Electroless plated maghemite for three-dimensional magneto photonic crystals

Photonic crystals (PCs) are n-dimensional (⁠$n=1−3$⁠) space modulated structures having a lattice parameter comparable with the light wavelength for manipulation of light.^1–4 In addition, magneto photonic crystals (MPCs) composed of the photonic structures with magnetic materials can modulate wave vector, phase, polarization direction and intensity of light with high switching speed by magneto optic (MO) effect. The MO effects in 1D-MPCs,^5,6 2D-MPCs^7–10 and 3D-MPCs¹¹ have been studied as light manipulation. Such MPCs have a great interest because they can potentially minimize nonreciprocal components, e.g. isolator, circulator and MO sensor. Specially, 3D-MPCs are promising material for manipulating light in 3D space. One of the fabrication ways of 3D-MPCs is filling the air-gap of opal photonic crystal with magnetic material.¹¹ However, solution or gas film forming method is required to permeate the gap. In addition, crystallization annealing for transparent magnetic oxide affect the photonic structure of the opal.

Ferrite film fabrication methods such as sputtering, vacuum evaporation, molecular beam epitaxy (MBE), liquid phase epitaxy (LPE) are anisotropic deposition and require high process temperature (over 380 ^oC) for crystallization of ferrites.^12–14 These characteristics make it difficult to fill the air-gap of opal with ferrite. On the other hand, ferrite-plating, one of electroless plating methods, is an attractive film-forming method which provides magnetite (Fe₃O₄) film on various substrates in aqueous solution at 24-90°C.¹² This method enables us to deposit magnetite uniformly on non-flat geometry such as opal structure. However, magnetite is less useful for magneto-optical applications because of low-transmissivity. In this study, we fabricated 3D-MPC composed of maghemite (γ-Fe₂O₃) using modified ferrite-plating method with isotropic and low temperature process.

Maghemite has the same spinel ferrite structure as magnetite and is also ferromagnetic.¹⁵ It can be obtained by carefully oxidizing the magnetite and is metastable. Typical ferrite-plating uses a base material (FeCl₂) and oxidant (NaNO₂ and CH₃COOK) to synthesize magnetite. In our process, we additionally used Y(NO₃)₃ and urea (CO(NH₂)₂) to control the oxidization. Isopropanol (IPA) was also employed to reduce the surface tension.

The maghemite was plated on flat grass substrate (Corning Eagle XG). A 300 mL of plating solution was pure water: 240 ml, IPA: 60 mL, FeCl₂: 16 mmol/L, Y(NO₃)₃: 0.5 mmol/L, NaNO₂: 3 mmol/L, CO(NH₂)₂: 15 mmol/L and CH₃COOK: 15 mmol/L. Fe(NO₃)₃, Y(NO₃)₃, Dy(NO₃)₃ and Bi(NO₃)₃ were examined to control the oxidation, and maghemite was stably produced with Y(NO₃)₃. Glass substrates were immersed in the solution and heated up to 80°C for 180 min. finally, the substrate was plated with 100-200 nm thick maghemite.

Figure 1(a) shows the X-ray photoelectron spectroscopy (XPS) spectrum of the fabricated film. A composition of the film was Fe_2.01O_2.99. Figure 1(b) is X-ray diffraction spectrum of the film. The film has typical spinel structure.¹⁶ These results indicate that the plated film was maghemite.

The optical absorbance of the plated maghemite film was measured and compared with plated magnetite film which was fabricated by typical ferrite-plating process. Figure 2(a) shows comparison of an optical absorbance of the maghemite film and the magnetite film. The absorbance of the maghemite was significantly lower within a wavelength range 550 nm or less. A comparison of Faraday rotation of the maghemite and magnetite is shown in Fig 2(b). The absolute value of the Faraday rotation of magnetite and maghemite was 0.50 deg./μm and 0.44 deg./μm at 450 nm respectively. The results of optical and magneto-optical investigation suggest that the plated maghemite is promising magneto-optic medium within a wavelength range 550 nm or less.

The 3D-MPCs composed of a SiO₂ opal photonic crystal and the maghemite was fabricated. The opal photonic crystal was fabricated from SiO₂ spheres of 300 nm in diameter by vertical deposition method on a glass substrate. The fabricated opal photonic crystal was 3 μm thick. Figure 3(a) shows absorbance spectra of the opal photonic crystal for various incident angle of light. The inset shows measurement setup. An absorption peak, which was considered to be due to the absorption at the photonic band gap, appeared in every spectrum. The absorption peak was shifted as a function of the incident angle. The incident angle dependence of absorption peak is shown in Fig. 3(b) and compared with theoretical value calculated by Equation (1),¹⁷ 

In the equation, D, m, n_avg, θ are diameter of SiO₂ sphere, the order of the Bragg diffraction, the average refractive index of the photonic structure and the angle between the incident light and the surface normal of the sample respectively. The experimental absorption peak showed good agreement with calculation.

After fabrication of SiO₂ opal photonic crystal, we filled the air-gap with maghemite by the ferrite plating. The maghemite was synthesized by the same condition described above. Figure 4 shows absorption spectra of fabricated 3D-MPC, First and second order bandgap was observed and shifted with increasing the incident angle of light. The wavelength of bandgap of 3D-MPC was red-shifted compared with the opal. This result could be considered that a part of air-gap (n=1) was filled with maghemite (n=2.63). Faraday rotation spectra of the fabricated 3D-MPC and single maghemite film are shown in Fig. 5. In the case of normal incident (0 deg.), Faraday rotation of the 3D-MPC was enhanced around 320 nm which coincides with edge of the second order band gap. This Faraday rotation peak was shifted with the band edge as a function of the incident angle. At the band edge of a PC, the group velocity of the light vanishes as the wavelength approaches the band edge. This small group velocity extends the time the photons need to travel through the structure. Thus, the interaction time between the light field and the magnetic material should be extended, and Faraday rotation was enhanced.

The 3D-MPC that composed of opal photonic crystal and electroless plated maghemite was fabricated and investigated. An electroless plating method of maghemite was developed and employed to fabricate the 3D-MPC. The fabricated maghemite showed significantly lower optical absorption compared with magnetite within a wavelength range 550 nm or less. The fabricated 3D-MPC shows photonic band gap and enhancement of Faraday rotation at the band edge of 320 nm. The photonic band gap and Faraday rotation was changed as a function of the incident angle of light.

This work was supported by JSPS KAKENHI Grant Number JP16K21569 and JP26220902.