JEREMY RAWSON makes very unusual magnets. It’s not just that his shiny, black
crystals will bend and flex. Or that they are rather light for magnets. Or even
that they will dissolve given the right solvent. No, the strange thing is that
Rawson’s crystals contain no trace of iron or any other metal—not even one
atom. Instead, they are made up of specially designed organic compounds, with
small, lightweight molecules mostly made of simple atoms such as carbon,
nitrogen and sulphur.
Other chemists too are investigating how to mimic metals’ magnetic properties
using organic materials. Their creations look more like plastics than
traditional magnets, and they offer other useful properties besides magnetism.
They can be transparent, flexible, lightweight and even insulating.
“Organic” magnets may not be able to replace iron magnets, but with their
unusual combination of properties, they could find their way into a range of new
applications. Researchers believe they could be used in the latest smartcard
technology such as electronic “wallets”, in gates and switches that control
light signals in optical computers, or for creating new kinds of optical
displays.
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Radical solution
The magnetic properties of metals—whether they are being used as
needles in compasses, the coatings on data storage tapes, or the decorations on
the refrigerator—stem from their electronic structure.
Rawson’s material, cooked up in the chemistry department at the University of
Cambridge in collaboration with Neil Bricklebank of Sheffield Hallam University
and Fernando Palacio of the University of Zaragoza in Spain, is based on the
dithiadiazolyl radical
(see Diagram).
A radical is an organic molecule
that contains a lone, unpaired electron, which is essential for making the
material magnetic. In Rawson’s material, the dithiadiazolyl radicals are linked
head-to-tail by strong interactions between the cyanide group and sulphur atoms
on neighbouring molecules. The resulting centimetre-long crystals are both
magnetic and flexible. “That’s one of the reasons why they’re so interesting,”
he says.FIG-21074802.jpg
Another unusual feature of Rawson’s material and other organic magnets is
that they are soluble in certain solvents, which could make processes such as
the production of magnetic films much easier. “We should be able to dissolve
these radicals in an organic solvent, pour the solution onto a plate and
evaporate the solvent to produce a thin layer of magnetic material,” he says.
Such processing could take place at room temperature. By contrast, standard iron
or chromium oxide magnets are insoluble and must be heated to hundreds of
degrees Celsius during manufacture. “It’s a big surprise,” says Joel Miller, who
is also designing organic magnets at the University of Utah. “The concept of a
magnet that you can dissolve in a solvent is foreign to most people.”
Rawson’s flexible magnetic material may look quite different from a standard
magnet, but the reason for its magnetic properties is the same. “Basically it
comes down to its electrons and how they communicate with one another,” says
Rawson.
Electrons have a property called spin, which gives them a small magnetic
field. Electrons spin in one of two directions. In most atoms, and in bonds in
molecules, they are found in pairs. And within the pair the electrons have
opposite spin, so the magnetic fields cancel out and the material doesn’t behave
as a magnet.
But if an atom or molecule has one or more unpaired electrons, the spins and
their resulting magnetic moments—a measure of the magnetic field
strength—do not cancel out. Iron atoms, for example, have four unpaired
electrons. Rawson’s dithiadiazolyl radicals each contain one unpaired electron.
Organic radicals are normally unstable, but the crucial lone electron in the
magnetic molecule is surrounded by nitrogen and sulphur atoms which stabilise
it.
For a material to be magnetic, the spins of the unpaired electrons must line
up and work together so that the magnetic moments are combined. But this
cooperation only happens if the unpaired electrons are close enough together for
their spins to interact with each other, causing them all to align in the same
direction. Normally, this cooperation only occurs locally, in small regions
known as domains. If all the domains throughout a piece of material line up in
the same direction, it forms a strong magnet. Otherwise their magnetic fields
will cancel out.
Critical temperature
Materials in which the electron spins can be lined up, such as iron, are
called ferromagnetic. Those in which the spins arrange themselves opposing each
other are said to be antiferromagnetic. Rawson’s material is a special case.
