Researchers in Maryland’s John Hopkins University have used
asymmetry to shrink the storage element of ring-type magnetic RAM
(MRAM) to less than 100x100nm per bit.
MRAM requires some form of magnetic structure to have two stable
magnetic states that can be manipulated predictably.
One structure for which read and write techniques are already
established is the ring in which magnetism can run around in ether
a clockwise or anticlockwise direction. These so called ‘vortex’
states are stable and rings can be packed closely as they produce
almost no external field.
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Frank Zhu
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“The vortex states of nanorings are robust states, much more
stable than thin films. In principle, these vortex states can be
[accidentally] erased, but a very large magnetic field is
required,” John Hopkins scientist Frank Zhu told Electronics
Weekly.
In switching between the vortex states, the ring passes through
a two-magnetic-domain state with domain walls on opposite sides of
the ring.
As the ring gets smaller, below around 1µm for the cobalt rings
proposed for vortex MRAM, this undesirable state frequently fails
to resolve into a vortex and jams in the ring as a third stable
state.
By 100nm diameter, vortices only form in 40 per cent of
transitions.
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Zhu's group makes nanorings by
depositing a layer of polystyrene
nanospheres, sputtering a layer of
cobalt, then eroding unprotected
cobalt with an ion beam. Tilting the
substrate to between 10 and 14°
during erosion forms asymmetric
rings |
However, by fabricating slightly asymmetric nanorings and applying
an external bias field which together manage the way transitional
domain walls move with respect to one another, Zhu has pushed
vortex formation to 98 per cent of transitions in 100nm rings.
For a future MRAM, Zhu thinks slightly larger rings are needed.
“The memory cell that I’m working on is about 100nm in diameter, It
could also be as small as 50nm in diameter,” he said. “For
application, I think 100nm is a little bit challenging to
fabricate, but 500nm and above should be no problem.”
Experiments so far, using bulk read and write to billions of
rings on a substrate, indicate a vortex MRAM would be fast. “The
read time is 1ns and write time is 0.8ns,” said Zhu.
www.jhu.edu