Intense laser pulses can fragment the magnetization of a material. In contrast to the precession of an intact magnetization that can be described with classical equations of motion, the quantum time-evolution of a fragmented magnetization requires a different approach. We apply optical techniques to examine the non-equilibrium dynamics for thin magnetic films in different experimental conditions and compare the results to model predictions and computer simulations. Films are deposited and characterized with instruments from the large suite of tools at the University of Louisville Micro and Nano Technology Center.

(1) 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.

Practical applications include minimizing the bit writing error rate in heat-assisted magnetic recording in materials with perpendicular magnetization.

Back-reflection echo of the strain wave in a thin nickel film (1 ps=10^-12 s).

Example of cumulative all-optical switching in Co/Pd superlattices. Dark and bright areas correspond to magnetization pointing up and down.

Micromagnetic simulations can shed light on the magnetization time-evolution.

The vertical stripe made in a Co/Ag heterostructure, disappears when crossing the horizontal stripe pattern, and reappears on the other side of the horizontal stack (above the top of this image).


(2) Measurements suggest that field-induced magnetic entropy and magnetization have different dynamics during ultrafast demagnetization. The external field decouples the entropy and magnetization dynamics, and adds magnetic entropy dynamics to the well-researched magnetization dynamics as another dimension to ultrafast demagnetization. Current research focuses on expanding these observations with further experiments, simulations, and models.

Thermal fluctuations inexorably increase in relative importance as devices decrease in size. Practical applications include using external fields to guide a dynamics set by thermal fluctuations along different dimensions.