May 1, 2009
Magnetism disappears at the nanoscale
The magnetic properties of ferromagnets dramatically change when these materials are shrunk down to the nanoscale. So say researchers at the University of Alicante in Spain who have found that magnetism can even disappear in strong magnets such as iron, cobalt and nickel. The phenomenon, which results from electron–electron interactions not present in bulk materials, will need to be considered when developing future nanoscale magnetic structures.
"Our finding shows that, at the nanoscale, electron–electron interactions, which we often neglect, can be extremely important," team leader Carlos Untiedt told nanotechweb.org.
In atomic scale structures, conventional magnetic interactions compete with an additional magnetic effect, known as the Kondo process. This occurs because the electronic environment surrounding a tip atom is different to that experienced by atoms in the bulk material. "We can tell this is going on because the competing Kondo process leads to a distinct feature in the electrical conduction that we can see," explained Untiedt.
For the Kondo effect to occur, we need an unpaired electron (and thus an unpaired spin) localized in space. In the present work, the unpaired electron is on a particular "junction" atom that is in contact with free (conduction) electrons. One of the conduction electrons could lower its kinetic energy by pairing up with the localized spin, but this is unlikely to occur because of electron–electron repulsion. Instead, the electrons perform a magnetic "dance" in which the localized spin gets flipped and a conduction spin is shuttled across the junction, says Untiedt.
The Alicante researchers were surprised to find this effect because, usually, the localized electron is either on a magnetic impurity or a quantum dot. "In our case we have an open system that is chemically homogenous but we still get Kondo physics," said Untiedt.
The finding means that scientists modelling magnetic properties of nanostructures will need to take these comparatively exotic strong correlation effects into account, he adds. "The atomic-scale details of magnetic surfaces can have a big impact on how magnetism works at these scales.
The work was published in Nature.
About the author
Belle Dumé is contributing editor at nanotechweb.org