50-year-old magnetic mystery solved; quantum structure obeys classical physics

Ohio State University physicists and their colleagues have demonstrated for the first time a type of magnetic behavior that was predicted to exist more than 50 years ago.

The behavior involves a special kind of energy transition among atoms in a very small magnet, called chromium-8 (Cr8). And while scientists have long thought that the effect was controlled purely by quantum mechanics, the magnet’s behavior appears to reflect the laws of classical physics.

The classical laws of movement and energy are ones that people experience in daily life, and they normally only apply to objects that are large enough to be seen with the naked eye. In contrast, the molecular magnet Cr8 is so small that quantum mechanics — the science that describes the interactions of subatomic particles — should rule its behavior.

The finding could help bridge the gap between quantum and classical approaches for understanding these tiny structures, and aid the future development of useful devices based on nanotechnology, such as very powerful, very small computers.

“This shows that we can understand important aspects of quantum behavior with classical thinking,” said Oliver Waldmann, a visiting scientist in the Department of Physics at Ohio State. “That’s a twist that I like.”

Waldmann and his colleagues published their results in a recent issue of the journal Physical Review Letters.

Materials such as Cr8 are called molecular magnets, because they are composed of only a small number of atoms that form a large molecule. The spins of the atoms’ electrons provide the magnetism, and the molecule itself acts as separate magnet.

In the case of Cr8, the structure contains eight electrically charged chromium atoms linked in a ring that measures less than one nanometer, or billionth of a meter, across.

The spins of four of the chromium atoms are magnetized in one direction — spin-up — and the other four in the opposite direction — spin-down.

The opposite spins cancel each other out, making Cr8 what’s known as an antiferromagnet. Researchers call the up-and-down spin structure a Néel structure, after the late French physicist Louis Néel, who in 1970 won the Nobel Prize for his discovery of antiferromagnetism.

In 1952, Princeton University physicist and Nobelist Philip W. Anderson predicted that when the atoms in an antiferromagnet become slightly canted out of their straight spin-up and spin-down positions, their energy transitions take on a wavelike structure.

But Anderson’s theory suggests that the magnet will generate a second kind of excitation called the Néel excitation when the electrons in its atoms are at their lowest possible energy state. This kind of Néel excitation has not been demonstrated, until now.

Waldmann performed the theoretical work that underpinned the experiment while he was at the University Erlangen-Nuremberg in Germany, and colleagues in Europe performed the experiment. Waldmann recently analyzed the data while working with Arthur J. Epstein, Distinguished University Professor of physics and chemistry at Ohio State.

To Epstein, the study demonstrates that magnets based on molecules with special internal molecular structures can produce new phenomena, such as Waldmann’s observation of the Néel excitation.

He added that using molecular magnets gives scientists the opportunity to use synthetic chemistry to tune magnetic properties, and introduce previously unknown properties into magnets.

“This enables both new fundamental science and new potential technologies,” Epstein said.

The scientists cooled a sample of Cr8 to only a few degrees Kelvin — colder than minus 450 degrees F — to lower the energy levels of electrons in the atoms as much as possible. Then they bombarded the material with neutrons to energize the electrons just enough for them to display Néel excitation.

The experiment was a tricky one, Waldmann said. Atoms sometimes absorb the neutrons, which would weaken signals from low-energy effects such as the Néel excitation.

The physicists chose Cr8 because the material would produce stronger signals, he said.

When Waldmann analyzed the spectrum of energy levels detected during the experiment, he saw that it matched the levels that were predicted by Anderson’s theory half a century ago.

“The pieces just fell into place,” Waldmann said. “Of course, I’d hoped for a long time that we would see the Néel excitation — we started this project four years ago — but when it actually happened, it was still a surprise.”

The find is more surprising still, given that the Néel excitation is a quantum mechanical effect, and the physicists could explain its properties using a classical approach.

That idea could bode well for experts who believe that quantum mechanical effects can be exploited to create a new kind of electronics.

Normal electronics encode computer data based on a binary code of ones and zeros, depending on the presence or absence of an electron within a material such as silicon. But in principle, the direction of a spinning electron — either “spin up” or “spin down” — can be used as data, too. And other directions of spin in-between up and down could theoretically provide further information, so a single electron could store many different pieces of data.

Such quantum computers could be much smaller than traditional electronics, and much more powerful. Instead of silicon chips, they would be built from arrays of tiny molecular structures similar to Cr8.

But working devices based on this technology could be decades away, and Waldmann said his work is only a basic step in that direction.

“Our work shows that these excitations can be understood with very basic reasoning, and this surely will help us to understand other effects that can be observed in such materials,” he said.

Co-authors on the Physical Review Letters paper include Tatiana Guidi of the Università Politecnica delle Marche in Italy; Stefano Carretta of the Università di Parma, also in Italy; Claudia Mondelli at the Institut Laue-Langevin in France; and Angela Dearden at the University of Manchester in the UK.

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Contact: Oliver Waldmann, (614) 292-3705; Waldmann.1@osu.edu
Arthur J. Epstein, (614) 292-1133; Epstein.2@osu.edu
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu