Magnetic helicoidal dichroism with XUV light carrying orbital angular momentum

Circularly polarized light is a fundamental tool in magnetic studies, notably for magnetization dynamics. It is less common in magneto-optics to exploit the orbital angular momentum (OAM) of value ℓ carried by light beams possessing a helical wavefront. After finding many applications in the visible range, recently OAM pulses with ultra-short duration and XUV wavelengths became available, widening the range of experiments that can be envisaged. We modelled the interaction of an XUV OAM beam with non-uniform magnetic structures, showing that the far field scattered intensity profile encodes the symmetry of the magnetic structure in a way that depends on the sign and value of ℓ. In analogy with magnetic circular dichroism, this effect, named magnetic helicoidal dichroism (MHD), can be observed by inverting the sign of either the orbital momentum or of the magnetization. We obtained experimental evidence of MHD by measuring ℓ-dependent resonant scattering from a magnetic vortex. The results of recent complementary experiments match well the theoretical predictions, confirming the potential of the new toolset provided by MHD for studying the laser-triggered ultrafast dynamics of complex magnetic materials.