Paul EVANS : Ultrafast probes of magnetic insulators: low-energy excitations and nanoscale dynamics

The dynamics and excitations of magnetic systems underpin future advances in quantum technologies and spintronics, including in magnetic computing, quantum transduction and communication, and spincaloritronics. A key example of this importance arises, for example, in the spin Seebeck effect (SSE) in which the temperature dependence of the inverse Hall signal has been linked to the temperature dependence of the wavevector-frequency dispersion of the excited magnons. There has been significant progress in characterizing and understanding the relevant magnons excitations, e.g. magnons, in the GHz regime in which phenomena can often be relatively straightforwardly described using ferromagnetic spin-wave modes.  There is an emerging frontier in extending the understanding of magnetic excitations to the THz range, in which the magnetic modes and their interaction with lattice excitations and with the electromagnetic field is not yet as well understood. The defining issue has been that the selective excitation and characterization of magnetic modes has been experimentally impossible.  Time-resolved hard x-ray scattering/diffraction experiments employing circular dichroism can probe the magnetism directly and use the time-domain response to probe magnetic dynamics and magnetic excitations. A similar approach in ferroelectric thin films and ferroelectric/dielectric superlattices probes modes linked to optical perturbation of the ferroelectric polarization.