Kiyoshi Aoyagi

Optical Absorption Lines of Ruby in a Strong Magnetic Field

The Zeeman effect of the B and R absorption lines of ruby was observed photographically with strong pulsed magnetic fields up to 230 kOe at 77°K. This was achieved by flashing a Xe-lamp at the time when a magnetic field reached its peak. The duration of the pulsed field was made to be about 1 m sec which was much longer than that of the pulsed light, about 20 µ sec. The g -values of the excited states responsible for the B and R lines were measured and the comparison was made with those reported by other authors. For the Zeeman patterns of the R lines with magnetic fields perpendicular to the optic axis, the Paschen-Back effect was calculated and compared with the observed patterns.

Exciton Absorption Lines in Antiferromagnetic Rare-Earth Orthochromites –With Particular Reference to YCrO3–

A method of analyzing absorption spectra of Frenkel excitons associated with single-Cr 3+ transitions in antiferromagnetic rare-earth orthochromites is presented for both cases with and without an external magnetic field. As the simplest example, exciton lines of YCrO 3 in the spectral region of ∼13,800 cm -1 are studied in detail both experimentally and theoretically. Four magnetic-dipole lines are assigned to Davydov-split exciton lines associated with one single-Cr 3+ excitation, and matrix elements of excitation transfer in various Cr 3+ -pairs are determined. It is considered that spin-forbidden excitation transfer between the sublattices of opposite spins is released by a small canting of spins not responsible for the weak ferromagnetism. When a magnetic field along the crystal a -axis is increased up to 50 kOe, spin reorientation from the a -axis to the c -axis is observed in the change of intensities and polarizations of the exciton lines.

Reflectivity and Electronic Structures of Layered Ionic Crystals, (CnH2n+1NH3)2CdCl4: n=1, 2, 3

The reflection spectra of (C n H 2 n +1 NH 3 ) 2 CdCl 4 were measured in the photon energy region of 0.5–25 eV at room temperature (RT) and liquid nitrogen temperature (LNT) by means of synchrotron radiation. It was found that absorption spectra of the present crystals resemble that of CdCl 2 crystals except for an additional peak at 5.10 eV (RT). The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10 eV (RT) to 5.48 eV (LNT). This absorption peak has doublet structure and the splitting energy is 0.1 eV, which is the same magnitude as the spinorbit splitting of a chlorine atom. The band is assigned to be an electronic transition from the chlorine 3 p valence band to a new-type conduction band which is composed of alkylammonium bases and is below the ordinary Cd 5 s conduction band in energy.

Absorption Spectrum of an Antiferromagnetic Rare-Earth Garnet Dy3Al5O12 above and below the Néel Temperature

The absorption spectrum of a Dy 3+ ion in dysprosium aluminum garnet in the paramagnetic state has been investigated in the region of 15,000 to 5,000 cm -1 . By observing the Zeeman effect in both pulsed and stationary magnetic fields, the g z -values of the lowest three levels of the 6 H 15/2 ground state split by crystalline-field and of the three levels of the 6 F 5/2 excited state split by crystalline-field have been determined. From the splittings of the absorption lines corresponding to the 6 H 15/2 → 6 F 5/2 transition in the antiferromagnetic state, the internal fields acting on the Dy 3+ ions in the ground and the 6 F 5/2 excited states have been estimated to be 6.2 and 6.8 kOe, respectively.

Optical Properties of Electron Trapped Center in Layered Ionic Crystals (CH3NH3)2CdCl4 and (C2H5NH3)2 CdCl4 Irradiated with X-Rays at 15 K

New optical absorption bands (IR bands) were found at around 10∼20 kcm -1 besides absorption band of Cl 2 - molecular center in two-dimensional crystals of (CH 3 NH 3 ) 2 CdCl 4 and (C 2 H 5 NH 3 ) 2 CdCl 4 irradiated with X-rays at 15 K. The absorption intensities increase with the X-ray dosage in proportion to that of the Cl 2 - center and decrease at temperatures higher than 25 K for (CH 3 NH 3 ) 2 CdCl 4 and 35 K for (C 2 H 5 NH 3 ) 2 CdCl 4 accompanying the decrease of Cl 2 - . The both bands are also bleached by the light irradiation on the IR bands at 15 K. The IR bands are assigned to an electron center where an electron is trapped by an alkylammonium head in the neighborhood of a Cl - vacancy.

