Imagine
a computer that doesn't lose data even in a sudden
power outage, or a coin-sized hard drive that could
store 100 or more movies.
Magnetic random-access memory, or MRAM, could make
these possible, and would also offer numerous other
advantages. It would, for instance, operate at much
faster than the speed of ordinary memory but consume
99 percent less energy.
The current challenge, however, is the design of
a fast, reliable and inexpensive way to build stable
and densely packed magnetic memory cells.
A team of researchers at The Johns
Hopkins University, writing in the Jan. 13 issue
of Physical Review Letters, has come up with one
possible answer: tiny, irregularly shaped cobalt
or nickel rings that can serve as memory cells. These "nanorings" can
store a great quantity of information. They also
are immune to the problem of "stray" magnetic
fields, which are fields that "leak" from
other kinds of magnets and can thus interfere with
magnets next to them.
"It's the asymmetrical design that's the breakthrough,
but we are also very excited about the fast, efficient
and inexpensive method we came up with for making
them," said paper co-author Frank Q. Zhu, a
doctoral candidate in the Henry A. Rowland Department
of Physics and Astronomy in the Krieger School of
Arts and Sciences at Johns Hopkins.
The nanorings are extremely small, with a diameter
of about 100 nanometers.
A single nanometer is one billionth of a meter. A
single strand of human hair can hold 1 million rings
of this size, Zhu says.
The asymmetrical design allows more of the nanorings
to end up in a so-called "vortex state," meaning
they have no stray field at all. With no stray field
to contend with, Zhu's team's nanorings act like
quiet neighbors who don't bother each other and,
thus, can be packed together extremely densely. As
a result, the amount of information that can be stored
in a given area is greatly increased.
Fabrication of the nanorings is a multistep procedure
involving self-assembly, thin film deposition and
dry etching. The key to creating the irregular rings,
Zhu said, is to -- while etching the rings with an
argon ion beam at the end of the process -- tilt
the substrate on which the rings are formed.
"In our previous study, we found that 100 nanometer
symmetric nanorings have only about a 40 percent
chance to get vortex state," Zhu said. "But
the asymmetric nanorings have between a 40 percent
and 100 percent chance to get vortex state. This
chance can be controlled on-demand by utilizing the
direction of magnetic field."
The research is funded by the National Science Foundation.
The Physical Review Letters paper will be available
online Jan. 12 here:
http://scitation.aip.org/dbt/dbt.jsp?KEY=PRLTAO&Volume=96&Issue=1
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