Ultrafast angular momentum relaxation dynamics studied with transient gratings*

The most debated question in the field of ultrafast magnetism in metallic ferromagnets, still not fully answered after almost 30 years of research is: how is angular momentum dissipated after ultrafast demagnetization? Several explanations have been put forward, and it is believed that a full picture requires multiple considerations which include the electronic, lattice and spin degrees of freedom in the materials. However, in many cases, the explanation is rather qualitative. In order to answer this question, we use a novel technique, the transient grating (TG) technique, which has been proven to be a sensible tool for studying ultrafast magnetization dynamics [1,2]. The time-dependent response induced by nanoscale extreme ultraviolet gratings in CoGd shows clearly different relaxation dynamics depending on the beam polarization and the detection geometry, suggesting a yet undisclosed intermediate relaxation mechanism for the angular momentum. Our all-optical TG experiments showed similar results in both cases, but at a longer timescale due to the micrometer level gratings. We will investigate this further by performing TG experiments at resonance and at off-resonance on different samples. We can only speculate at the moment, but new measurements comparing transient polarization and intensity grating may offer us a completely novel view of the physics at play, helping us uncover, quantitatively, the role of the two most prominent scattering channels, i.e. the phonon and magnon systems.