Due to its unique properties and potential applications, electron spin has become central to modern physics and technology. Spintronics and magnonics utilizes electron spin to enable faster, lower-power and non-volatile data processing. Applications such as MRAM, spinbased transistors, and logic devices demonstrate the advantages of spintronics over traditional charge-based electronics, especially as Moore’s Law slows. Spintronics also drives progress in magnetic storage, sensing, imaging and quantum computing. In recent years, light–matter interactions have emerged as a powerful tool to probe and control spin systems on ultrafast timescales, which have created a new paradigm in nanoscale magnetism and data transfer. Understanding the laser-induced ultrafast spin dynamics on femto-, pico-, and nanosecond timescales in ferro-, antiferro- and ferrimagnetic systems is still one of the most challenging and open question, which will address the important questions of how fast the magnetization in a material can be reoriented and what physical processes impose fundamental limits on this speed. Ultimately, light is the fastest means to alter the state of a material since laser pulses can now be generated with extremely short temporal duration down to a few tens of attoseconds.
Following the demonstration of ultrafast demagnetization in ferromagnetic Ni1, several theoretical and experimental approaches have attempted to elucidate the underlying physics of this highly non-equilibrium regime within a few hundred femtoseconds (fs) after pulsed laser excitation. The ultrafast laser pulses are also capable of generating magnons, which can be observed as precession dynamics on the nanosecond (ns) timescale2,3. This precession dynamics (GHz range), governed by the Landau-Lifshitz-Gilbert (LLG) equation4, represents a close-to-equilibrium regime characterized by the anisotropic magnetic properties, precession frequencies and damping of magnetic materials. Considerable efforts have been made to understand the underlying mechanisms and to explore the characteristic timescales of such highly non-equilibrium and close-to-equilibrium regimes. I will discuss the proposed microscopic mechanisms and our results on ultrafast demagnetization on fs timescales (highly non-equilibrium regime) for ultrathin ferromagnetic heterostructures, studied by the all-optical time-resolved magneto-optical Kerr effect (TR-MOKE) technique. I will then discuss the generation of coherent and incoherent magnons in such systems after laser induced strong ultrafast demagnetization5,6. Recently, it been considered that, on short time scales, the inertia term in the LLG equation becomes important, leading to additional nutation of the magnetization vector, which is governed by the ILLG equation7. However, experimental observation of nutation is still in its infancy8,9. I will discuss the optically driven magnetization dynamics in the yet unexplored timescale between ultrafast demagnetization and the precession dynamics, where, the direction of the magnetization and angular momentum are transiently separated due to inertia, leading to additional nutation oscillations superimposed on the usual precession, with higher frequencies (sub-THz range)10. A unified picture of strong ultrafast demagnetization via nutation to precession, over a broad timescale in a single experiment will enable the integration of different magnetic processes in one device and the control of one effect by another. Finally, in the last part of my talk, I will discuss how magnetoelastic coupling between surface acoustic waves and spin waves can amplify intrinsic magnon modes resonantly and generate new magnon modes in ferromagnetic nanostructures . I will also briefly discuss the ultrafast phenomena in some emerging quantum materials, such as non-collinear antiferromagnets and Weyl semimetals, studied using TR-MOKE and THz spectroscopy (TDS) techniques.
References:
1. E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996).
2. M. van Kampen et al, Phys. Rev. Lett. 88, 227201 (2002).
3. B. Koopmans et al., Phys. Rev. Lett. 95, 267207 (2005).
4. T. Gilbert IEEE Trans. Magn. 40, 3443 (2004).
5. A. De et al., Phys. Rev. B 109, 024422 (2024).
6. A. De et al., Phys. Rev. B 112, 144407 (2025).
7. M.-C. Ciornei et al., Phys. Rev. B 83, 020410(R) (2011).
8. K. Neeraj et al., Nat. Phys. 17, 245 (2021).
9. V. Unikandanunni et al., Phys. Rev. Lett. 129, 237201 (2022).
10. A. De. et al., Phys. Rev. B (Editors’ suggestion) 111, 014432 (2025).
11. A. De. et al., Nanoscale, 13, 10016 (2021).