Invited Paper SA-MoA7
Revealing Spin Texture Dynamics in Complex Materials via Time-resolved Resonant Soft X-ray Scattering

Monday, October 19, 2015, 4:20 pm, Room 112

Self-organized mesoscale spin textures emerge in complex materials due to coupling between charge, spin, and lattice degrees of freedom, and play a significant role in establishing the exotic properties of these materials. Here we focus on two examples: (1) topologically-protected spin vortices (Skyrmions) in the insulating multiferroic Cu₂OSeO₃ which result from a combination of symmetric spin-exchange interactions, and antisymmetric exchange resulting from a Dzyaloshinskii-Moriya (DM) interaction, and (2) helical spin states in the lanthanide metal Dy which result from competition between spin-orbit coupling, magneto-elastic effects, and long-range exchange coupling mediated by the indirect RKKY interaction.

A key scientific challenge is to understand the origin of these ordered spin textures, and the fundamental mechanisms and time-scales for manipulating these phases, in order to develop a knowledge base for potential technology applications. Resonant X-ray scattering (RXS) is a powerful direct probe of charge, spin, and orbital ordering in complex materials. Current synchrotron X-ray sources and new X-ray free-electron lasers enable RXS to be applied in the time domain. This provides an important new route to disentangle the cause-effect interactions that drive the formation and evolution of spin textures in complex materials.

Optical pump and resonant X-ray scattering studies of the skyrmion phase in Cu₂OSeO₃ reveal six-fold symmetric magnetic peaks that appear as satellites around the (001) Bragg peak. Transient optical excitation at 2.3 eV (above band gap) suppresses the spin ordering on a ~40 ps time scale – significantly faster than from excitation below gap at 1.5 eV, indicating an electronically-driven collapse of the skyrmion phase. We will discuss the fluence dependence of the conical and skyrmion phases and the motion of the ordering wavevector in response to excitation above and below the insulating gap.

We also report on recent studies of an epitaxially grown Y/Dy/Y multilayer film to understand the dynamics of the core-level spin helix in response to excitation of the conduction electrons responsible for the exchange interactions. Ultrafast optical excitation at 1.5 eV results in ultrafast injection of unpolarized spins into the 500 nm Dy film via nonequilibrium diffusion. The subsequent dynamics of the helical phase, revealed by time-resolved resonant X-ray scattering, differ significantly from those observed in ferromagnetic materials due to the relationship between the core spins and conduction electron Fermi surface nesting.