Western Kentucky University
Department of Physics and Astronomy

Colloquium

Dr. Serban Smadici

University of Louisville
Department of Physics and Astronomy

"Ultrafast magnetism and thermal transport in nanostructures"

February 04, 2019 @ 4:00 pm

Abstract

Precession of an out-of-equilibrium magnetization can be described with classical equations of motion. However, when intense laser pulses fragment the magnetization of a material in addition to setting it into an out-of-equilibrium state, the resulting time-evolution requires a different approach. We apply optical techniques to examine the non-equilibrium magnetization and temperature dynamics in thin metallic films in different experimental conditions and compare results to model predictions and computer simulations. The research revealed that all-optical switching in thin metallic samples on glass substrates is driven by demagnetizing fields and thermal gradients, following full thermal demagnetization. Measurements made possible by heat accumulation probed simultaneously the equilibrium thermal demagnetization and the non-equilibrium ultrafast demagnetization dynamics in a temperature range from room temperature to above the Curie temperature. The magnetization time-dependence can be modeled for small deviations from equilibrium by applying the fluctuationódissipation theorem, relating fluctuations in the equilibrium state to the rate of magnetization relaxation toward equilibrium. Further work will examine different aspects of the dynamical non-equilibrium magnetic state in thin films and patterned media. Practical applications include minimizing the bit writing error rate in heat-assisted magnetic recording in materials with perpendicular magnetization.