Abstract
While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction (DMI) are candidates as novel information carriers, their dynamics on the fs-ps timescale is little known. Since with the bulk Heisenberg exchange and the interfacial DMI two distinct exchange mechanisms are at play, the ultrafast dynamics of the chiral order needs to be ascertained and compared to the dynamics of the conventional collinear order. Using an XUV free-electron laser we determine the fs-ps temporal evolution of the chiral order in domain walls in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe experiment. Upon demagnetization we observe that the dichroic (CL-CR) signal connected with the chiral order correlator mzmx in the domain walls recovers significantly faster than the (CL + CR) sum signal representing the average collinear domain magnetization mz2 + mx2. We explore possible explanations based on spin structure dynamics and reduced transversal magnetization fluctuations inside the domain walls and find that the latter can explain the experimental data leading to different dynamics for collinear magnetic order and chiral magnetic order.
Highlights
While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction (DMI) are candidates as novel information carriers, their dynamics on the fs-ps timescale is little known
As the ferromagnetic alignment minimizes the Heisenberg exchange energy, while the chiral order results from the DMI, the ultrafast dynamics of both orders needs to be probed individually
Using samples with interfacial DMI and perpendicular magnetic anisotropy exhibiting labyrinth-like domain patterns, we measure in the same experiment both the sum signal corresponding to the ferromagnetic order in the domains and the difference signal corresponding to the average chiral order in the domain walls
Summary
 jmz ðQÞj2 þjmx ðQÞj2à ð1Þ with mz;xðQÞ denoting the Fourier transform of the magnetization profiles. Averaging these signals in addition along Qr (radial average) represents averages over 7.1 × 105 pixels containing in total between 1 × 105 and 5 × 105 photons With this the numerical integration of the radial profiles for each time delay leads to the data shown, which demonstrates the evolution of the total intensity of the sum and the difference signal as a function of delay time normalized to the unpumped total intensity. The time constant τr2 = 312 ± 18 ps of the difference signal is significantly smaller than the sum signal (τr2 > 900 ps) demonstrating a faster recovery of the dichroic signal that reflects the chiral magnetic order in the domain walls. We point out that a simple reduction of the magnetization by a single demagnetization factor everywhere in space reduces both ICL+CR(Q) and ICL-CR(Q) by the same factor
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