Picosecond magnetization dynamics in nanomagnets: Crossover to nonuniform precession

The picosecond magnetization dynamics of arrays of square ${\mathrm{Ni}}_{88}{\mathrm{Fe}}_{12}∕{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}$ bilayer nanoelements were studied by optical pump-probe measurements. Experimentally observed modes were found to fall upon two branches, with a crossover from the high- to low-frequency regime as the element size was reduced to less than $220\mathrm{nm}$. Micromagnetic simulations revealed that the branches are associated with center and edge modes. The edge mode is found to dominate as the element size is reduced so that the magnetic response to a pulsed field becomes less spatially uniform.

Figure 1

(Color online) TR Kerr signals obtained from the element arrays and their FFT spectra are presented in the left and right columns, respectively. In each panel, curves from top to bottom correspond to bias field values of 772, 589, 405, 267, and $152\mathrm{Oe}$, respectively; although for the $220\mathrm{nm}$ element there are no data for the field value of $267\mathrm{Oe}$.

Figure 2

(Color online) FFT spectra of the out-of-plane component of the average dynamic magnetization of the center element in the model arrays are shown for different values of the static field. In each panel, curves from top to bottom correspond to bias field values of 772, 589, 405, 267, and $152\mathrm{Oe}$, respectively.

Figure 3

(Color online) Extracted mode frequencies are plotted against element size for different bias fields. The symbols represent data points and the curves are simulations. A small horizontal shift has been applied to the experimental frequencies at different bias fields for clarity. The error bars represent the full width at half maximum of the spectral mode peaks.

Figure 4

The spatial distribution of the magnitude of the demagnetizing field in the model element arrays is shown in column (a) of the upper panel. White corresponds to the maximum of the demagnetizing field magnitude. Columns (b), (c), and (d) show the spatial distribution of the out-of-plane magnetization component at the marked antinodes in the TR average magnetization shown in panel (e). White corresponds to positive $M_z$. The static field of $589\mathrm{Oe}$ was applied horizontally.