Abstract
We study ultrafast magnetization quenching of ferromagnetic iron following excitation by an optical versus a terahertz pump pulse. While the optical pump (photon energy of 3.1 eV) induces a strongly nonthermal electron distribution, terahertz excitation (4.1 meV) results in a quasithermal perturbation of the electron population. The pump-induced spin and electron dynamics are interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure allows us to push the time resolution down to 130 fs, even though the driving terahertz pulse is about 500 fs long. Remarkably, the MOKE signals exhibit an almost identical time evolution for both optical and terahertz pump pulses, despite the 3 orders of magnitude different number of excited electrons. We are able to quantitatively explain our results using a nonthermal model based on quasielastic spin-flip scattering. It shows that, in the small-perturbation limit, the rate of demagnetization of a metallic ferromagnet is proportional to the excess energy of the electrons, independent of the precise shape of their distribution. Our results reveal that, for simple metallic ferromagnets, the dynamics of ultrafast demagnetization and of the closely related terahertz spin transport do not depend on the pump photon energy.Received 29 April 2021Revised 15 September 2021Accepted 4 October 2021DOI:https://doi.org/10.1103/PhysRevX.11.041055Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDemagnetizationElectron relaxationSpin dynamicsTechniquesMagneto-optical Kerr effectTerahertz techniquesCondensed Matter, Materials & Applied Physics
Highlights
To push writing of magnetic information to ultrafast timescales, it is essential to understand the response of magnetically ordered solids to a quasi-instantaneous perturbation [1,2]
The pump-induced spin and electron dynamics are interrogated by the magneto-optic Kerr effect (MOKE)
For simple metallic ferromagnets, the dynamics of ultrafast demagnetization and of the closely related terahertz spin transport do not depend on the pump photon energy
Summary
To push writing of magnetic information to ultrafast timescales, it is essential to understand the response of magnetically ordered solids to a quasi-instantaneous perturbation [1,2]. In ferromagnetic metals like Fe, Co, and Ni, the resulting ultrafast demagnetization (UDM) is known to proceed on a timescale of 100 fs and yields insights into fundamental material parameters such as the electron-spin equilibration time. UDM is an important process in all-optical magnetization switching [2]. It is driven by the same force as ultrafast spin transport [6,7,8,9] and, relevant for applications such as generation of spin torque [10] and terahertz (THz) electromagnetic pulses [11,12,13].
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