Enhancement of magnetic circular dichroism in bi-layered ZnO-Bi:YIG thin films

Magneto-optic (MO) effect is widely used in optical applications. Recently, enhancement of MO effect is reported in hybrid structures of MO materials with metallic^1–3 and dielectric materials.^4–6 This kind of structures attracts much interest, since they have advanced functional capabilities for optical devices. Zinc oxide wide band material is one of the attracting materials to be combined with MO materials because of its large exciton bonding energy (60 meV).⁷ This strong exciton will interact with MO materials and modulate the MO response. If the MO response was modulated by exciton, it will be potentially controlled by a voltage because the exciton wavelength in semiconductor systems can be altered by electro-optic effect.^8–12 Although many magnetic studies on ZnO is devoted to diluted magnetic semiconductors,¹³ a bi-layered system of ZnO and MO materials would open novel possibilities for manipulating light. Current studies on the hybrid structure of MO materials with other functional materials generally employs a localization of light. This approach is effective to enhance the MO effect because of enlargement of the interaction time between the MO materials and the light. However, it is hard to modulate the localized light (photon) by electric or magnetic field. Our structure aims to utilize the exciton (electron) which is intrinsically interacted with electric field.

In this study, ZnO-Bi:YIG bi-layer was fabricated and magneto-optically investigated. Figure 1 shows schematic image of the ZnO-Bi:YIG bi-layer and generation of an exciton.

In order to fabricate the bi-layer, Bi:YIG was used as a MO material. A precursor of Bi:YIG films were deposited on a glass substrate (Corning Eagle XG) by RF-magnetron sputtering method. Then, the films were sintered at 680^oC for 20 min. Composition and thickness of the Bi:YIG film were Bi_0.5Y_2.5Fe₅O₁₂ and 50 nm respectively. The Bi:YIG film was plated with ZnO by means of electroless plating method^14,15 for preventing the Bi:YIG from heat damage. Prior to the ZnO deposition, the Bi:YIG was rinsed with acetone, aqueous potassium hydroxide (8mol/L) and pure water. Then, it was catalyzed by the wet process composed of two-step Sn/Pd or three-step Sn/Ag/Pd activation. The activation process was carried out by the sequential immersion of the substrate into the aqueous solution of SnCl₂ (2g/l), AgNO₃ (0.1g/l) or PdCl₂ (0.1g/l) for 1min. each. The activated surface of Bi:YIG was entirely covered with Pd particles. Zn(NO₃)₂ and dimethylaminborane (DMAB) were dissolved into the pure water to prepare an aqueous 0.05 mol/L Zn(NO₃)₂ – 0.02 mol/L DMAB solution. The resulting solution was heated to 68^oC, and the catalyzed Bi:YIG was immersed into the solution for 20 min. to deposit ZnO. The obtained film was rinsed with pure water and dried.

Figure 2 shows scanning electron microscope (SEM) surface images of the fabricated ZnO-Bi:YIG bi-layers. In the case of (a) Sn/Pd activation, each ZnO crystal had hexagonal column shape which reflects a wurtztite structure of ZnO. For the (b) Sn/Ag/Pd activation, ZnO was more densely deposited.

Figure 3 is absorption spectra of the ZnO-Bi:YIG bi-layer. The spectra show absorption peak around 368 nm (3.37 eV) which is theoretical wavelength of exciton absorption of ZnO. In the case of Sn/Ag/Pd activation, absorption peak was observed at shorter wavelength than the intrinsic value of ZnO. It could be caused by a strain of the crystallographic structure or quantum confinement effect.

Figure 4 shows a Faraday rotation and absorbance of the Bi-layer that activated by (a) Sn/Pd and (b) Sn/Ag/Pd catalyzation. The Faraday rotation was measured by a magneto-optical effect measurement system (NEOARK BH-M800UV-F-10) and subtracted the effect of single Bi:YIG film. Although the activation process and the micro structure was different, the Faraday rotation of both films showed same two small peaks beside 368 nm. This result indicates that these peaks are not coming from micro structure. Faraday rotation represents the difference of the phases of left and right circular components of the light transmitted through MO materials. It is caused by a resonance between circular components of light and spin of electron. When frequency of the light is slightly different from a resonance frequency of electron, phase of the circular components is altered. Therefore, two peaks of Faraday rotation are obtained beside the resonance frequency (wavelength) of electron. According to the result, the center wavelength of the two Faraday rotation peaks was around 368 nm which coincide with the absorption peak and the exciton wavelength of ZnO. This result indicates the contribution of the exciton for the Faraday rotation.

MCD spectra of the fabricated ZnO-Bi:YIG bi-layers are shown in fig. 5. The MCD spectra were measured by the magneto-optical effect measurement system (NEOARK BH-M800UV-F-10) and the effect of single Bi:YIG film was subtracted. MCD is the difference of the absorbance for left and right circularly polarized light, induced in magnetized materials. It is caused by a resonance between light and electron as Faraday rotation described before. Generally, MCD shows the peak at the resonance frequency of light and electron. The spectra showed a clear peak around 368 nm which was not obtained in a single film of ZnO or Bi:YIG. The wavelength of the MCD peaks matched the measured exciton absorption peak and typical exciton wavelength of ZnO quite well. The intensity of MCD peak was much higher in the case of Sn/Ag/Pd activation. It will be because of high ZnO density. The second peak right before 368 nm is the remaining MCD signal of Bi:YIG.

The bi-layered ZnO and Bi:YIG was fabricated and magneto-optically investigated. To prevent the Bi:YIG form heat damage, electroless deposition method of ZnO was employed. The fabricated bi-layer showed absorption peak at exciton wavelength of ZnO. The Faraday rotation and MCD were modulated around absorption peak. Especially MCD showed clear peak at the absorption peak which coincide with exciton wavelength of ZnO. These results will be giving us the possibility to control the MO response via exciton.

This work was supported by JSPS KAKENHI Grant Number JP16K21569, JP26220902, and Toyohashi University of Technology - Kosen collaborative research grant number 10105.