Hitoshi Ohta

Development of high-pressure, high-field and multifrequency electron spin resonance system

The electron spin resonance (ESR) system which covers the magnetic field region up to 16 T, the quasicontinuous frequency region from 60 to 700 GHz, the temperature region from 1.8 to 4.2 K, and the hydrostatic pressure region up to 1.1 GPa has been developed. This is the first pulsed high-field and multifrequency ESR system with the pressure region over 1 GPa as far as we know. Transmission ESR spectra under hydrostatic pressure can be obtained by combining a piston-cylinder-type pressure cell and the pulsed magnetic field ESR apparatus. The pressure cell consists of a NiCrAl cylinder and sapphire or zirconia inner parts. The use of sapphire or zirconia as inner parts enables us to observe ESR under pressure because these inner parts have high transmittance for the electromagnetic wave with millimeter and submillimeter wavelengths. We have successfully applied this system for the pressure dependence measurements of an isolated spin system NiSnCl(6)6H(2)O up to 1.1 GPa. It was found that the single ion anisotropy parameter D of this compound strongly depends on pressure. The parameter D is approximately proportional to the pressure up to 0.75 GPa, and the relation between D and the pressure can be used for the pressure calibration of this high-field and high-pressure ESR system.

Electron spin-resonance studies of conduction electrons in phosphorus-doped silicon nanocrystals

The properties of conduction electrons in P-doped Si nanocrystals embedded in insulating glass matrices have been studied by electron spin-resonance spectroscopy. For heavily P-doped samples, a broad conduction electron signal is observed at low temperatures. The width of the signal is found to be much broader than that of P-doped bulk Si crystals. The temperature dependence of the signal intensity obeys the Curie law even when the P concentration is very high. This suggests that in P-doped nanocrystals donor levels do not merge into the conduction band even at very high P concentration, and also provides evidence that Si nanocrystals smaller than a certain threshold size do not become metallic, at least when they are prepared under an equilibrium condition.

ESR Measurements of KCuCl3 under Multi Extreme Conditions

ESR measurements of a quantum spin gap system KCuCl3 have been performed under multi extreme conditions which consist of high field, high pressure and low temperature. The ESR mode due to the direct transition from the singlet ground state to the triplet excited states is observed and we have succeeded in observing the decrease of the gap energy by applying pressure directly.

Anomalous polarization characteristics of magnetic resonance in a quasi-one-dimensional CuGeO3: Co magnet

Doping of CuGeO3 by 2% Co was found to cause a new magnetic resonance, which has anomalous polarization characteristics. In the Faraday geometry, where a microwave field Bω is directed along certain crystallographic directions, this mode is suppressed, which indicates that the character of magnetic oscillations in this mode differs strongly from standard spin precession. This resonance coexists with the EPR on Cu2+ chains and is likely to be caused by an unknown collective mode of magnetic oscillations of an antiferromagnetic quantum S = 1/2 spin chain.

Magnetism of C60-based molecular complexes: High-field-magnetization and magnetooptical study

Magnetization study of the C60 · TMTSF · 2CS2 molecular complex in magnetic fields up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)2C60 at T=1.8 K for the frequency range 60–90 GHz in magnetic fields up to 32 T provide experimental evidence that paramagnetic centers with reduced g-factor values g<1 control the magnetic properties of these solids. A model is suggested in which the renormalization of the g factor is due to the dynamic Jahn-Teller effect involving negative C60 ions that appear as defects in the crystalline structure with a weak charge transfer.

Analysis of the temperature dependence of the high-frequency EMR spectra of Mn ions in the lithium-ion battery material LiMn2O4

This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn3+ and Mn4+ ions in the lithium manganese spinel LiMn2O4. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis of the temperature dependence of the HF-EMR spectra due to Mn3+ and Mn4+ ions in LiMn2O4 is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn3+ and Mn4+ ions in accounting for the characteristics most suitable for application of LiMn2O4 as a cathode material may be gained.

High Field ESR Measurements of S = 1/2 Diamond Chain Substance Cu3(CO3)2(OH)2 at the Magnetization Plateau Region

High field ESR measurements of the distorted S = 1/2 diamond chain substance Cu 3 )(CO 3 ) 2 (OH) 2 have been performed in the frequency region from 50 to 1000 GHz at 1.9 K using the pulsed magnetic field up to 36 T. The ESR signals of the direct transition from the singlet ground state to the excited state have been observed for B∥a, b and c axes. As the direct transition is essentially forbidden in ESR, the signal can be observed due to the Dzyaloshinsky-Moriya (DM) interaction. The angular dependence of the direct transition intensity has been measured in order to investigate the D vector of the DM interaction. This paper shall discuss the D vector in connection with the theoretical calculation for the anisotropic ESR signal intensity of the direct transition.

High-frequency ESR measurements of the Co spinel compound SiCo2O4

The new Co spinel compound SiCo2O4 has been synthesized using the high-pressure synthesis technique. High-frequency ESR measurements of powder samples of SiCo2O4 have been performed using a pulsed magnetic field up to 16 T in the temperature range from 1.8 to 80 K. A broad absorption line of the EPR spectrum, which is observed at 80 K, becomes sharper as the temperature decreases towards the Neel temperature TN = 15 K. On the other hand, below the Neel temperature, TN, two absorption lines appear at around 2 T due to the critical field resonances. The spin dynamics of the Co spinel compound SiCo2O4 are discussed in connection with other frustrated systems.

High Field ESR Measurements of the Organic Magnet F5PNN

We have performed high field ESR measurements of a F5PNN (2‐pentafluorophenyl‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1 H‐imidazol‐1‐oxyl 3‐oxide) powder sample, which forms a one dimensional (1D) bond alternating chain along the (1 0 1) direction, in order to clarify the magnetic properties at low temperature. The frequency dependence of our ESR measurements shows an anomaly in the width of the absorption line at Hc = 3.7 T, where the magnetization data shows a bond alternating ratio change from α = 0.4 to 0.5, and an abrupt broadening of the absorption line around the saturation field found from the magnetization measurements.