Magneto-optic enhancement and magnetic properties in Fe antidot films with hexagonal symmetry

The magneto-optic and magnetic properties of hexagonal arrays of holes in optically thin iron films are presented. We analyze their dependence on the hole radius and compare the results to a continuous iron film of the same thickness. We observe a large enhancement of the magneto-optic Kerr rotation with respect to that of the continuous film, at frequencies where surface-plasmon excitations are expected. The spectral position of the Kerr maxima can be tuned by the size and the distance between the holes. Additional simulations are in very good agreement with the experiment and thus confirm the effect of the surface plasmons on the Kerr rotation. The altering of the magnetic properties by the hole array is also visible in the hysteretic behavior of the sample where a significant hardening is observed.

Figure 1

(Color online) (a) Atomic force micrograph of a 100-nm-thick Fe film with hexagonal arrays of holes. The pitch size of the array is $a=470\text{nm}$ and the hole size is $d=248\text{nm}$. A large defect-free area is visible. The inset shows a 3D representation of the atomic force micrograph. (b) Schematic of the experimental setup for the longitudinal Kerr measurements. The angle of incidence of the light is $24°$.

Figure 2

(Color online) Relative reflectivity (top) and magneto-optic polar Kerr (bottom) measurements for two different Fe hole diameters with holes diameter of 248 nm (open circles) and 297 nm (open squares), respectively. The Kerr spectrum of a continuous Fe film (filled circles) is shown as reference. The Kerr spectra are recorded at the saturation state of the samples at magnetic field of 1.64 T. The magneto-optic response of the antidot samples is strongly affected and enhanced at $\sim 2.8\text{eV}$, and at $\sim 3.8\text{eV}$ as a direct response to surface-plasmon excitation.

Figure 3

(Color online) Calculated reflectivity and polar Kerr rotation versus photon energy. The array parameters for the hole arrays are as follows: $a=470\text{nm}$ as pitch size for all arrays, $d=150\text{nm}$, $d=200\text{nm}$, $d=248\text{nm}$, and $d=297\text{nm}$. The two bigger hole diameter correspond to the experimental spectra shown in Fig. 2. The spectrum for the Fe film is also shown. A continuous enhancement of the polar Kerr rotation as the reflectivity decreases is observed as the hole diameter increases.

Figure 4

(Color online) (a) Longitudinal and (b) polar Kerr hysteresis loops for a Fe thin film (closed circles) and for two antidot Fe samples with $d=248\text{nm}$ (open circles) and 297 nm (open squares) hole diameter. The lattice constant of the antidot array is $a=470\text{nm}$ for both samples. The $y$ axis is normalized to unity for a better comparison. The magnetic behavior is dominated by the size of the holes. Higher coercivity and the appearance of out-of-plane magnetization components with increasing hole size is visible.

Figure 5

(a) Magnetic-force micrograph of a nanohole array $\left(d=248\text{nm}\right)$ with a 100-nm-thick Fe film without applied external magnetic field. The color scale describes different magnetization orientations. (b) Micromagnetic simulation of the same structure as in (a) in remanence. The spin orientation is denoted by the arrows and the color scale, showing mainly an in-plane orientation affected by the holes.