Can relativity explain coherent ultrafast magnetism?

Since the field of ultrafast magnetism was started by pioneering publications about 20 years ago, it has matured into a diverse area of research. The field comprises fundamental aspects of spin dynamics in ferromagnetic materials as well as novel concepts for applications towards ultrafast spintronics and magnetic recording. Here we take a look at a fascinating recent paper in the Journal of Physics: Condensed Matter special issue on ultrafast magnetism by Ritwik Mondal, Marco Berritta and Peter M Oppeneer. Read on to find out more in the authors’ words:

The keywords for next-generation information storage devices are faster and smaller. Twenty years ago, Jean-Yves Bigot and coworkers observed the laser-induced demagnetization of a ferromagnetic film within 300fs, a speed that was previously unthought-of. Their discovery proved that it could be possible to achieve magnetic memory storage at orders of magnitude higher writing rates.

Ultrafast demagnetization is observed in a typical pump-probe setup, shown in Figure 1, where the material is excited by a fs-pump laser and a fs-probe measures the material’s response. Another remarkable finding was made by Bigot, Vomir and Beaurepaire in 2009 when they observed that laser-induced demagnetization shows a non-negligible difference when the polarizations of pump and probe laser beams are parallel as compared to perpendicular, an effect which they called coherent ultrafast magnetism. However, the underlying fundamental mechanisms of laser-induced demagnetization and coherent ultrafast magnetism are still unknown and debated.

Figure 1. Sketch of the ultrafast pump-probe experiments. Femtosecond pump excitation of the material leads to an ultrafast demagnetization, measured with the probe beam. The sub-picosecond demagnetization behavior strongly depends on the polarizations of the pump and probe lasers, a phenomenon called coherent ultrafast magnetism. © Ritwik Mondal, Marco Berritta and Peter M Oppeneer.

Our approach to this issue is to use ultra-relativistic theory. We derive a new relativistic interaction Hamiltonian that involves the interaction of the laser with the spin of electrons in a magnetic material. From this Hamiltonian we deduce that this relativistic spin-photon interaction explains the appearance of laser-induced opto-magnetic fields within the material. In a typical pump-probe experiment when both pump and probe beams are linearly polarized, and their polarizations are parallel, the opto-magnetic field is zero. However, it is nonzero when they are perpendicular. Thus, this relativistic spin-photon Hamiltonian explains coherent ultrafast magnetism occurring within the first 100 fs.

About the Authors

Ritwik Mondal received his BSc in Science at Calcutta University, Kolkata, India (2010) and MSc in Physics at Indian Institute of Technology, Mumbai, India (2012). He is currently a PhD student at Uppsala University. His research interest focuses on the relativistic theory of light-matter interactions and their effects on fast and ultrafast magnetization dynamics.



Marco Berritta is a post-doctoral researcher at Uppsala University. His research focuses on the first-principles study of laser and transport-induced magnetizations as well as on the microscopic origin of fundamental parameters ruling the dynamics of magnetism in materials at the atomic scale.



Peter M Oppeneer is a professor at Uppsala University. His research focuses on theory of ultrafast laser-induced demagnetization, ultrafast spin transport, and theory of femtosecond magneto-optical spectroscopy.



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Categories: Journal of Physics: Condensed Matter, JPhys+

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