Magnetism—large and small. |
A little piece of iron wire is magnetic—just like a huge
iron rod. When it comes to material properties, size usually does not matter.
Surprisingly, researchers from Austria
and India
have now discovered that some materials show very unusual behavior, when they
are studied in the form of tiny crystals. This could now lead to new materials
with tailor-made electronic and magnetic properties.
Different size changes material properties
Material properties such as electrical conductivity, magnetic properties, or
the melting point do not depend on an object’s size and shape. “In India, however,
an experiment recently showed that special manganese oxides—so called
manganites—exhibit completely different properties, when their size is reduced
to tiny grains,” Karsten Held explains.
A team of scientists from the Vienna University of
Technology (Austria) and the
S.N. Bose National Centre Kolkata (India) investigated this phenomenon—and
the new effect could be explained in computer simulations. In a crossover from
large crystals to smaller crystals, the distribution of the electrons changes,
and so does their energy. This, in turn, changes the electrical and magnetic
properties of the crystal. “The phenomenon of quantum entanglement plays a very
important role here,” says Professor Karsten Held. “We cannot think of the
electrons as classical particles, moving independently of each other, on
well-separated paths. The electrons can only be described collectively.”
By changing their size, the properties of the manganite-crystals
can now be harnessed. Larger crystals are insulators, and they are not
magnetic. Tiny crystal pieces on the other hand turn out to be metallic
ferromagnets.
Important for industrial applications
Phase transitions, at which important material properties change, play a major
role in technological applications: “When data is read from a hard-drive with a
reading head, a transition between a conducting and a non-conducting state is
used,” Karsten Held explains. Similar effects can be seen in manganite
crystals: “We knew that magnetic properties of manganites depend on the temperature
and the magnetic field,” says Tanusri Saha-Dasgupta, a material scientist at
the S.N. Bose National Centre Kolkata . “But now we know that these transitions
can also be controlled by altering the size of the crystals.” By changing the
granular size of the crystals, the scientists can influence the critical
temperature and magnetic field strength, at which the phase transition takes
place. For technological applications, this opens up exciting new
possibilities.
Huge computational effort
The manganite crystals studied by the Austro-Indian research team are only some
three to fifteen billionths of a meter wide—but still they consist of hundreds
or thousands of atoms. Simulating their behavior on a computer is therefore
still a great challenge. “The quantum mechanical equations we are dealing with
here can only be solved with extremely powerful computer clusters,” says
PhD-student Angelo Valli. “Fortunately, the computer cluster VSC at the Vienna
University of Technology provides us with remarkable computing power.”
