FERMI | DiProI Tracking the magnetization dynamics of a magnetic vortex by magnetic helicoidal dichroism Small magnetic objects, with lateral size confined at the micrometer or nanometer scale, are the basis of a wide range of technological applications, from sensors to data storage. New approaches, based on a variety of methodologies, are being developed for controlling and measuring the magnetic properties of these small objects and their dynamics. A set of techniques traditionally used for measuring magnetization, and more recently also for controlling it, is based on the interaction with polarized light beams, i.e. on magneto-optics. Magnetic circular dichroism (MCD), defined as the difference in the response to the interaction with a right- or left-handed circularly polarized beam, is one of the most frequently employed techniques for characterizing the magnetic state of a material. The response of matter to a light pulse is a function of its polarization, i.e. of the dynamic behavior of its electric field. According to the quantum interpretation of light as a photon, the polarization defines a spin angular momentum (SAM) of σℏ per photon, with σ = 0 for linear polarization and σ = ±1 for left or right circular polarization. In addition to the SAM, photon beams can carry also an orbital angular momentum (OAM) of Lℏ/ photon, associated to an azimuthally varying phase term exp(iLφ) in the Laguerre-Gaussian representation of the electromagnetic radiation. In the visible and near-infrared (IR) range, OAM beams have found many applications in fields as different as biology, telecommunication, imaging and quantum technologies. Recently, the generation of OAM beams at shorter wavelengths, from extreme ultra-violet to hard x-ray, is also finding an increasing number of applications. As it happened for the SAM, the handedness imposed by the OAM has been exploited to perform x-ray spectroscopic studies of chiral systems in general, and of magnetic materials in particular. The concept of magnetic helicoidal dichroism (MHD) has been introduced to describe the dependence of the resonant absorption and scattering from a magnetic material on the OAM value of the incident photon beam, opening new perspectives for highresolution element-selective studies of magnetic structures. This new method is particularly suited for monitoring the magnetization dynamics of magnetic vortices. In 2021, an electromagnetic theory of MHD was proposed and rapidly confirmed by an experimental proof-ofconcept, where this type of dichroism was observed. Following these promising results, a wide collaboration involving several European laboratories has shown that it is possible to perform time-resolved MHD experiments at the FERMI free-electron laser source in Trieste (Italy), which provides short (<100 fs duration) and intense light pulses at wavelengths going from the ultra-violet to the soft x-ray range. In particular, it has been possible to image the picosecond-scale temporal evolution of the response of a magnetic vortex after the excitation by a femtosecond IR laser pulse. It is well known that the magnetization of a material excited by an ultrafast and intense infrared (IR) laser pulse is temporarily reduced due to an energy redistribution between spin, charge carriers and phonons. Nonetheless, the mechanism driving this phenomenon remain poorly understood at the atomic and microscopic scales. The timeresolved MHD technique has made it possible to follow step-by-step the changes in the magnetization structure of a micrometric magnetic vortex during this transient phase of de-magnetization – re-magnetization following the IR excitation. A complex behavior is observed, which depends on both time and depth into the object, involving a temporary reversal of the magnetization direction at the surface of the vortex. These experiments made it possible to follow with a wealth of details the evolution of a magnetic vortex perturbed by a short IR laser pulse, which is an important step for a better understanding of the writing and processing of magnetically encoded information. The results have been published recently in Physical Review Letters, highlighting the relevance and potential of time-resolved MHD as a new technique for characterizing the magnetic properties of complex magnetic micro-structures. Elettra Highlights 2024–2025 86
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