Availability of Ti:Sapphire femtosecond laser pulse sources in the 1990 enabled the investigation of picosecond ( 1 ps = 10-12 s) magnetic dynamics. By 1996, a high intensity 0.1 ps long pulsed excitation had been shown to drive an unexpected subpicosecond demagnetisation of nickel. New discoveries such as coherent and uncoherent excitation of ferromagnetic and antiferromagnetic resonance, generation of THz radiation and switching of magnetisation without applied magnetic field, lay the foundations of the emerging domain of ultrafast magnetism.
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.
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).