Pump - probe spectroscopy of magnetisation
A widespread experimental arrangement uses an intense pump pulse to trigger the magnetisation dynamics while the magnetisation is measured with a delayed probe pulse using the magneto-optical Faraday or Kerr effect. These are linear effects consisting of a rotation of the linear polarisation of light after transmission or reflection that is proportional to the magnetisation of a material. The spin dynamics is acquired by repeating the experiment as a function of the pump-probe pulse delay. Results can vary greatly vary depending on the type of material - dielectric, semiconducting or metallic - and the type of magnetic order - ferromagnetic, antiferromagnetic or ferrimagnetic - that is studied.
In ferromagnetic metals, the absorption of the light pulse by the free electrons results in an increase of magnetic energy and a complete or partial loss of magnetic order, usually in less than ten picoseconds. Interestingly, the pump can also transiently alter the magnetic anisotropy and trigger the ferromagnetic resonance (FMR) where spins precess around an effective magnetic field. The interest of this method is that it can measure high FMR frequencies up to 10-100 THz without the use of a coplanar waveguide at a rather low cost compared to vector network analysis FMR. On the low side, frequencies are limited to about 0.1 GHz and the method is unreliable if the resonance is highly damped.
Figure 1: Optically induced precession of the magnetisation in CoFeB.
Single pulse all-optical switching in Mn2RuxGa
Light can control magnetism in numerous ways. One surprising discovery was the single pulse all-optical switching (SP-AOS) of ferrimagnetic amorphous Gd(FeCo)3 films in 2007 . SP-AOS consists of deterministic toggling of the orientation of the magnetisation following a light pulse of sufficient energy. It is a thermal effect that, surprisingly, is independent of applied magnetic field and the polarization of the pulse. The electrons in the metallic films strongly absorb the light and are heated above 1000 K. At such temperatures, the magnetisation should be greatly diminished as the atomic spins are randomized; multiple magnetic domains should re-emerge during the cooling if the Curie temperature was crossed.
For years, amorphous Gd(FeCo)3 was optically unique. Then in 2019, we discovered that SP-AOS occurs in films of the compensated ferrimagnet Mn2RuxGa (MRG). A big difference between the magnetic sublattices was believed to be a requirement for SP-AOS. This requirement is not met in MRG where both magnetic sublattices are formed by manganese in different sites of the crystal lattice. Using a pump and probe approach, we determined system can be re-switched using a second light pulse just 10 ps later, the fastest ever observed.
Fig.1. (a) Toggling of magnetisation observed by Kerr microscopy after irradiation by multiple pulses delayed by 1 s . (b) Ultrafast toggling with two pulses delayed by 9 to 12 ps .
 C. Banerjee, N. Teichert, K. E. Siewierska, Z. Gercsi, G.Y.P. Atcheson, P. Stamenov, K. Rode, J.M.D. Coey and J. Besbas, Nat. Commun. 11, 4444 (2020).
 C. S. Davies, G. Bonfiglio, K. Rode, J. Besbas, C. Banerjee, P. Stamenov, J. M. D. Coey, A. V. Kimel and A. Kirilyuk Phys. Rev. Res. 2, 032044(R) (2020).
 C. Banerjee, K. Rode, G. Atcheson, S. Lenne, Stamenov, J. M. D. Coey and J. Besbas Phys. Rev. Lett. 126,177202 (2021).