Jul 12, 2007
Are all nanoparticles magnetic?
Ferromagnetism may be a universal feature of all nanoparticles, according to work by researchers in India. Chintamani Rao and co-workers of the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore have found that nanoscale metal oxides and nitrides show room-temperature magnetism because of surface effects. The team found that even nanoparticles of high-temperature superconductors can become ferromagnets – a surprising result since superconductivity and ferromagnetism are usually incompatible.
"We have shown that, independent of whether an oxide or some other inorganic material is intrinsically magnetic, it can still exhibit magnetic hysteresis at room temperature because of surface effects," Rao told nanotechweb.org. "The same applies even to superconducting materials in the non-superconducting state."
Rao and colleagues decided to study the magnetism of nanoparticles because of a discovery made in their lab – that nanoparticles of many non-magnetic oxides, including cerium oxide and aluminium oxide, showed magnetic hysteresis at room temperature. The origin of the ferromagnetism in these nanoparticles is thought to come from magnetic moments arising from oxygen vacancies at the surfaces of the particles.
In the new study, the researchers prepared oxide nanoparticles of different sizes ranging from 2–150 nm and heated them to varying temperatures. They then measured the magnetic hysteresis of the samples using a magnetometer. The results clearly showed that small nanoparticles at room temperature showed surface magnetism.
This group also found that nanoparticles of superconducting yttrium barium copper oxide (YBCO) showed ferromagnetism at room temperature (see figure). According to the scientists, the magnetism is confined to the surface of the nanoparticles since bulk YBCO at room temperature is paramagnetic. "It is also interesting to note surface ferromagnetism in nanoparticles of ferroelectric BaTiO3," said Rao.
Nanoparticles of metal nitrides, such as niobium nitride as well as metal chalcogenides (such as cadmium selenide) were found to exhibit surface ferromagnetism too.
The team now plans to do more experiments to try and understand the nature of the surface ferromagnetism using spectroscopic and other local probes.
The work was published in arXiv and Solid State Communications.
About the author
Belle Dumé is contributing editor at nanotechweb.org