Femtosecond Single-Shot Imaging of Nanoscale Ferromagnetic Order in $\mathrm{Co}/\mathrm{Pd}$ Multilayers Using Resonant X-Ray Holography

We present the first single-shot images of ferromagnetic, nanoscale spin order taken with femtosecond x-ray pulses. X-ray-induced electron and spin dynamics can be outrun with pulses shorter than 80 fs in the investigated fluence regime, and no permanent aftereffects in the samples are observed below a fluence of $25\mathrm{mJ}/{\mathrm{cm}}^2$. Employing resonant spatially muliplexed x-ray holography results in a low imaging threshold of $5\mathrm{mJ}/{\mathrm{cm}}^2$. Our results open new ways to combine ultrafast laser spectroscopy with sequential snapshot imaging on a single sample, generating a movie of excited state dynamics.

Figure 1

The experimental setup. (a) Scanning electron microscopy image of a 15-reference gold holography mask, showing the aperture and the references. Samples with 5 references contain the inner ring of references only. The sample aperture diameter is $1.45\mu \mathrm{m}$ (two markers), and the references are 100 nm in diameter. (b) A CCD camera located 490 mm downstream (numerical aperture of 0.028) records the spectrohologram in the far field. A tungsten carbide beam stop is used to block the direct transmitted beam to prevent damage to the detector. (c) Reconstruction of the initial magnetic domain state from a low-fluence-accumulated spectrohologram with 58% circularly polarized x-ray pulses ($<2\mathrm{mJ}/{\mathrm{cm}}^2$). The dark and light regions are 100–150 nm wide domains with opposite out-of-plane magnetization directions.

Figure 2

Single-shot reconstruction of the nanoscale ferromagnetic ordering. (a) X-ray spectrohologram from a 5-reference sample containing $1.5\times {10}^5$ detected photons from an 80 fs pulse. The scale bar unit is photon count per pixel. The corners shown here correspond to a momentum transfer of $0.054{\mathrm{nm}}^{-1}$. (b) Real space magnitude of the complex autocorrelation after Fourier inversion of (a). The center of the image is dominated by the self correlation of the object and references. The complex conjugate pairs of object-reference cross correlations are separated due to the off-axis geometry of the references, and the smaller, bright spots are the cross-correlation pairs between references. (c) The final averaged reconstruction from (b) with a 10%–90% resolution of 80 nm, which is limited by the reference size.

Figure 3

Fluence dependence of charge-magnetic interference intensity for 80 and 360 fs single shots. The errors are shot noise dependent and the bars shown are representative of other data points with similar fluences and photon statistics. For 80 fs pulses, the CMI intensity remains proportional to the incident x-ray fluence. This relation holds for low fluence 360 fs pulses, while a reduction of CMI intensity was observed at higher fluences indicating that x-ray-induced demagnetization occurred during the longer pulse durations. The reduction of the CMI intensity is in agreement with calculations (red line) based on optically induced ultrafast demagnetization with comparable amounts of energy deposited.

Figure 4

Fluence thresholds and image correlations in single-shot images with 360 fs pulses. (a) Fluence plot of the single-shot series. The blue markers represent pulses that are above the statistical threshold of $5\mathrm{mJ}/{\mathrm{cm}}^2$ for spectroholographic imaging. The red marker represents the pulse with sufficient fluence to induce irreversible changes in the sample. Gray markers indicate pulses that are below the imaging threshold. Error bars indicate uncertainty due to shot noise. (b)–(c) Average reconstruction from shots 12 to 23 and reconstruction from shot 24. (d)–(e) Average reconstruction from shots 25 to 31 and reconstruction from shot 33. Line cuts show good agreement of the domain patterns shot to shot.