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 Johns Hopkins has come up
with one possible answer: tiny asymmetrical 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. Chia-Ling
Chien, a professor of physics and astronomy, headed up the
research team, whose findings are scheduled for the Jan. 20
issue of Physical Review of Letters.
"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
Krieger School's Henry
A. Rowland Department of Physics and Astronomy.
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 could hold 1
million rings of this size, Zhu said.
The asymmetrical design allows more of the nanorings
to end up in a so-called "vortex state," meaning that 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 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 asymmetrical rings, Zhu said,
is tilting the substrate on which the rings are formed
while etching them with an argon ion beam at the end of the
process.
"In our previous study, we found that 100 nm 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."
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