The world's thinnest transparent magnetic semiconductor comes out

The Japanese Institute of Materials Research (NIMS) has developed a new thin film-like transparent magnetic semiconductor (Fig. 1). If a multilayer structure is used, it has a strong magneto-optical effect in the ultraviolet to visible region. It is prepared to be applied to a large-capacity optical communication and optical information processing element using short-wavelength light having a wavelength of less than 500 nm.

The raw material is photocatalyst TiO2

The semiconductor developed by NIMS is Ti0.8Co0.2O2 and Ti0.6Fe0.4O2 which are made by adding Co and Fe magnetic elements to Ti1-δO2 (a thin film of TiO2, a well-known photocatalyst material). Ti0.8Co0.2O2 is representative). The thickness is about 1 nm, "as the transparent magnetic material, the thinnest among the materials developed so far" (Nagasaki, researcher, soft chemistry research group, NIMS Institute of Physics). When this material is overlapped by 10 layers, the result shows a strong Faraday effect and a Kerr Effect for the light from the infrared to the visible light (wavelength of 280 to 380 nm).

Both the Faraday effect and the Kerr effect are magneto-optical effects exhibited by the interaction of the magnetic body and the light. The former refers to the phenomenon that the polarization plane of the light of the magnetic body rotates, and has been applied to fields such as an optical isolator used for optical fiber communication. The Kerr effect is a phenomenon in which the polarization plane of light reflected by the magnetic body rotates, and has been applied to data reading of MO discs.

The tensor tensile effect is 10 times that of existing materials

Ti0.8Co0.2O2 generally has the following four characteristics:

(1) The wavelength (response wavelength) having a magneto-optic effect is short, and is 280 to 380 nm;

(2) The magneto-optical effect is high;

(3) The characteristics of the magnetic elements added and the number of layers to be added can be controlled;

(4) It is easy to implement a multilayer structure.

As a characteristic wavelength (1), the Ti0.8Co0.2O2 is a garnet material such as YIG and GdBiIG (1.3 to 1.5 μm) which has been put into practical use in the field of optical isolators, and existing magnetic semiconductors such as CdMnTe. (0.6 ~ 1μm) should be short (Figure 2). Therefore, it is expected to be applied to optical communication and optical information processing elements using optical fibers having a wavelength of 500 nm or less, such as an optical isolator having a larger transmission capacity and an MO optical disc having a higher data recording density.

The characteristic (2) means that the ten-layer Ti0.8Co0.2O2 exhibits a tensile strength (Faraday rotation angle) of 10,000 degrees. This shows that the light is rotated by 10,000 degrees for every 1 cm of light traveling in Ti0.8Co0.2O2. It is approximately 10 times as large as GdBiIG having a response wavelength of 1.3 to 1.5 μm.

Control features by changing the number of added elements and stacks

The feature (3) means that the Faraday rotation angle and the response frequency can be controlled by changing the types of the magnetic elements added and the number of layers to be stacked. For example, by alternately laminating Ti0.8Co0.2O2 and Ti0.6Fe0.4O2 and changing the number of different layers, it is expected to increase the response wavelength. "The data obtained shows that a strong magneto-optical effect can be produced under the blue light (wavelength of 450 nm) used in the new generation of optical discs" (Nagata).

The feature (4) means that the multilayer structure can be easily formed by the following steps. First, the glass substrate is immersed in a positively charged polymer solution. Then, the glass substrate was taken out from the polymer solution and immersed in a Ti0.8Co0.2O2 solution.

Since Ti0.8Co0.2O2 has a negative charge, it is adsorbed to the polymer film on the glass substrate by electrostatic attraction. According to such a procedure, a single layer laminate can be formed, and as described above, the number of layers capable of obtaining desired characteristics can be formed. In order to change the characteristics of the garnet material and the existing magnetic semiconductor material, it is necessary to change the composition and crystal structure of the laminated film.