Magnetic Instability Regions in Patterned Structures: Influence of Element Shape on Magnetization Reversal Dynamics

We report a time-resolved imaging study of the influence of shape on magnetic instabilities in patterned magnetic structures. We find that in rectangular structures magnetization reversal initiates at the ends and interior simultaneously, while in structures with tapered ends the reversal begins in the middle of the structures and spreads out to the ends. The degree of tapering is important for both the switching field and the time required for full reversal. A model based on the concept of local instability regions yields good agreement with the observed location of the reversal onsets.

Figure 1

(a) Schematic of the time-resolved PEEM setup. The current pulse train $J(t)$ is synchronized with the x-ray intensity $I_0(t)$ with a variable delay, $\Delta t$, allowing the x-ray pulses to be scanned across the positive-to-negative field transition. In (b), the time-dependent field on the dots is plotted, as estimated from the current at the output of the waveguide using the waveguide equation $B=J/2A$. The transition time obtained (10%–90%) is 360 ps.

Figure 2

Scanning electron micrograph (top left image) and selected PEEM magnetic domain images for the five $2\times 7\mu \mathrm{m}$ elongated dots at various times after the field reversal at $\Delta t=0\mathrm{ps}$. The domain images are the differences of PEEM images taken at the Ni ${\mathrm{L}}_3$ resonance with left and right circularly polarized radiation. Thus, the brightness in the XMCD images is approximately proportional to $M_x$.

Figure 3

(a) Gray scale image of the form factor $N_{xx}-N_{yy}$ for sample 1. $N_{xx}-N_{yy}$ is a measure of the effective demagnetizing field: black indicates that the field is along the long axis (i.e., stable) and white along the short axis (i.e., unstable) of the structure. (b) Average local field as a function of reversed domain size for domains nucleated in the center and the end of the structure. (c) Nucleation fields for nucleating reversed domains at the center and at the ends of the structures for the five different shapes. Note that the external applied field is $-25\mathrm{Oe}$.

Figure 4

Hysteresis loops for each structure measured with MOKE.