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Researchers from the University of Amsterdam and the FOM Foundation have found an explanation for this 'colossal magnetoresistance effect'. This knowledge is important for the development of new materials, such as for ABS sensors, as well as for a new type of electronics: oxide electronics. The researchers published their results on 11 September in 'Nature Physics'.

Up until now it was a mystery: certain materials change from being an isolator to a conductor under the influence of a magnetic field. Researchers from the University of Amsterdam and the FOM Foundation have found an explanation for this 'colossal magnetoresistance effect'. They discovered that the charge carriers in materials with this effect are not electrons but polarons. The conduction of these polarons depends on the structure of the material and is highly sensitive for a small external magnetic field. This knowledge is important for the development of new materials, such as for ABS sensors, as well as for a new type of electronics: oxide electronics. The researchers published their results on 11 September in 'Nature Physics'.

Colossal magnetoresistance

Colossal magnetoresistance is mainly observed in manganese oxide compounds. These compounds are built up from layers: layers with the potential to conduct (drawn yellow in the diagram) and insulating intermediate layers (green). If the conducting layers are always separated by two insulating intermediate layers then the crystal never conducts. However, when blocks of 3 or more conducting (yellow) layers alternate with intermediate layers then the substance conducts well. In compounds where exactly two conducting layers are positioned one above the other (the so-called bilayer manganites) something unusual occurs: a small disruption caused by the application of a magnetic field then has a crucial effect on the resistance.

Polarons

The researchers investigated what exactly causes the transition from insulator to conductor in these manganites. They discovered that the charge carriers in the material play a pivotal role. In most materials, electrons are responsible for the conduction. The easier these can move through the crystal lattice, the higher the conductance of the material. In bilayered manganites – the materials with the greatest magnetoresistance effect – polarons, and not electrons, were found to be the charge carriers. Polarons are electrons that deform the crystal lattice and therefore hinder their own moment through it. The movement of polarons through a lattice can be compared to running across a soft mattress; your presence deforms the mattress thus slowing you down.

Soft or firm

The crystal lattice of manganese oxides with just a few conducting (yellow) layers is very 'soft'. The polarons become trapped and consequently no conduction occurs. The greater the number of conducting layers stacked one on top of the other, the less the deformation of the lattice (the material becomes 'harder'). The charge carriers can then move easily and the material conducts. The bilayer manganites fall exactly in between these two scenarios. At high temperatures and without a magnetic field they are 'soft'. Then the polarons are trapped. However, at a low temperature or in a magnetic field the network becomes 'firmer' and the polarons move more easily. This transition from trapped to moving polarons clarifies the colossal magnetoresistance effect.

The researchers collaborated with colleagues from the University of Oxford and the German synchrotrons Bessy (HZB in Berlin) and SLS (PSI in Villingen). The research was partly financed by the Foundation for Fundamental Research on Matter (FOM) and the European Union.