12-02 :: February/March 2012

nanotimes News in Brief

Researchers in Japan have observed strong cross- coupling of dynamic excitations in multiferroic materials called rare-earth perovskites. The work has revealed optical properties in opto-electronic materials similar to those found in common dichroic glass, and may make possible new types of optical devices.

At the microscopic level, the electric and magnetic field structures inside any material are complicated. Small displacements of electrons from their equi- librium positions cause electric polarization and create associated electric fields. Similarly, electrons have either a ‘spin up’ or ‘spin down’ magnetic configuration that can produce magnetic fields, or dipoles. These electric and magnetic fields can be aligned randomly, or they can form complex, large- area structures useful for devices, explains Youtarou Takahashi from the Japan Science and Technology Agency and the RIKEN Advanced Science Insti- tute, who led the team that observed the dynamic cross-coupling.

Takahashi and his colleagues also demonstrated an immediate and remarkable consequence of this cross-coupled excitation: their rare-earth perovskite absorbed more of the light that passed through it in one direction than it did light passing through in the opposite direction. In a regular material, the absorp- tion strength would be identical in each direction since only the strength of the electric polarization, internal to the material – not its direction – would affect absorption. When cross-coupling occurs, however, the directions of the absorbing material’s internal electric and magnetic fields also matter. Because reversing the direction of light travel re- verses at least one of these internal field directions,

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A schematic diagram of directional dichroism behavior. A rare-earth perovskite (black blocks) absorbs light (purple arrows). The internal magnetic (M) and electric (P) fields of the perovskite interact with the light according to their orientation relative to the light. As a result, the strength with which the perovskite absorbs light depends on the light’s direction. Absorption strengths are equal for configurations connected by a ‚=‘. © 2012 Y. Takahashi et al.

absorption will vary for different directions of travel (see Fig.)

Takahashi, Y., Shimano, R., Kaneko, Y., Murakawa, H. & Tokura, Y. Magnetoelectric resonance with electroma- gnons in a perovskite helimagnet In: Nature Physics AOP, December 04, 2011, DOI:10.1038/NPHYS2161: http://dx.doi.org/10.1038/NPHYS2161

A team of University of Texas at Arlington resear- chers have developed a method that uses magnetic carbon nanoparticles to target and destroy can- cer cells through laser therapy – a treatment they