetoresistance
(MR) effect can be used to decode binary data and has been employed in
reading heads in computer hard drives. Giant magnetoresistance (GMR), a
stronger version of MR, affords even greater data-decoding sensitivity.
Prototype hard-drives with read heads using GMR have achieved areal data
densities of 3 Gbits/sq.in. Tunnel junction magnetoresistance (JMR) is
yet another approach to transforming a tiny magnetic field into a change
in resistance. Unlike the all-metal GMR sensor, a room-temperature JMR
sensor consists of two metal (ferromagnetic) layers separated by an insulating
layer. A JMR trilayer junction tested recently at MIT is only 20 nm thick
and the signal (the fractional change in resistance) was 23%, compared
to a signal of less than 7% for a 40-nm-thick, 4-layer GMR prototype. MIT
physicist Jagadeesh Moodera (moodera@slipknot.mit.edu; 617-253-5423) suggests
that the more compact size, relatively larger signal, and the low sub-nanoamp
operating current of the JMR sensor could make for easier engineering of
devices and lower production costs. An areal density of more than 10 Gbits/sq.in.
is possible, he says. (J.S. Moodera et al., Applied Physics Letters, 29
July.)
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