Exciton Absorption Lines in Antiferromagnetic DyCrO3

The absorption spectrum of DyCrO 3 in the red region was studied with and without a magnetic field. From the Zeeman spectrum of a Dy 3+ ion, the anisotropy axis of a Dy 3+ ion was determined to be directed along the axis deviated by ±30° from the b -axis in the a b -plane. The magnitudes of the excitation transfer between the Cr 3+ ions with parallel spin (0.25 cm -1 ) and with antiparallel spin (0 cm -1 ) were obtained by analyzing the exciton lines associated with the Cr 3+ t 2 g 3 4 A 2 g → t 2 g 3 2 E g transition. Remarkable shifts of the exciton lines were observed in magnetic fields parallel to the b -axis. Magnetic-field dependence of these shifts indicated that they were induced by some Dy 3+ –Cr 3+ interaction. An electric-dipole line creating a Cr 3+ exciton and a Dy 3+ spin flip was also observed, from which a negative dispersion of the Cr 3+ exciton was found to be at least -16 cm -1 .

ESR and optical studies on Cl2− in single crystals of (CnH2n+1NH3)2CdCl4 with n=1, 2 and 3

The crystals of (C n H 2 n +1 NH 3 ) 2 CdCl 4 ( n =1, 2, 3) have been irradiated with X - and γ-rays at 77 K. From ESR and optical absorption spectra, it is found that Cl 2 - is created in the layer of CdCl 6 . With angular dependence of hyperfine coupling constant of Cl 2 - , the direction of its molecular axis is determined. The presence of any absorption bands corresponding to vacancy centers, such as F center in alkali halides, has not been detected in the irradiated crystals. Metallic luster as a silver mirror has been observed in the crystals which have been warmed up to room temperature after irradiation at 77 K.

Metal Precipitation in Transparent Layer Ionic Crystals (CnH2n+1NH3)2MCl4 with n=1, 2 and M=Mn, Cd

Layer ionic crystals (CnH2n+1NH3)2MCl4 with n=1, 2 and M=Mn, Cd have been irradiated with X- or γ-rays at 77 K. After warming the crystals up to room temperature, metallic luster as a silver mirror has been observed in them. Taking into account optical absorption spectra and elimination of chlorine from the crystals, it is concluded that the metallic luster is caused by metal precipitation in the crystals. The mechanism of the precipitation is considered with the concept of color center formation and destruction in low dimensional crystals.

Exciton Absorption Lines in Antiferromagnetic LuCrO3

Optical properties of R -excitons in LuCrO 3 as well as their change during the magnetic-field-induced spin reorientation have been investigated. It is shown that the polarization characters of the exciton lines agree with the theory of the Frenkel excitons in antiferromagnetic rare-earth orthochromites. By using the available theory, parameters related to the excitation transfer matrix elements of Cr 3+ ions in LuCrO 3 are determined. The energy shift and intensity change of the exciton lines during the spin reorientation are discussed and compared with those in YCrO 3 .

Production of Strong Magnetic Field of Rectangular Pulse, and Its Application to Study of Relaxation Process in Ruby

An apparatus is described which produces a strong magnetic field in a form of a rectangular pulse. The apparatus is composed of a coil for producing the magnetic field, a capacitor for storing electrical energy, and a pulse forming net-work. The maximum field obtained so far is 174 kOe. The rise time of the field is 200 µs. The field lasts for 700 µs. The overshoot of the magnetic field is less than 2 per cent of the average field strength. As an example of the application of the rectangular field a relaxation process in ruby is studied by employing an optical technique.