Polaron glass in colossal magnetoresistive oxides researched. (General Developments).

The magnetic properties of the lanthanum manganese oxide class of materials have attracted tremendous interest recently because of the dramatic increase in conductivity these systems exhibit when the magnetic moments order ferromagnetically, either by lowering the temperature or applying a magnetic field. This huge increase in the carrier mobility, which has been given the name "colossal magnetoresistivity" (CMR), is both of scientific and technological interest. In particular, it is anticipated that the half-metallic behavior these materials exhibit will provide fully spin-polarized electrons for use in spintronics applications, for sensors in a variety of applications such as in the automotive industry, and may also provide the next generation of read/write heads for the magnetic data storage industry.

The colossal magnetoresistivity originates from a magnetically driven insulator-metal transition, where the magnetic, electronic, and structural degrees of freedom are intimately intertwined. A research collaboration involving scientists from NIST, the University of Maryland, and Argonne National Laboratory, has recently used neutron scattering measurements to discover that the transition from the low temperature ferromagnetic-metallic state to the paramagnetic-insulator state is caused by the formation of combined structural/magnetic polarons, which have a size of about one nanometer. The formation of these nanoscale polarons truncates the ferromagnetic phase, and thus explains the first-order nature of the transition. These polarons form a well-defined thermodynamic glass phase above the ferromagnetic ordering temperature, which then melts into a polaron fluid at higher temperatures.

There is a strong similarity between these nanoscale polarons observed in the CMR materials, the polar nanoregions that cause the dramatic piezoelectric response of relaxor ferroelectrics, and the formation of stripes in the high temperature superconducting cuprates. Recent progress in our understanding of these intrinsic nanoscale structures has enabled a deeper understanding of the fundamental properties and shared concepts of these perovskite-based materials.

CONTACT: Jeffrey Lynn, [phone]; [e-mail].