VP12-08 Simulation of ultrafast magnetization dynamics in ferromagnetic nanoparticles
Creators: Pawel Steblinski
Tomasz Blachowicz
Andrea Ehrmann
Publisher: name: IEEE
Year: 2025

Ultrafast magnetization dynamics of thin-film magnetic samples can be investigated by pump-probe experiments, i.e. a strong pump laser induces a heat pulse which leads to a reduction of the magnetic anisotropies in the sample and thus a change in the direction of the superposition of external magnetic field and internal anisotropies. The resulting magnetization vector change leads to a damped precession which can be measured by the weak probe laser [1-3]. Experimental investigations by pump-probe measurements pose challenges regarding the overlap of both laser spots and the delay between both measurements. Modeling this process can thus be supportive to understand ultrafast magnetization dynamics of thin film samples or magnetic nanostructures [4]. Here we report a new model based on micromagnetic simulations, using the finite element method, which we applied to investigate ultrafast magnetization dynamics in Fe nanoparticles, embedded in an MgO layer to ensure adequate thermal conditions (Fig. 1). Heat transport was calculated by the two-temperature model. The simulations are based on solving the Landau-Lifshitz-Bloch (LLB) equation. As an example, Fig. 2 shows the y-component of the spin precession in the Fe nanoparticle depicted in Fig. 1, applying an external magnetic field of 35.8 kA/m at an angle of 30° to reach saturation. Angle-dependent simulations of the precession frequencies (Fig. 2b) reveal a clear difference between such nanoparticles and the values calculated by the Kittel equation [5] for demagnetization factors representing a large thin film [6]; instead they show angle-dependent anisotropies strongly dependent on the sample orientation (Fig. 2b-d). Such simulations thus allow for estimating the ultrafast magnetization dynamics of diverse samples and optimizing samples for ultrafast magnetization reversal.References: [1] E. Carpene, E. Mancini, D. Dazzi, C. Dallera, E. Puppin, S. De Silvestri, Ultrafast three-dimensional magnetization precession and magnetic anisotropy of a photoexcited thin film of iron, Phys. Rev. B 81, 060415(R) (2010) [2] E. Carpene, E. Mancini, C. Dallera, E. Puppin, S. De Silvestri, Femtosecond laser spectroscopy of spins: Magnetization dynamics in thin magnetic films with spatio-temporal resolution, Thin Solid Films 519, 1642-1648 (2010) [3] S. Monda, A. Barman, Laser Controlled Spin Dynamics of Ferromagnetic Thin Film from Femtosecond to Nanosecond Timescale, Phys. Rev. Appl. 10, 054037 (2018) [4] T. Korn, F. Giesen, J. Podbielski, D. Ravlic, C. Schueller, D. Grundler, Time-resolved study of the increased magnetization precession frequency in Fe wires, J. Magn. Magn. Mater. 285, 240-244 (2005) [5] C. Kittel, Introduction to Solid State Physics 4th Edition, John Wiley and Sons (New York, London, Sydney, Toronto) (1971) [6] M. Bauer, Grundlagen und Strategien für schnelles Schalten der Magnetisierung, PhD thesis, Kaiserslautern University (2000)
Ultrafast magnetization dynamics of thin-film magnetic samples can be investigated by pump-probe experiments, i.e. a strong pump laser induces a heat pulse which leads to a reduction of the magnetic anisotropies in the sample and thus a change in the direction of the superposition of external magnetic field and internal anisotropies. The resulting magnetization vector change leads to a damped precession which can be measured by the weak probe laser [1-3]. Experimental investigations by pump-probe measurements pose challenges regarding the overlap of both laser spots and the delay between both measurements. Modeling this process can thus be supportive to understand ultrafast magnetization dynamics of thin film samples or magnetic nanostructures [4]. Here we report a new model based on micromagnetic simulations, using the finite element method, which we applied to investigate ultrafast magnetization dynamics in Fe nanoparticles, embedded in an MgO layer to ensure adequate thermal conditions (Fig. 1). Heat transport was calculated by the two-temperature model. The simulations are based on solving the Landau-Lifshitz-Bloch (LLB) equation. As an example, Fig. 2 shows the y-component of the spin precession in the Fe nanoparticle depicted in Fig. 1, applying an external magnetic field of 35.8 kA/m at an angle of 30° to reach saturation. Angle-dependent simulations of the precession frequencies (Fig. 2b) reveal a clear difference between such nanoparticles and the values calculated by the Kittel equation [5] for demagnetization factors representing a large thin film [6]; instead they show angle-dependent anisotropies strongly dependent on the sample orientation (Fig. 2b-d). Such simulations thus allow for estimating the ultrafast magnetization dynamics of diverse samples and optimizing samples for ultrafast magnetization reversal.References: [1] E. Carpene, E. Mancini, D. Dazzi, C. Dallera, E. Puppin, S. De Silvestri, Ultrafast three-dimensional magnetization precession and magnetic anisotropy of a photoexcited thin film of iron, Phys. Rev. B 81, 060415(R) (2010) [2] E. Carpene, E. Mancini, C. Dallera, E. Puppin, S. De Silvestri, Femtosecond laser spectroscopy of spins: Magnetization dynamics in thin magnetic films with spatio-temporal resolution, Thin Solid Films 519, 1642-1648 (2010) [3] S. Monda, A. Barman, Laser Controlled Spin Dynamics of Ferromagnetic Thin Film from Femtosecond to Nanosecond Timescale, Phys. Rev. Appl. 10, 054037 (2018) [4] T. Korn, F. Giesen, J. Podbielski, D. Ravlic, C. Schueller, D. Grundler, Time-resolved study of the increased magnetization precession frequency in Fe wires, J. Magn. Magn. Mater. 285, 240-244 (2005) [5] C. Kittel, Introduction to Solid State Physics 4th Edition, John Wiley and Sons (New York, London, Sydney, Toronto) (1971) [6] M. Bauer, Grundlagen und Strategien für schnelles Schalten der Magnetisierung, PhD thesis, Kaiserslautern University (2000)