Magnetic dynamics at the nanoscale revealed by transient magnetization gratings
Fundamental aspects of magnetic dynamics, such as the generation and the lifetime of optically induced spin currents in magnetic thin-films, need to be understood for further developing science and technology based on magnetic processes. Extreme ultraviolet (EUV) and X-ray free electron lasers (FELs) offer a prospect of controlling and studying magnetism on ultrafast timescales and on a spatial scale much finer than what is possible with optical sources.
In particular, the newly developed EUV transient grating (TG) setup at FERMI provides the unique capability to excite and probe spatially periodic responses with nanoscale periods. In this work we use the interference between two crossed FEL pulses to generate transient magnetization patterns with spatial periods (Λ) as short as 44 nm (Figure 1, a) on a thin, 9-nm-thick, magnetic film containing of Co0.81Gd0.19 alloy with perpendicular magnetic anisotropy. The dynamics launched by such magnetization grating is monitored via transient diffraction of a third, time-delayed FEL probe pulse resonant with the dichroic M-edge of cobalt. After each FEL shot, the sample is brought back to the initial magnetic state by the field H.
Figure 1, b shows the dependence of the EUV TG signals on the pump−probe time delay at Λ = 87.2 nm. One can see that in a saturating magnetic field (± 40 mT), the signal is much larger than at zero field and consists in a signal rise on a sub-ps time scale, followed by a decay on the time scale of tens of ps. The zero field instead signal shows a rise time close to the FEL pulse duration and a ∼0.5 ps decay, which is consistent with the creation (on a timescale faster than the FEL pulse duration) of an electronic excitation grating that then decays via electron-phonon interactions. Since this nonmagnetic contribution to the TG signal is independent on the magnetic field, the signal measured in the saturating field must almost entirely come from the transient magnetization grating. The initial fast response of the EUV TG signal is indeed consistent with the known thermalization time (∼200 fs) of the Co sublattice, while the subsequent slower demagnetization has not been previously observed, suggesting a different ultrafast magnetic mechanism in nanoscale patterns of magnetization. The decay time of the magnetization grating is longer at longer TG period (Figure 1, c) and does not follow the Λ2dependence, expected when the mechanism for the TG decay is purely diffusive.
The EUV TG technique is well suited for studying spin transport, and we expect that the hallmark of spin diffusion will be a dependence of the demagnetization dynamics on the TG period. The dynamics of magnetization for various magnetic elements can be observed by tuning the probe wavelength to their absorption edges. We envision that our work will stimulate further research on nanoscale magnetic transient gratings, with the prospect of further exciting opportunities that cannot be anticipated at this early stage.
Figure 1. (a) Schematic illustration of the experiment. (b) EUV TG signal as a function of the time delay between the TG excitation and the probe pulses, recorded without and with positive and negative external magnetic field; the EUVr TG periodi is Λ = 87.2 nm. (c) Dynamics of the transient magnetization grating signal for Λ= 43.6 nm and Λ = 87.2 nm. Solid lines are fits to an exponential function with an offset correction.
This research was conducted by the following research team:
Dmitriy Ksenzov1, Alexei A. Maznev2, Vivek Unikandanunni3, Filippo Bencivenga4, Flavio Capotondi4, Antonio Caretta4, Laura Foglia4, Marco Malvestuto4, Claudio Masciovecchio4, Riccardo Mincigrucci4, Keith A. Nelson2, Matteo Pancaldi3, Emanuele Pedersoli4, Lisa Randolph1, Hendrik Rahmann1, Sergei Urazhdin5, Stefano Bonetti3,6, and Christian Gutt1
1 Department Physik, Universität Siegen, Siegen, Germany
2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
3 Department of Physics, Stockholm University, Stockholm, Sweden
4 Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy.
5 Department of Physics, Emory University, Atlanta, Georgia, United States
6 Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venice, Italy
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