Time-Resolved Measurement of Propagating Spin Waves in Ferromagnetic Thin Films

We measure the propagation of spatially localized spin waves in NiFe thin films through local inductive detection of the dynamic magnetization. A pulsed magnetic field excites a linear superposition of spin wave modes with a distribution that is predominantly driven by the spatial dependence of the in-plane excitation field. The results of numerical micromagnetic calculations exhibit excellent agreement with experiment and show that a comprehensive account of spatial nonuniformity and propagation is necessary to accurately measure the intrinsic damping rate.

Figure 1

Schematic of the experimental geometry. (a) Two ACPS transmission lines overlay ferromagnetic squares having uniaxial anisotropy in the $x$ direction. The conductor dimensions in the narrow region are $w_2=3\mu \mathrm{m}$ and $w_1=9\mu \mathrm{m}$, and they are separated by $2\mu \mathrm{m}$. The gap, $g$, ranges from 2.5 to $50\mu \mathrm{m}$. The fields $H_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}$ and $H_{\mathrm{s}\mathrm{a}\mathrm{t}}$ are produced by a set of Helmholtz coils. (b) Schematic of the magnetostatic wave packet, $M_y$, and the in-plane field, $H_{\mathrm{w}}$, from the equal and opposite currents flowing through the ACPS. The dotted line is the Gaussian amplitude and the solid line is the full shape of $M_y$.

Figure 2

Inductive voltage as a function of distance $g$ from the excitation waveguide for devices with 100 nm NiFe at a 100 Oe bias field. The label “$0\mu \mathrm{m}$” indicates the response from the excitation ACPS. The arrows show when the excitation ACPS is pulsed for all devices.

Figure 3

Bias field dependence of the global fit parameters. (a) Wave vector. The dashed line indicates the peak location in the Fourier transform of the in-plane excitation field. (b) The initial $1/e$ point of the Gaussian wave packets. The inset shows an example of the agreement between experiment (open circles) and the fit (solid line) for the 100 nm NiFe device at 100 Oe with $g=30\mu \mathrm{m}$.

Figure 4

Exponential decay time versus bias field for a device with a large CPW overlaying a 1 mm square of 100 nm thick NiFe, as described in the text. The inset shows experimental data (open circles) and the micromagnetic simulation (solid line) for a 16 Oe bias field.