Time-resolved imaging of domain pattern destruction and recovery via nonequilibrium magnetization states

The destruction and formation of equilibrium multidomain patterns in permalloy $\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)$ microsquares has been captured using pump-probe x-ray magnetic circular dichroism (XMCD) spectromicroscopy at a new full-field magnetic transmission soft x-ray microscopy endstation with subnanosecond time resolution. The movie sequences show the dynamic magnetization response to intense Oersted field pulses of approximately 200-ps root mean square (rms) duration and the magnetization reorganization to the ground-state domain configuration. The measurements display how a vortex flux-closure magnetization distribution emerges out of a nonequilibrium uniform single-domain state. During the destruction of the initial vortex pattern, we have traced the motion of the central vortex core that is ejected out of the microsquare at high velocities exceeding 1 km/s. A reproducible recovery into a defined final vortex state with stable chirality and polarity could be achieved. Using an additional external bias field, the transient reversal of the square magnetization direction could be monitored and consistently reproduced by micromagnetic simulations.

Figure 1

Time-resolving full-field magnetic transmission x-ray microscopy (TR-MTXM) setup. The grating condenser illuminates a $20\text{−}\mu \mathrm{m}$ diameter spot on the sample that is imaged by the micro zone plate with an outermost zone width of $50\mathrm{nm}$ onto a gateable 2D detector. The sample structures can be excited by short magnetic field pulses ${\mathbf{h}}_{\mathrm{pulse}}$ formed by a transient current through a coplanar waveguide.

Figure 2

Image sequence of the magnetization dynamics of two $2\times 2\text{−}{\mu \mathrm{m}}^2$ permalloy squares (I + II) exposed to a short magnetic field pulse of $\tau =186\mathrm{ps}$ duration and ${\mu }_0{h}_0\approx 82\mathrm{mT}$ peak amplitude at $\Delta t=0\mathrm{ns}$. The XMCD images recorded at different pump-probe delays $\Delta t$ show the Landau vortex domain pattern before the pulse, its destruction followed by a homogeneous magnetization during the pulse and the domain pattern recovery after the pulse has vanished. Each XMCD frame is generated by subtracting two images with $180\text{−}\mathrm{s}$ exposure time at opposing photon helicities. Images within the full width at half maximum (FWHM) of the pulse are highlighted by orange frames and key images discussed in the text are marked by red frames.

Figure 3

Spatially averaged inverted gray level of the permalloy squares I (red circles) and II (black squares) in the XMCD images of Fig. 2 corresponding to the total magnetization component $M_x$ with respect to time. Along with the data is plotted the $M_x$ output of a micromagnetic simulation (blue solid curve).

Figure 4

Trajectory of the vortex core motion in permalloy square II with the corresponding frames of Fig. 2 during the domain pattern destruction at the rising slope of the excitation pulse ${\mathbf{h}}_{\mathrm{pulse}}$.

Figure 5

Magnetization reversal of the $2\times 2\text{−}{\mu \mathrm{m}}^2$ permalloy square I of Fig. 2 in a static external magnetic field counteraligned to the transient excitation pulse field ${\mathbf{h}}_{\mathrm{pulse}}$. Each XMCD frame is generated by subtracting two images exposed for $360\mathrm{s}$ at opposing photon helicities. Images within the pulse FWHM are highlighted by orange frames.

Figure 6

Simulated magnetization response of a $2\times 2\times 0.03\text{−}{\mu \mathrm{m}}^3$ permalloy square to the excitation pulse ${\mathbf{h}}_{\mathrm{pulse}}$ of $\tau =210\mathrm{ps}$ duration with an additional opposing external magnetic bias field of ${\mu }_0{H}_{\mathrm{ext}}=38\mathrm{mT}$ tilted by an angle of $5^{\circ }$ with respect to the $x$ axis. The output of the nmag simulation displays the experimentally accessible $M_x$ component of discretized magnetization vectors $\mathbf{M}$. Black indicates an orientation along the $x$ axis and white into the opposite direction. The red framed images at $0.35$ and $0.40\mathrm{ns}$ also show the simulated decay back to the alignment along the bias field direction where no XMCD data are available.

Figure 7

Spatially averaged inverse XMCD gray level of permalloy square I in Fig. 5 (black squares) with the averaged $M_x$ output of the micromagnetic simulation in Fig. 6 (blue solid curve).