Optical Magnetoelectric Effect in a Submicron Patterned Magnet

The optical magnetoelectric effect, which is a nonreciprocal directional dichroic response, has been demonstrated in a submicron patterned magnet by monitoring the diffracted visible or near-infrared light intensity. An artificial magnetic superstructure is composed of chevron shaped “$>$” islands made of the ferromagnetic permalloy ${\mathrm{N}\mathrm{i}}_{80}{\mathrm{F}\mathrm{e}}_{20}$ with a pitch of $1\mu \mathrm{m}$ on silicon substrate, in which both space inversion and time reversal symmetry are broken simultaneously. On the basis of the light-polarization angle and magnetic field $H$ dependence, and also comparing the results with the those of the submicron square patterns, we show that the optical magnetoelectric effect emerges as the finite change ($\sim {10}^{-3}$ at room temperature in $H$ of 500 Oe) of the diffracted intensity.

Figure 1

Experimental setup for the observation of optical magnetoelectric effect by diffraction measurements. Four Bragg diffractions of the order $n=\pm 1$ appear at the diffraction angle $\varphi$ of $\sim 40°$ when the cw light from the laser diode (the wavelength $\lambda$ of 785 nm) is irradiated on the submicron patterned magnet sample (see the right panel) in normal incidence. Diffracted light is lock-in detected by a silicon photodiode at positions $A$ and $B$. Static magnetic field $H$ is applied perpendicular to the plane of scattering. A spin-polarized SEM image at room temperature of the noncentrosymmetric chevron shaped “$>$” patterns, which was recorded after the removal of $H$ of 500 Oe, is shown in the right panel. The relationship between the color and the direction of the in-plane remnant magnetization is represented by the color wheel; the direction of the remnant magnetization is also guided by black arrows on one of the chevron shaped patterns in the spin-polarized SEM image. The black background area corresponds to the silicon substrate. The white scale bar is $1\mu \mathrm{m}$. Left upper inset shows the configuration of toroidal vector $T$, which is defined as the outer product of the spontaneous polarization $P_{\mathrm{s}}$ and the magnetization $M$; chevron shaped patterns are expected to produce $P_{\mathrm{s}}$ and $M$ due to the noncentrosymmetric shape and the ferromagnetic nature, respectively. The propagation vector $k_{\mathrm{i}}$ of incident light is set parallel or antiparallel to $T$.

Figure 2

Light-polarization angle $\theta$ dependence of the relative change $\Delta I/I_0$ of the diffracted light ($\lambda =785\mathrm{n}\mathrm{m}$) intensity for Bragg spots of the order $n=\pm 1$ with the reversal of the magnetic field $H$ for (a) the centrosymmetric square patterns and (b) the noncentrosymmetric chevron shaped “$>$” patterns. $\theta$ is defined as the angle of the incident light $E$ vector relative to the applied $H$, as depicted in the right lower panel. The measurements were performed at room temperature in $H$ of $\pm 500\mathrm{O}\mathrm{e}$. Data for Bragg spots of $n=\pm 1$ taken at positions $A$ and $B$ (see Fig. 1) are represented by closed red and blue circles, respectively. Respective solid lines are least-squares fits to the data assuming the form of (a) ${sin}^2\theta$ and (b) ${sin}^2\theta +\mathrm{\text{const}}$ $(\sim -1\times {10}^{-3})$. SEM images of respective samples are shown in the right panels. The white scale bar is $1\mu \mathrm{m}$. Gray and black areas correspond to permalloy patterns and silicon substrate, respectively.

Figure 3

Magnetic field $H$ dependence of the relative change $\Delta I/I_0$ of the diffracted light ($\lambda =785\mathrm{n}\mathrm{m}$) intensity with the reversal of $H$ for the noncentrosymmetric chevron shaped “$>$” patterns. $H$ dependence of the TMOKE and the optical ME effect are shown by closed and open circles, respectively. For the respective estimates, see text. Solid lines are least-squares fits to the data assuming the linear response to $H$.