 |
|
|
|
Chemistry professor Mark Turnbull and his students create new compounds in order to study complex magnetic properties. Dr. Turnbull also sponsors students, like chemistry minor Lesley Mathews, who want to pursue internships in the local area. |
|
 |
Having a Magnetic Moment
Professor Mark Turnbull's research
Don't count on being able to retrieve data stored on old computer diskettes. They and other
types of electronic storage media are not archival. That's because they rely on magnetism to
help store data, and the magnetic properties of a material can change over time. Even dropping
a diskette can cause it to lose its magnetic stability.
Chemist Mark Turnbull is designing new chemical
compounds that can be used to test theories of magnetism. He is interested in how the molecules
in a compound can be arranged to exhibit different kinds of magnetic properties. How strong
is the magnetic force? At what temperature does it lose its magnetism? How is the magnetic
force affected by the distances and angles between atoms?
Turnbull is particularly interested in low-dimensional magnets. These materials display
magnetic interactions in only one or two dimensions, or some increment in between, instead
of the usual three dimensions. Many of the studies done to date on square and ladder magnets,
two kinds of low-dimensional magnets, have been conducted at Clark.
Testing new compounds
Turnbull and undergraduate Stephanie Amaral '02 collaborated on two papers exploring
one-dimensional magnets, also called linear chain magnets. In one article they described
their attempt to grow a crystal that would exhibit the properties of a one-dimensional magnet.
As a kid, you may remember seeing ads for kits allowing you to grow crystals at home.
Using a similar process, Turnbull and Amaral were able to grow, in solution, a crystal
made from copper nitrate and 2,6-dimethylpyrazine (containing the elements copper, nitrogen,
oxygen and hydrogen) that exhibited one-dimensional magnetic properties.
The material they isolated as crystals was different from the original powders. Although
the crystal was not stable when removed from the solution, Turnbull and Amaral were still
able to determine the structure of the material and understand why its magnetic properties
had changed. Their attempt was the most successful so far to fill a gap in a particular
family of compounds that are being explored for their potential one-dimensional properties.
Further studies are in progress in Turnbull's lab.
In an earlier paper, Amaral and Turnbull teamed with physics colleague* Professor Chris Landee,
undergraduate Bill Jensen and graduate student Matt Woodward to report on another compound,
(2-methylpyrazine)copper(II) nitrate. This crystal was a variation on a related crystal already
shown to have excellent one-dimensional magnetic properties. The researchers wanted to
compare the structure and magnetic behavior of the two materials. They accomplished this
using x-ray diffraction† to measure the lengths and angles of the bonds between
atoms (for example, between copper and nitrogen), and a SQUID magnetometer to measure the
magnetic properties of the two substances. As in all scientific research, the search for new
magnetic materials requires patience and perseverance as all possibilities are systematically
explored.
Turnbull and his students use special
software
that allows them to create visual models of different compounds whose magnetic properties
they'd like to explore. Their knowledge of the properties of different atoms, how and if
they can be combined, and the forces they exert on each other, helps them to choose
appropriate materials for further study in the lab.
Magnetism begins with electrons
Electrons are particles that, along with protons and neutrons, make up an atom.
Each electron generates its own tiny magnetic field, called a magnetic moment. The
number of electrons, and the way in which the north and south "poles" of their magnetic
moments align, determines the magnetic properties of the material. What's interesting is
that the alignment of the magnetic moments is not necessarily fixed, and can be altered
by changes in temperature and other types of energy.
All substances are either diamagnetic or paramagnetic. A diamagnetic material has an even
number of paired electrons whose magnetic moments cancel each other out. Diamagnets do not
generate a magnetic field.
On the other hand, paramagnets†† contain one or more unpaired electrons
whose magnetic moments are not canceled. At high temperatures (high relative to the
magnetic exchange energy), the moments all point in random directions. But, as the temperature
drops, the moments begin to align with each other. When the north and south "poles" of the
unpaired electrons line up in the same direction and generate a magnetic field, a ferromagnet
is the result. Ferromagnets are what we think of as true magnets--they stick to your
refrigerator.
Turnbull is particularly interested in studying two other types of paramagnets, antiferromagnets and ferrimagnets. In the former, the magnetic moments align opposite to each other (antiparallel). The same is true of ferrimagnets, but here the magnet moments aligned in one direction are stronger than the magnetic moments aligned in the other direction.
Very few known materials are ferromagnetic at room temperature. Because materials that generate magnetic fields have so many potential applications, scientists want to develop new compounds that are magnetic at, or close to, room temperature, and that combine their magnetic behavior with other kinds of properties, such as transparency or flexibility.
*While it is common to have both chemists and physicists working together in the study of molecular magnetism, the Turnbull/Landee collaboration is unusual in that both are at the same institution. Physicists have an understanding of the forces exerted within and between atoms, while chemists understand how to use atoms as building blocks to make compounds.
** A magnet is a substance that generates a magnetic field
†X-ray diffraction involves exposing a material to known wavelengths from the x-ray region of the electromagnetic spectrum. The angle and intensity of the x-rays as they are reflected from the material gives chemists information about its atomic structure.
†† Paramagnets are usually composed of metal ions or free radicals, both of which are characterized by unpaired electrons.
|
|
|
Additional Resources
|
|
|
Magnets |
|
