Researchers discover largest high-field magnetoresistance

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PostPosted: Tue Aug 19, 2008 12:00 pm    Post subject: Researchers discover largest high-field magnetoresistance Reply with quote

Researchers discover largest high-field magnetoresistance

Belle Dumé

Researchers in France have discovered the largest high-field magnetoresistance in super-lattices of cobalt-iron nanoparticles at low temperatures. Although the result, which was unexpected, will not lead to any applications just yet, it will be crucial for fundamental studies of transport in nanoparticle arrays. Magnetoresistance is the change in the resistance of a material caused by a magnetic field, and allows hard disks to read out magnetic data.

Julian Carrey of the Laboratoire de Physique et Chimie des Nano-Objets in Toulouse and colleagues discovered a magnetoresistance (MR) of up to 3000% in millimetre-sized super-lattices of CoFe nanoparticles surrounded by an organic layer. Theory predicts that the MR in such materials could be enhanced thanks to charging effects due to the small size of the nanoparticles. However, until now, measured MRs in these materials were small – limited to just a few tens of per cent.
MEB micrograph of the CoFe samples.
Credit: J Carrey.

By measuring the magnetotransport properties of these materials, the scientists also found that the amplitude of the MR increased exponentially when the voltage decreased. "It is not impossible that the MR could have been even higher at smaller voltages in our sample but the current was too low to be measured," Carrey said.

"Our results were surprising because the MR amplitude of standard 3d metals – iron, cobalt and nickel – is not expected to exceeds a few tens of per cent when the standard mechanisms of magnetoresistance are assumed," Carrey said.

The team is yet unsure as to where the large MR is coming from. It may be that localized electronic states are involved – in the process of electronic transfer between the particles. "However, this is rather speculative," explained Carrey. "We also observed a new mechanism of magnetoresistance related to the Coulomb blockade properties of the sample, which is also unexplained. So our experiments raise more questions than they answer." Coulomb blockade is the increased resistance at small bias voltages of an electronic device composed of nanoparticles.

At present, the effect has only been observed at temperatures between around 2 and 10K, so any direct applications are still a way off. "But these results may stimulate more experimental or theoretical work on transport in arrays of nanoparticles," added Carrey.

Since the scientists suspect that localized states between the nanoparticles are responsible for the MR, they will now try to determine the exact nature of these states and control their presence and density. "This will also be a challenge for the chemists in our lab," quipped Carrey.

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