Magnonic Crystal as a Medium with Tunable Disorder on a Periodical Lattice

We show that periodic magnetic nanostructures represent a perfect system for studying excitations on disordered periodical lattices because of the possibility of controlled variation of the degree of disorder by varying the applied magnetic field. Magnetic force microscopy images and ferromagnetic resonance (FMR) data collected inside minor hysteresis loops for a periodic array of Permalloy nanowires were used to demonstrate correlation between the type of FMR response and the degree of disorder of the magnetic ground state.

Figure 1

(a): SEM image of the CPW line and of two nanowires arrays (inset). (b): SEM image of the alternating width nanowire array ($w_1=260\mathrm{nm}$, $w_2=220\mathrm{nm}$ and edge-to-edge separation $g=60\mathrm{nm}$). (c): Full loop 2D FMR absorption spectra for the array. (d): Normalized M-H loop for the array.

Figure 2

(a-1) to (a-6): FMR absorption spectra inside the minor loops with $H_{\max }=128\mathrm{Oe}$, 163 Oe, 177 Oe, 192 Oe, 199 Oe and 220 Oe. Inset to (a-4): example of 1D simulation. (b-1) to (b-6): MFM images for the same $H_{\max }$ at remanence. Insets: Fourier transforms of the respective MFM data. (c) Frequency of the fundamental mode at remanence (solid line) and the ratio $r_0/r_1$ (dashed line) as a function of $H_{\max }$.

Figure 3

(a)–(c): The magnetic ground states for $H_{\max }=163\mathrm{Oe}$, 177 Oe and 192 Oe, respectively. (d)–(f): the respective calculated profiles of dynamic magnetization. Red solid line: AFM mode; blue dashed line: FM mode. A 1D numerical model has been used in this calculation.