Fabrication of biogenic guanine crystal/ferromagnetic film hybrid plate for micro-optical MEMS

Abstract

Bio-photonic crystals in fish and so on have attracted much attention due to their unique biological nano-sturcture. Guanine crystals of fish scales are the thin plate having the interesting properties such as high reflection and optical biaxiality. It has been demonstrated that the magnetic field controls the light reflection intensity from the guanine crystal plates [1], [2]. This reflection anisotropy is generated by a diamagnetic orientation of the guanine crystal with the magnetic filed more than 100 mT (goldfish guanine case). From the viewpoint of optical devices application, the reduction of magnetic field amplitude is effective. In this study, we have prepared a hybrid thin film consisting of diamagnetic guanine crystal plates and soft ferromagnetic material, and demonstrated that the hybridization enables to orient the guanine plate with the magnetic field of several mT. A ferromagnetic thin film was developed by sputtering method. After the chamber was evacuated to a pressure below 5×10-7 Pa, the sputtering was carried out with DC power of 50 W at sputter gas pressures of 0.53 Pa in an Ar gas atmosphere. A permalloy film with a thickness of 5–30 nm was formed on the dried surface of the stacked biogenic guanine crystal plates considering the typical thickness of guanine plates of goldfish is around 100 nm. Figure 1(a) shows the SEM image of guanine crystal plate after depositing the permalloy film of 5 nm in thickness. The permalloy entirely covers the guanine crystal surface but the edge of guanine plate is clearly seen. Next, the hybrid plate has been detached from the substrate and the dynamic behavior under the magnetic field has been investigated. Figure 1(b)-(d) shows optical microscope images of the hybrid plate having the 5-nm-thick permalloy film under the magnetic field generated by a three-axis Helmholtz coil. The hybrid plate still tends to float in water. When the rotating field of x-y plane is applied, the hybrid plate rotates according to the magnetic field as shown in Fig.1 (b) and (c), and the reflected light from the plate can be confirmed. When z-direction field is applied, hybrid plate easily stands up (see Fig.1 (d)) unlike pure guanine crystal which is oriented so that the long axis of guanine plate takes orthogonal to the magnetic field due to the gravity [2]. In the pure guanine case, it has been reported that more than 2 T magnetic field is needed to make the guanine crystal stand up against the gravity [3]. These hybrid plate motions arise from the strong shape anisotropy of permally film which fixes the magnetization direction parallel to the long axis direction of guanine plate due to the elongated shape. Critical magnetic field amplitudes for controlling the plate motion, which are 0.6 mT for in-plane motion and 3 mT for standing up, drastically decrease compared to the diamagnetic orientation of the pure guanine crystal.