Ultrafast Demagnetization Dynamics at the $M$ Edges of Magnetic Elements Observed Using a Tabletop High-Harmonic Soft X-Ray Source

We use few-femtosecond soft x-ray pulses from high-harmonic generation to extract element-specific demagnetization dynamics and hysteresis loops of a compound material for the first time. Using a geometry where high-harmonic beams are reflected from a magnetized Permalloy grating, large changes in the reflected intensity of up to 6% at the $M$ absorption edges of Fe and Ni are observed when the magnetization is reversed. A short pump pulse is used to destroy the magnetic alignment, which allows us to measure the fastest, elementally specific demagnetization dynamics, with 55 fs time resolution. The use of high harmonics for probing magnetic materials promises to combine nanometer spatial resolution, elemental specificity, and femtosecond-to-attosecond time resolution, making it possible to address important fundamental questions in magnetism.

Figure 1

(a) Experimental setup. (b) Scanning electron micrograph of the Permalloy grating with a period of $2\mu \mathrm{m}$. (c) Sample geometry—the blue arrow depicts the $p$-polarized soft x-ray beam whose plane of incidence is shown by the dashed black box. The black arrows indicate the two directions of magnetization reversal, which are perpendicular to the plane of incidence of the probe light. (d) Typical harmonic spectrum reflected from the Permalloy grating and recorded on a CCD.

Figure 2

Experimental data showing the HHG spectrum (lower plot, right axis) and measured magnetic asymmetry (dashed lines in upper plot, left axis) for different incident angles. No asymmetry is measured for $s$-polarized probe light (black line). The vertical lines denote the Fe, Ni, and Al absorption edges.

Figure 3

Hysteresis loop of the magnetic asymmetry measured near the Ni (top) and Fe (bottom) $M$ edges, with the selected energies indicated on the right.

Figure 4

Demonstration of the near-surface sensitivity of $T$-MOKE using high-harmonic beams. The measured asymmetry near the Fe (circle) and Ni (triangle) edges is plotted as a function of Pd capping layer thickness. The curves correspond to an exponential decay length of $\sim 3\mathrm{nm}$ in each case.

Figure 5

Ultrafast, element-selective measurement of demagnetization following laser-pulse excitation of both Ni and Fe components of Permalloy. The magnetic orientation decreases for Fe and Ni on similar time scales of 400 fs. For this measurement, the time resolution is 55 fs. Error bars of the measurements are represented at the 2.1 ps data points.