Although it is antiferromagnetic in one direction— say, up and
down—the spins all point slightly to one side, say, the right, thanks to
the way the molecules are linked together
(see Diagram). This produces a
magnetic field in that direction of about one-thousandth the strength of that in
iron. Although this is nowhere near strong enough for the new material to
replace iron magnets, it may be strong enough for some specialised devices and
applications.FIG-21074802.jpg
But before researchers start making novel devices there is a major obstacle
to overcome—getting the organic magnets to work at a high enough
temperature. Like superconducting materials, organic magnets only work at very
low temperatures. The first metal-free magnet, made in 1991, worked only at 0.6
kelvin. Rawson’s latest material is magnetic up to 36 K, which is still not very
warm but a vast improvement on previous efforts
(see Diagram).FIG-21074801.jpg
The critical temperature above which a magnet loses its magnetism is called
the Curie temperature. In a magnet, heat causes the unpaired electrons to stop
spinning in sync, so that their magnetic moments cancel out. As the spins flip
out of alignment, magnetism disappears. For iron, the Curie temperature is more
than 1000 K. By contrast, organic magnets are very susceptible to heat. But
Miller looks at it another way. “It’s not a question of why are the Curie
temperatures of these materials so low,” he says, “but how come they’re so high.
The one property of these types of materials that no one would have predicted
was magnetism.”
Even so, researchers trying to make useful organic magnets will have to find
a way of raising the Curie temperature to above room temperature. The key may
lie in tweaking the molecular structure to bring the unpaired electron closer to
its neighbours. The result would be increased spin interactions, which require
greater thermal energy to break up. In iron, for instance, the unpaired
electrons sit on adjacent atoms and so are very strongly interacting.
For organic magnets, however, “it’s a balancing act”, says Rawson. Squeeze
the unpaired electrons on the molecules too close together, and you may lose
magnetic behaviour completely. “The spare electrons simply form bonds,” he says.
In Rawson’s organic radicals, the unpaired electrons are kept far enough apart
by fluorine atoms.
Rawson is confident that he’ll be able to raise the Curie temperature of his
organic magnets further. He plans to replace the sulphur atoms with selenium
atoms, which he hopes will make the interactions between radicals even stronger,
and increase the angle at which the spins are tilted in one direction. This
should increase the magnetic field and make it less susceptible to heat.
Another option is to design radicals that have more double and triple bonds
between atoms. These form a “delocalised” system that “spreads out” the unpaired
electron over the molecule, stopping it forming a bond and so allowing radicals
to pack closer together in the magnetic material.
Tiny tubes
Rawson is working with Phil Langley and JĂĽrg Hulliger at the department
of chemistry and biochemistry in the Institute for Inorganic, Analytical and
Physical Chemistry in Bern, looking into applications for his materials. They
have succeeded in locking the magnetic molecules inside tiny tubes a few
nanometres across. They think that these miniature magnetic sausage rolls could
be used to halve the wavelength of light passing through them. Such devices
could transform infrared light into ultraviolet light, a feat that could allow
twice as much information to be packed onto a CD as is possible with
conventional infrared lasers. These devices could also be made to act as
switches by altering their magnetic state, making them useful for processing
signals in optoelectronic circuits.
Meanwhile, Richard Bushby at the University of Leeds is investigating an
alternative way of making organic magnets with higher Curie temperatures.
Instead of linking together radicals, Bushby and his colleagues are creating
unpaired electrons in a ready-made polymer. They start by adding small
quantities of oxidising agent to polyarylamines—polymers built from
benzene-like molecules and cross-linked with nitrogen atoms. The oxidising agent
pulls single electrons from the polymer, leaving lots of unpaired electrons in
the material. Bushby hasn’t yet managed to make a magnet, but he thinks that the
close-knit polymer chains will strengthen the spin interactions between unpaired
electrons. He reckons that magnets made from such materials should have higher
Curie temperatures.
Best of both worlds
Other chemists tackling the problem prefer to take an easier way out and
cheat—by adding a metal atom. “I suspect that organometallic rather than
organic molecular magnets are more likely to deliver the goods in the
short-term,” says Rawson. Organometallic magnets provide the best of both
worlds. Including a metal atom such as iron, manganese or vanadium inside an
organic molecule gives a greater number of unpaired electrons and stronger spin
interactions. The result is higher Curie temperatures and stronger magnets,
while the organic nature of the bulk of the molecules means that they still have
properties such as flexibility and solubility.
