Abstract
The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving atomistic spin dynamics simulations. Our model is in agreement with the observed lattice dynamics and previously reported electron and magnetization dynamics. Our approach reveals that the spin system is the dominating heat sink in the initial few hundreds of femtoseconds and implies a transient non-thermal state of the spins. Our results provide a clear picture of the microscopic energy flow between electronic, magnetic and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems.
Highlights
The discovery of ultrafast demagnetization in ferromagnetic nickel in 1996 by Beaurepaire et al [1] induced a paradigm shift in the field of magnetism
Our results provide a clear picture of the microscopic energy flow between electronic, magnetic, and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems
The main advantage of the atomistic spin dynamics (ASD) simulations is the improved description of the spin system and its magnetization dynamics compared with the s-temperature model (TTM)
Summary
The discovery of ultrafast demagnetization in ferromagnetic nickel in 1996 by Beaurepaire et al [1] induced a paradigm shift in the field of magnetism. To obtain a consistent model for the microscopic energy flow and the magnetization dynamics, it is paramount to compare theoretical results with the response of all three subsystems, including the lattice. Dvornik et al introduced an energy-conserving model that goes beyond a thermal description of the spin system by employing micromagnetic simulations [42], but no direct comparison with experimentally measured lattice dynamics has been made yet. For this comparison, we perform spin-resolved density functional theory (DFT) calculations to obtain the electron-phonon coupling parameter Gep as well as the electronic and lattice heat capacities. III A, we compare the experimental results with the commonly used two-temperature model (TTM) and a modified TTM with strong electron-spin coupling (s-TTM) The latter is the minimal extension of the TTM that considers magnetic degrees of freedom.
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