Oldest Known Magnet’s Secrets Revealed Under High Pressures
FEBRUARY 1, 2008
The more we know about the ways materials function, the better our chances of modifying these materials for new applications, or even using them as the basis for entirely new materials. That holds true even for materials that have been studied and used for thousands of years, such as magnetite (or lodestone), the oldest known magnetic material. Now, researchers using two high-brightness x-ray beamlines at the Advanced Photon Source (APS) at the U.S. Department of Energy’s Argonne National Laboratory have uncovered new information about the coupling between magnetic and electrical properties of this venerable and highly useful material. The research appeared in the February 1 online edition of Physical Review Letters.
A major ore of iron, magnetite (Fe3O4), was discovered more than 2,000 years ago by the Chinese and Greeks and was used as the first compass material for navigation. Magnetite plays vital roles in a myriad of natural phenomena ranging from the biological and geological sciences to materials science. Tiny crystals of magnetite are found in the skulls of migratory birds, which may explain how they use the Earth’s magnetic field for navigation. Magnetotactic bacteria use tiny nanoparticles of magnetite to follow the geomagnetic field towards favorable oxygen-poor, nutrient-rich habitats in aquatic environments. Magnetite also has found its way into numerous modern-day technological applications such as magnetic recording media and ferrofluids used in magneto-rheological applications.
Despite being known and studied for a few thousands years, magnetite remains at the forefront of condensed matter research because a strong interplay between charge, spin, and orbital degrees of freedom displayed by magnetite’s electrons in this strongly correlated electron system makes the derivation of a microscopic picture of its coupled electrical, magnetic, and structural properties a rather difficult task.
Using a novel approach aimed at unraveling the secrets of magnetite, a collaboration of researchers from the Carnegie Institution of Washington, Argonne National Laboratory, and the Kirensky Institute of Physics combined diamond anvil cell techniques with high-brilliance circularly-polarized x-rays to study the intricate coupling between electronic structure and magnetism in magnetite. Using special instrumentation recently developed at X-ray Operations and Research beamline 4-ID-D at the APS, the researchers probed the magnetic state of magnetite up to applied pressures of 20 GPa (200,000 atmospheres). While it was known that high pressure modifies the overlap of electronic wavefunctions and affects electron mobility in an unusual manner, the presence or absence of correlated magnetic effects and their impact on such electrical anomalies was unknown up to now, due to the difficulty in directly probing magnetic ordering under such extreme conditions. The researchers used a specialized diamond anvil cell featuring perforated diamond anvils to allow transmission of relatively low-energy x-rays needed for measurement of the x-ray magnetic circular dichroism at the Fe K- absorption edge (1s electron excitation at 7.112 keV).
Their measurements revealed the presence of a sharp magnetic transition in the same pressure range where electrical anomalies occur in magnetite, providing unambiguous evidence of a strong interplay between the electronic and magnetic degrees of freedom. Furthermore, by combining the dichroism results with x-ray emission data taken at beamline 16-ID-D of the High Pressure Collaborative Access Team at the APS, and theoretical cluster calculations, the researchers were able to pinpoint the origin of the magneto-electrical anomaly to changes in the electronic configuration of octahedrally-coordinated iron sites.
“The results clearly demonstrate that electron mobility is strongly tied to the precise electronic orbital- and spin-configuration in this strongly correlated electron system,” said Yang Ding of HPSynC/Carnegie Institution of Washington, who led the research team. “By squeezing electronic orbitals together we can trigger electronic transitions, allowing us to understand the relevant interactions in magnetite.”
“We think that this new ability to directly probe magnetic ordering at extreme pressure conditions will prove to be a very valuable tool for the materials and geological sciences alike,” said physicist Daniel Haskel of the Argonne X-ray Science Division, who led development of the new instrumentation. “While other powerful techniques are available, including x-ray emission and Mossbauer spectroscopies, the x-ray magnetic circular dichroism technique is readily applied to most magnetic materials without the need for isotope enrichment, and provides a true measure of long-range magnetic order.”
Other authors in the paper are Ho-kwang Mao, Sergei G. Ovchinnikov, Yuan-Chieh Tseng, Jonathan C. Lang and Yuri S. Orlov.