Miller is one of the researchers focusing on organometallic materials. His
latest offerings include vanadium tetracyanoethylene (TCNE), which has a Curie
temperature of more than 100 °C and a magnetic strength per metal atom
almost equal to that of iron. So far so good—but there is a drawback.
Vanadium-TCNE spontaneously reacts with air and catches fire. These materials
are “pretty reactive”, Miller admits, but no one has put much effort into
solving this problem yet.
Vanadium-TCNE has a rigid three-dimensional structure in which each TCNE
group binds to up to four vanadium atoms, and each vanadium can bind to other
TCNE groups. But making it is a problem, as solvent molecules left over from
processing bind to the vanadium atoms. This breaks up spin interactions and
lowers the Curie temperature. “We are working on making these vanadium magnets
using gas phase chemistry,” says Miller. He simply mixes the reactants as
vapour, and a magnetic film forms as they react.
Even if Miller succeeds, he accepts that his organometallic magnets won’t be
able to replace powerful conventional magnets. “They will never contain the same
density of spins as metal magnets,” he says. “The organic parts are just extra
fat.” However, the prospects for organometallic magnets are good, says Miller.
He predicts that their future lies in speciality uses, such as electromagnetic
shielding on high-voltage power lines. A thin layer of Miller’s magnetic
material might reduce the strength of the electromagnetic field experienced on
the ground below.
Colourful credits
Meanwhile, Olivier Kahn of the Laboratory of Molecular Science in the
Institute of Condended Matter Chemistry at the University of Bordeaux I, France,
is investigating organometallic magnetic materials that could be used to store
data. He is focusing on materials known as iron-1,2,4-triazoles, in which iron
atoms are held in place by an organic framework. This structure stabilises the
spins of the unpaired electrons on the iron atoms, allowing them to spin in one
of two directions. In other words, the material can be stabilised in two
different magnetic states—one a low-energy state and the other a
higher-energy state. So the spins of the unpaired electrons on iron atoms could
store bits of information. Kahn predicts that such materials would be excellent
for data storage. “Europe is at the forefront of this technology,” he says.
Another approach is to develop these materials into smartcards in which
credits can be switched on or off. Kahn is collaborating with the electronics
company Philips and France Télécom on this idea. The trick is to
find a material in which the domains can easily be switched from the low-energy
to the higher-energy state. “At the moment, heat is the easiest way of changing
their state,” says Kahn. In the case of a phonecard, the telephone would attach
to a small heating head inside the card reader that switches on when the card is
being used. Heating the magnetic material changes the number of credits left on
the card.
By carefully adjusting the chemical structure of the iron triazoles to give
more or less bonding and so a more or less rigid organic framework, Kahn has
produced materials that switch states at close to room temperature. The
materials also change colour as they switch from one state to another. “The
molecules turn from purple to white as they switch, so that stored data can be
read directly,” says Kahn. Such a system could form the basis of an electronic
wallet that changes colour according to how much money is available.
Kahn’s next goal is to try and produce magnetic materials that are stable at
room temperature and that can be switched using light. In the long term, says
Kahn, such materials could be used as active elements in new flat-screen
displays. He and the chip maker Motorola are currently working on this idea.
According to Peter Day of the Royal Institution in London, who is also
researching organometallic magnetic materials: “They may also have potential for
high-speed light modulation.” Such a property would be useful in directing
information in optical computers.
Conventional electronics could also benefit from organic magnets. Researchers
are trying to coat thin magnetic films directly onto electrodes to produce tiny
thin-film electromagnets. Like their larger counterparts, these would be
switched on and off with electricity, and could be useful in gates or switches
for intricate electronic circuits.
Chris Murray, a materials chemist at IBM’s T. J. Watson Research Laboratory
in Yorktown Heights, New York, believes that research into how these organic
magnets work is helping our understanding of the collective properties of groups
of molecules. Having pushed back the boundaries of what makes a magnetic
material, today’s researchers are only just beginning to realise the wide range
of attractions offered by organic magnets. As Murray says: “There’s a lot of
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