Novel schemes for optically controlling ferromagnetic order at a femtosecond time scale [1] receive great scientific interest. In the strongly non-equilibrium regime, it has become possible not only to quench magnetic order, but even to deterministically switch the magnetic state by a single femtosecond laser pulses. Moreover, it has been shown that pulsed laser excitation can induce spin currents over several to tens of nanometers. This development triggered a merge of the fields of ‘femto-magnetism’ and spintronics – opening up a fascinating playground for novel physical phenomena. In this lecture I will discuss the underlying principles, but also envision their exploitation in THz magnonics and integrated spintronic-photonic memories.

After a brief review of the field, mechanisms for ultrafast loss of magnetic order upon fs laser heating [2] as well as all-optical switching will be explained. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer [3]. These fs spin currents are absorbed within a few nanometers, providing ideal conditions for exciting and exploring THz spin waves [4]. Finally, it will be argued that synthetic, layered ferrimagnets provide an ideal platform for combining fs optical control with advanced spintronic functionality. It will be shown how magnetic bits can be written ‘on-the-fly’ by fs laser pulses in a so-called magnetic racetrack, where they are immediately transported by a dc current [5]. Such schemes may lead to a novel class of integrated photonics, in which information is transferred back and forth between the photonic and magnetic domain without any intermediate electronic steps.

 [1] E. E. Fullerton, H. A. Dürr, A. V. Kimel and B. Koopmans, Chapter VI: Interfacial effects in ultrafast magnetization dynamics, in F. Hellman et al., Interface-induced phenomena in magnetism, Rev. Mod. Phys. 89, 025006 (2017).
[2] B. Koopmans, G. Malinowski, F. Dalla Longa, D. Steiauf, M. Faehnle, T. Roth, M. Cinchetti, and M. Aeschlimann, Explaining the paradoxical diversity of ultrafast laser-induced demagnetization, Nature Materials 9, 259 (2010).
[3] A. J. Schellekens, K. C. Kuiper, R. R. J. C. de Wit, and B. Koopmans, Ultrafast spin-transfer torque driven by femtosecond pulsed-laser excitation, Nat. Commun. 5, 4333 (2014).
[4] M. L. M. Lalieu, R. Lavrijsen, R. A. Duine, and B. Koopmans, Investigating optically excited terahertz standing spin waves using noncollinear magnetic bilayers, Phys. Rev. B 99, 184439 (2019).
[5] M. L. M. Lalieu, R. Lavrijsen, and B. Koopmans, Integrating all-optical switching with spintronics, Nat. Commun. 10, 110 (2019).