Towards a New Mechanism in Magnetoresistive Compound
November 09, 2012
Magnetoresistive materials enable the sensitive measurement of magnetic fields to be made, as their electrical resistance changes depending on the angle and strength of magnetic fields. Moreover, as they are easy to miniaturize, they are used in the manufacture of numerous types of sensors which, for instance, read data in computer storage media or determine the steering angle in power steering systems in cars.
Scientists at the Peter Grünberg Institute and RWTH Aachen have now discovered a novel, temperature-dependent behaviour in a magnetoresistive compound of the rare earth metal gadolinium and the semiconductor silicon: at temperatures of 54 Kelvin, the resistance of this material diminishes down to a tenth, and in stark contrast to this, at even lower temperatures, it can then as much as quadruple in strength. Normally, magnetoresistive materials always react either with a simple increase or decrease in resistance.
Moreover, the scientists discovered an extraordinary shrinkage which took place as the material cooled; at first, its volume reduced as expected. However, at temperatures under 54 Kelvin, it began to increase again. This is precisely the temperature under which the material becomes magnetic. “This combination of phenomena is extraordinary and prompted us to look at this in more detail,” explained Dr. Haifeng Li, researcher at the outstation of the Jülich centre for neutron research at the Institut Laue-Langevin in Grenoble, France.
With the help of scattering experiments, Li and his colleagues found indicators pointing to a new mechanism to explain this remarkable behaviour. Quasiparticles in the conduction band of the material, known as magnetic polarons, are the likely cause. “You can imagine a polaron as being a kind of magnetic cloud moving with the conduction electrons through the material,” Li clarifies. The cloud can move quickly or slowly depending on the angle of the external magnetic field as electrical resistance reduces or increases.
In contrast, the magnetoresistive effects on ferromagnetic materials observed up to now almost always result from the direction of magnetization being influenced by an external magnetic field and electrical resistance being changed by the angle between magnetization and the direction of the current.
The researchers now hope that their findings will help them to discover further interesting materials of this type, which may also be suitable for use at high temperatures in industrial applications. The results are available in the open access online journal “Scientific Reports” published by the prestigious Nature Publishing Group (DOI: 10.1038/srep00750).
Possible magnetic-polaron-switched positive and negative magnetoresistance in the GdSi single crystals; H. Li et al.; Scientific Reports Volume: 2, Article number: 750, DOI:10.1038/srep00750