Single pulse all-optical switching (AOS) of magnetization by fs laser pulses has been proposed as a candidate for writing data in future ultrafast memory applications. However, in this regard it is limited by being a symmetrical toggle mechanism, where the final state is always the opposite of the initial state. We show that optically induced spin currents generated in a ferromagnetic reference layer can significantly impact single pulse AOS in an adjacent all-optically switchable Co/Gd free layer. Using this mechanism, we experimentally demonstrate deterministic all-optical magnetization writing, where the final state of the free layer depends purely on the writing procedure and the magnetization direction of the reference layer. This is realized by using either one, or a sequence of two laser pulses, with carefully selected energies. Moreover this demonstrates that the strong threshold nature of AOS makes it a sensitive probe for elusive optically generated spin currents.

A. Stupakiewicz¹, C.S. Davies^2,3, K. Szerenos³, D. Afanasiev⁴, K. S. Rabinovich⁵, A. V. Boris⁵, A. Caviglia⁴, A. V. Kimel³, and A. Kirilyuk^2,3
1Faculty of Physics, University of Bialystok, 1L Ciolkowskiego, 15-245 Bialystok, Poland.
²FELIX Laboratory, Radboud University, 7 Toernooiveld, 6525 ED Nijmegen, The Netherlands.
³Radboud University, Institute for Molecules and Materials, 135 Heyendaalseweg, 6525 AJ Nijmegen, The Netherlands.
⁴Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.
⁵Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only.

Antiferromagnets have great potential for the development of fast magnetic recording devices due to their fast dynamics and insensitivity to external magnetic fields. In this context, we explore the possiblity of ultrafast all-optical magnetisation switching in antiferromagnets by studying the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density functional theory and atomistic spin dynamics simulations, we show how an ultrashort, circularly polarised, high-intensity laser pulse can switch the order parameter of the antiferromagnet. The magnetic moments induced by the inverse Faraday effect are staggered, oriented opposite to the local magnetisation, and can hence effectively switch the orientation of the magnetic sublattices within a matter of a few hundred femtoseconds. We find reliable switching of the Néel vector between two perpendicular configurations with multiple successive laser pulses and even deterministic single-pulse switching might be possible.