Scientists solve riddle of strangely behaving magnetic material

The compound of lanthanum, cobalt and oxygen (LaCoO₃) has been a puzzle for over 50 years, due to its strange behavior. While most materials tend to lose magnetism at higher temperatures, pure LaCoO₃ is a non-magnetic semiconductor at low temperatures, but as the temperature is raised, it becomes magnetic. With the addition of strontium on the La sites the magnetic properties become even more prominent until, at 18 percent strontium, the compound becomes metallic and ferromagnetic, like iron.

"They knew that we could calculate x-ray absorption and magnetic dichroism, so we started doing that. It is a case where we fell into doing what we thought was a routine calculation, and it turned out we discovered a totally different explanation," said Harmon. "We found we could explain pretty much everything in really nice detail, but without explicitly invoking that local model," said Harmon.

The scientists found that a small rhombohedral distortion of the LaCoO₃ lattice structure, which had largely been ignored, was key.

"We found that the total electronic energy of the lattice depends sensitively on that distortion," explained Harmon. "If the distortion becomes smaller (the crystal moves closer to becoming cubic), the magnetic state of the crystal switches from non-magnetic to a state with 1.3 Bohr magnetons per Co atom."

Ames Laboratory scientists Bruce Harmon and Yongbin Lee partnered with the researchers at the Argonne National Laboratory and the University of California, Santa Cruz to publish a paper in Physical Review Letters, "Evolution of Magnetic Oxygen States in Sr-Doped LaCO₃."

This new understanding may help the further development of these materials, which are easily reduced to nanoparticles; these are finding use in catalytic oxidation and reduction reactions associated with regulation of noxious emissions from motor vehicles.

The research is supported by the U.S. Department of Energy Office of Science through the Ames Laboratory.