Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation

Ultrashort, coherent x-ray pulses of a free-electron laser are used to holographically image the magnetization dynamics within a magnetic domain pattern after creation of a localized excitation via an optical standing wave. We observe a spatially confined reduction of the magnetization within a couple of hundred femtoseconds followed by its slower recovery. Additionally, the experimental results show evidence of a spatial evolution of magnetization, which we attribute to ultrafast transport of nonequilibrium spin-polarized electrons for early times and to a fluence-dependent remagnetization rate for later times.

Figure 1

Experimental setup. The black and white areas of the sample represent the in- and out-of-plane magnetized domains. (a) Scanning electron microscope image of the elliptical object hole with two gold nanoislands. (b) Calculated squared electric field $|E{|}^2$ after excitation with $p$-polarized, $\lambda =780\text{−}\mathrm{nm}$ light, incident under 45°.

Figure 2

Reconstructions of the magnetic domains for unpumped (a) and pumped samples for selected time delays (b)–(d). In the upper area of the elliptical object hole, the magnetic contrast is reduced. (e)–(h) Calculated variance of $72\times 72\text{−}{\mathrm{nm}}^2$ areas of the magnetic contrast. For normalization, see text.

Figure 3

(a) Normalized integrated variance as a function of time (line is a guide to the eye). Dotted line: double exponential function describing time-resolved MOKE measurements with ${\tau }_{\mathrm{RE}}=2.6\mathrm{ps}$. The demagnetization time constant has been set to 100 fs. (b) Normalized radial integrated variance as a function of radius for different time delays with corresponding error functions, indicating the size of the demagnetized area. Inset: variance matrix showing the area of the radial integration. (c) Inflection point, $x_0$, and width, ${\mathrm{w}}_{\sigma }$, of the radial integrated variance as a function of time delay.

Figure 4

(a) Measured time-resolved MOKE for fluences of 1.9 (diamonds), 2.9 (circles), 3.9 (plus signs), 4.8 (stars), and $5.8\mathrm{mJ}{\mathrm{cm}}^{-2}$ (crosses). (b) Remagnetization times as a function of fluence; (red dot) extrapolation for an excitation of $9\mathrm{mJ}{\mathrm{cm}}^{-2}$. (c) Calculated lateral magnetization assuming weighted remagnetization times ${\tau }_{\mathrm{RE}}$ between 0 and 2.6 ps for nondemagnetized and maximal demagnetized regions, respectively. (d) Inflection point, $x_0$, and (e) $1\sigma$ width, $w_{\sigma }$, of the magnetization profile extracted from (c) and corresponding data from Fig. 3(c) (squares). (f) Real-space domain pattern [cf. Fig. 2] with simulated local reduction of the XMCD contrast and softening of the domain walls. The red line indicates the position of profile shown in (g) with (dashed line) and without (solid line) domain wall broadening. (h) Corresponding radial integrated variance with (dashed line) and without (solid line) domain wall broadening.