IEEE Spectrum August, 2011 - 41

3. CONSOLIDATE
Use heat and pressure to
compact the mixture into
a dense magnet.

We are also trying to build composite magnets from the bottom up. But
we make nanosize magnetic particles
using a variation on a standard metallurgical technique: ball milling. When
a powdered substance is stirred, tumbled, or shaken in a vial filled with steel
balls, the particles usually break up but
sometimes stick together. The trick is
to do the milling in a liquid that resembles gasoline, to which a small amount
of a fish-oil-like surfactant is added.
That greatly reduces the tendency of
the particles to stick together, because
the surfactant molecules surround the
particles, keeping them apart.
Such surfactant-assisted ball milling can produce magnetic particles
that are only a few nanometers across.
We have also managed to obtain
samarium-cobalt flakes that are a few
tens of nanometers thick, which should
work well as the high-stability component in a composite nanomagnet. We
can easily produce large quantities of
these magnetic flakes, but we continue
working to control their sizes better
and to figure out ways to protect them
from oxidation, which acts fiendishly
fast with things this tiny.
Our ideas for how these particles
could be combined with others of high
saturation magnetization and how the
resulting mixture could be consolidated
into dense nanocomposite magnets are
admittedly somewhat sketchy. Twice, in
2002 and in 2007, groups that included
researchers from Georgia Tech, IBM,
Iowa State University, Louisiana Tech
SPECTRUM.IEEE.ORG

University, the University of Texas, and
Chiba Institute of Technology, in Japan,
employed similar techniques to fabricate nanocomposite magnets. But they
used two different iron-platinum alloys,
which resist oxidation. Because those
particles were assembled randomly and
because the chosen high-saturation particles had a magnetization that wasn't
all that high, the magnets they made
didn't perform well.
One relatively simple way to assemble these two kinds of particles is by
depositing the high-saturation ones as
a thin coating on high-stability cores.
The easy direction of magnetization of
the cores would have to be aligned in an
applied magnetic field before the mixture was compacted. The particle coatings would then merge into a network
of thin layers. In an experiment carried
out in cooperation with Electron Energy
Corp. of Landisville, Penn., we tested
this idea by coating relatively coarse
copper particles with iron nanoparticles.
After compacting them at high temperature, we observed just such a network of
thin iron layers inside a piece of copper.
Copper isn't magnetic, though, so the
result wasn't a permanent magnet. But
this experiment provided a convenient
way to test our ideas and procedures.
The problem with this approach is
that it works well only when the cores
dwarf the particles in the coating. This
means that only a small amount of the
high-saturation material can be added.
So any nanocomposite magnet made
this way, assuming that all other difficulties are trumped, would not be
much better than magnets made from
whatever material is used for the cores.

e and other researchers
clearly need to explore
other approaches if we
truly want to make real
strides. One possibility is to use surfactants more cleverly so that one set of
magnetic particles becomes negatively
charged on their surfaces while the other
set becomes positively charged. Mixing
the two kinds of particles in a liquid in the
presence of a magnetic field might then
give the desired arrangement, with particles of one type alternating with particles
of the other, with the easy directions of
magnetization neatly lined up.

The problem with using a surfactant
to assist in assembling nanoparticles is
that the molecules of the coating would
be very much out of place in the fi nal
magnet. At best, they would dilute its
magnetization. At worst, they might
even react with the rare earth element,
destroying its stabilizing effect. So we
would expect that the surfactant molecules would have to be removed after
assembly, either by dissolving them
chemically or by heating the mixture until these molecules decompose
or evaporate.
Assuming that the desired nanoparticle assemblies could be arranged
in this way, the results still wouldn't
make for very good permanent magnets: They would neither be mechanically strong nor sufficiently dense.
Somehow you'd have to compact them,
a step that we don't think would be
any easier than the preceding ones. If
you use only pressure to do that, for
example, the pressures would need
to be impractically high, because
hard alloys like samarium-cobalt are
exceedingly difficult to pack together.
You'd need to use explosives to achieve
the required amount of squish, and
that doesn't make for an orderly
assembly line! A combination of pressure and heat would probably work
better, but heating might coarsen the
particles or initiate unwanted chemical reactions.
This list of obstacles gives you an
idea why progress in the not-so-young
quest for nanocomposite magnets has
been advancing so slowly. But thanks
to more than a few incremental successes, we not only understand what
we want to achieve, we also clearly see
a path to getting there. There is a feeling within the community of researchers working on this problem that any
day now our efforts will be rewarded
with an impressive next-generation
supermagnet. And even if that great
magnet of the future proves a long
time coming, we are still bound to find
a way to make it, one tricky step at a
time. Get your refrigerator ready. ❏
J OIN TH E D IS CUS SIO N Post your
comments at http://spectrum.ieee.org/
magnets/0811

AUGUST 2011 * IEEE SPECTRUM * NA

http://spectrum.ieee.org/ http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum August, 2011