Solid-state chemistry of inorganic materials: for complete understanding of curious magnetic and optical properties of inorganic solids based on their electronic structures

In recent years, many breakthroughs have been achieved in the area of inorganic materials, such as the discovery and invension of quasi-crystals, high-Tc superconductors, colossal magnetoresistive materials, and bule LEDs.

We are studying the fundamentals of solid-state chemistry for better understanding of the inorganic materials. Our research includes spintronics, magneto-optics, non-linear optics, and random photonics.

1. Magnetic and magneto-optical properties of oxides

In conventional electronics, principles of operation of devices only involve holes and electrons in solids, and spins of electrons are neglected.  Recently, transistors or magnetic memories which elucidate the states of spins inside solids have been proposed. This new concept has created a new research field called "spintronics".  Also, photons traveling through the magnetic materials interact with spins inside the solids, resulting in the rotation of the axis of light polarization. Therefore, by use of "transparent magnet", light signals can be manipulated externally, which can be applied for in many fields including optical communications.  We are studying magnetic and optical properties of wide band-gap oxides which are transparent to the visible light for basic understanding of the relation between structures and properties, and searching for new materials for applications in the fields of spintronics and magneto-optics.  

We have succeeded in preparation of ferrimagnetic semiconductor thin films based on a solid solution of FeTiO3-Fe2O3 system by using a pulsed laser deposition method.  The thin films are single-crystalline, and have Curie temperature higher than room temperature.  The type of carrier, that is, n-type and p-type in the semiconductor thin films can be readily controlled by tuning the composition.  We have also demonstrated that a spin-polarized carrier is present in the thin films on the basis of the observation of anomalous Hall effect.

We have found that ZnFe2O4 thin films prepared by a sputtering method exhibit ferrimagnetic properties with large saturation magnetization (32 emu/g at room temperature) and high Curie temperature (higher than room temperature) although stable phase of ZnFe2O4 prepared by a solid-state reaction is an antiferromagnet with very low Néel temperature of 10 K or so.  We have revealed that this anomalous ferrimagnetism in the sputtered ZnFe2O4 thin films are ascribed to the site exchange between Zn2+ and Fe3+ ions.  Namely, the Fe3+ ions occupy both tetrahedral and octahedral sites, and the strong superexchange interaction between them leads to the high magnetization and high Curie temperature.  We have confirmed that such a structure is valid on the basis of measurements and analyses with X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) along with first principle calculation of electronic structure.  Further, we have demonstrated that the ZnFe2O4 thin films manifest large Faraday effect in a short wavelength region such as 400 nm or so.


2. Structure control and second-order nonlinear optics in dielectrics 

A periodic arrangement of the dipole moment can be induced in the structure of solids by the application of strong electric field or the irradiation of light waves with different wavelengths.  In particular, in amorphous dielectrics such as heavy-metal oxide glasses, periodic structure of ion polarizations and electron-hole pairs can be written inside the random network of ions.  Using this scheme, symmetry-dependent phenomena, such as second-order nonlinear optical effect can be obtained even in glasses.  Second-order nonlinear optical effects stems from the fact that the polarization induced in solids is proportional to the square of the electric field. Interesting phenomena such as second-harmonic generation (SHG) in which the irradiated light is converted to the frequency-doubled one occur.  We have measured second-order nonlinear optical properties of dielectrics after inducing a modulation in electronic structures and microscopic morphologies in order to find materials with giant optical nonlinearity.  In particular, we have paid attention to tellurite and chalcogenide glasses which have large third order nonlinearity, because the second-order nonlinear susceptibility is proportional to both third-order nonlinear susceptibility and dc electric field induced in the glass by poling process.  We have carried out thermal, optical, and electron-beam poling, and achieved large second-order nonlinear susceptibility in WO3-TeO2 and GeS2-Ga2S3-KBr glasses and stable second-order nonlinearity in ZnO-TeO2 glasses.  Also, we have measured second-harmonic generation in many kinds of tellurite glasses thermally poled at various temperatures, and found that the second-harmonic intensity increases, takes a maximum, and then decrease with an increase in the poling temperature, and that the poling temperature giving the maximum second-harmonic intensity is proportional to the glass transition temperature.  Furthermore, we have demonstrated that the ferroelectric phase of BaTiO3 is stabilized when BaO-TiO2-TeO2 glass is thermally poled although only heat-treatment of the glass leads to precipitation of paraelectric BaTiO3.  Subsequently, the glass-ceramic material fabricated via thermal poling shows large second-order nonlinearity.