Probing magnetization dynamics with nanometer scale sensitivity

Magnetic order can be efficiently manipulated by light. For instance, a femtosecond laser pulse can quench or even deterministically reverse magnetization on an ultrafast time scale. However, studying such light-spin interactions on the nanometer scale in the lateral dimension and on the relevant femtosecond time scales has proved difficult.

Weder et al. have developed an approach to probing magnetism at such high resolution with femtosecond, resonant small-angle X-ray scattering (SAXS) in the extreme ultraviolet spectral range. The technique allowed them to study local and transient magnetization profiles with nanometer scale sensitivity of a ferromagnetic cobalt/palladium multilayer after localized excitation.

The researchers confined the laser pulses within the magnetic film with near-field metallic masks that imprinted a corresponding nanoscale transient modulation of the magnetization. The key idea was to tailor the near-field mask in such a way to induce a symmetric grating with a line-to-space-ratio around one, such that diffraction efficiencies of even orders – such as second or fourth order – are identical or close to zero.

Tiny changes in this ratio due to potential electron diffusion after optical excitation resulted in strong relative changes of the even-order diffraction intensities. An array of gratings with periodicities differing by only a few nanometers consequently led to a qualitatively different dynamic response. These experimental results together with simulations suggest an upper boundary of 3 nm for a potential lateral increase of the initially optically excited area due to hot-electron transport.

This technique has applications for all-optical magnetic data storage, since it can study the localization of bits written with short laser pulses on the nanometer scale.

Source: “Transient magnetic gratings on the nanometer scale,” by D. Weder, C. Von Korff Schmising, C. M. Günther, M. Schneider, D. Engel, P. Hessing, C. Strüber, M. Weigand, B. Vodungbo, E. Jal, X. Liu, A. Merhe, E. Pedersoli, F. Capotondi, J. Lüning, B. Pfau, and S. Eisebitt, Structural Dynamics (2020). The article can be accessed at http://doi.org/10.1063/4.0000017 .