Physicists have moved a step closer to creating a new generation of
"spintronic" devices that exploit the spin of electrons as well as their
charge. Jing Shi and colleagues at the University of Utah in the US have
made the first organic "spin valve" - a device that changes resistance
depending on the applied magnetic field. Previous spin valves were made from
metals or insulators (Z Xiong et al. 2004 Nature 427 821).
A spin valve consists of a thin layer of metal or insulator sandwiched
between two ferromagnetic electrodes. The spin of the electrons passing
through the device can be flipped by an external magnetic field, which
changes the resistance of the two ferromagnetic layers. This effect, known
as magnetoresistance, has already been used to make highly sensitive
magnetic-recording devices and memory chips.
Extending these spin-dependent effects to semiconductor materials has,
however, proved difficult. Shi and co-workers have now built a spin valve
with a 100 nanometre thick organic semiconductor made from aluminium and
hydroxyquinoline. The semiconductor was sandwiched between a layer of cobalt
and an alloy of lanthanum, strontium and magnesium (see figure).
To test their device, the Utah team first calculated the current that flowed
through the semiconductor when the two electrodes were magnetized in the
same direction - or parallel - and then in opposite directions - or
anti-parallel. Shi and colleagues found that that the current increased by
as much as 40% when the magnetization of the electrodes was switched from
anti-parallel to parallel. This constitutes giant magnetoresistance.
At present, the device only works at low temperatures - between about -260°C
to about -40°C - but Shi's team says that the experiment is "a proof of
concept that sets the stage for more practical applications". The long-term
aim is to make the device work at room temperature. The group believes that
organic semiconductors have many advantages over conventional
semiconductors, such as those made from silicon. They are simpler to make,
are flexible and their resistance can be tuned by doping.
Belle Dumé is Science Writer at PhysicsWeb
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