HELIMAG

Recently developed laser-driven sources based on High order Harmonic Generation (HHG) in gases deliver photon pulses with durations in the femtosecond (fs) to attosecond (as) range, carrying a spin or an orbital angular momentum (SAM & OAM). The former is associated with a circular polarization of the light and carry ±ħ angular momentum. The latter has an inclined wavefront that rotates helically around the propagation axis and carries any integer value of angular momentum, in ħ units. These sources can be structured spatially and in polarization at the scale of the optical cycle. While SAM has been widely used to probe and control magnetic samples, OAM remains largely ignored in this context. At the same time, the rapid development of spintronics, the technology that makes use of both charge and spin of the electron, lets envisage the design of more energy efficient and faster electronic devices. However, the associated physics is not perfectly understood yet, in particular at ultra-short time scales, (fs and as), and at small transverse dimensions (micrometer et nanometer). In this context, two types of structures are particularly interesting, namely i) magnetic vortices (MV) and ii) all-optical switching devices (AOS), the magnetization of which is controlled by fs laser pulses. We recently predicted and observed the existence of a specific dichroism of magnetic structures, called Helicoidal Magnetic Dichroism (MHD). It appears as a change in the light intensity profile of a beam carrying an OAM, after reflection off a magnetic structure. This change, which does not require a polarimetric measurement to be observed, depends on both the absolute value and on the sign of the OAM, and, in the case of MV, on their chirality and polarization. Therefore, MHD is potentially a very powerful tool to investigate magnetic structures of interest for spintronics. HELIMAG will have three objectives. First, through the development of theory and optimized measurements, we will establish MHD as a method of choice for the study of magnetic structures. Second, we will theoretically and experimentally study the control of AOS by beams carrying an OAM with Inverse Helicoidal Dichroism (IHD) and will quantify the benefit provided by unrestricted values of the OAM, as well as by the possibility of structuring the field. Finally, we will monitor and control ultrafast spin dynamics by combining IHD-triggered dynamics with MHD diagnostics. To achieve these objectives, INSP and SPINTEC will design and prepare several types of samples: simple magnetic layers and their alloys (vortex-shaped or not), multilayers, skyrmion networks, antivortex, epitaxial multilayers presenting a modulation of magnetization, etc. The light sources of LIDYL, carrying OAM or SAM, will be focused on these structures by means of specific optics designed at SOLEIL and produced in collaboration with LCF. In a second phase, on homogeneous samples, we will design by automatic methods structured light fields amplifying or directing the transfer of angular momentum to magnetic structures. Finally, we will use these methods to control and monitor the transfer of angular momentum to magnetic structures. The corresponding theory will be developed at LIDYL for the optical part, at Spintec for micromagnetism. Helimag will have a fundamental scope, establishing a new technique for the analysis and control of magnetic samples, in static and dynamic regimes. The context of spintronics gives it an important societal scope. Although ambitious, Helimag coordinates the efforts of a consortium with unique complementary skills to keep the risk at a reasonable level.