Susumu Okubo

Dense radial growth of silver metal leaves in a high magnetic field

Growth patterns of silver metal leaves from AgNO 3 solutions were investigated in a high magnetic field of 8 T as experimental research of diffusion-limited aggregation (DLA) with magnetohydrodynamic drifts of ions. Silver metal leaves grew into a typical DLA form with the fractal dimension of 1.62 in the absence of magnetic fields, and they grew into a dense radial form in the field of 8 T, where the crossover in the fractal dimension from 1.69 to 2 was observed with increasing length scale.

Submillimeter Wave ESR Study of Spin Gap Excitations in CuGeO3.

Transitions between the ground singlet state and the excited triplet state have been observed in CuGeO 3 by means of submillimeter wave electron spin resonance. The energy gap at zero field is evaluated to be 570 GHz (2.36 meV) and this value is nearly identical to the gap at the zone center observed by inelastic neutron scattering. The absorption intensity shows strong field orientation dependence, but it shows no significant dependence on magnetic field intensity. These features have been explained by considering the existence of Dzyaloshinsky-Moriya (DM) antisymmetric exchange interaction. The doping effect on this singlet-triplet excitation has been also studied. A drastic broadening of the absorption line is observed by the doping of only 0.5 % of Si.

Submillimeter wave ESR system using the pulsed magnetic field and its applications to one dimensional antiferromagnetic system

Submillimeter wave ESR measurements using the pulsed magnetic field is a powerful spectroscopic technique covering the wide frequency and magnetic field range with many advantages compared to the conventional X-band ESR measurements. Recent submillimeter wave ESR results showed that it is especially powerful to study the low dimensional antiferromagnets, that is, quantum spin systems. In accordance, the submillimeter wave ESR facilities in Japan are also presented. As an example of the study of quantum spin system, the investigation of the S=1/2 one dimensional antiferromagnets BaCu 2 (Si 1-x Ge x ) 2 O 7 (x=0, x=0.65), which have the staggered field effect in the system, is also presented in connection with the recent ESR theory by Oshikawa and Affleck (OA). Although both x=0 and x=0.65 systems show one dimensional behavior from the magnetic susceptibility, the temperature and frequency dependences of the mixed crystal x=0.65 system are well interpreted by the OA theory while they are not for the pure x=0 system. The antiferromagnetic resonance measurements of x=0 system (T N =8.9 K) have been performed up to 30 T at 1.8 K, and the magnetic phase transitions at 7 T and 16 T for H//α and H//c, respectively, are suggested for the first time.

High Field ESR Study of the S=1/2 Diamond-Chain Substance Cu3(CO3)2(OH)2 up to the Magnetization Plateau Region

High field ESR measurements of Cu 3 (CO 3 ) 2 (OH) 2 have been performed in the frequency region from 50 to 900 GHz and in the temperature region from 1.8 to 265 K using the pulsed magnetic field up to 36 T. The large g -shifts below 23 K and the characteristic temperature dependence of the linewidth are discussed in connection with the two maxima of magnetic susceptibility observed at 5 and 23 K. Our ESR results together with the magnetization and theoretical results suggest that the ground state of the system is in the spin fluid (SF) phase at low field and low temperature. From the frequency–field dependence measurements at 1.8 K, the direct transition, which is closely related to the magnetization plateau from 16 to 26 T at 1.5 K, is observed for the first time. The observed frequency–field diagram suggests that the exchange interaction in the dimer is estimated to be about 50 K (1057 GHz).

Electric Polarization Induced by Néel Order without Magnetic Superlattice: Experimental Study of Cu3Mo2O9 and Numerical Study of a Small Spin Cluster

We clarify that the antiferromagnetic order in the distorted tetrahedral quasi-one-dimensional spin system induces electric polarization. In this system, the effects of low dimensionality and magnetic frustration are expected to appear simultaneously. We obtain the magnetic-field–temperature phase diagram of Cu 3 Mo 2 O 9 by studying the dielectric constant and spontaneous electric polarization. Around the tricritical point at 10 T and 8 K, the change in the direction of electric polarization causes a colossal magnetocapacitance. We calculate the charge redistribution in a small spin cluster consisting of two magnetic tetrahedra to demonstrate the electric polarization induced by the antiferromagnetism.

Development of high-pressure and high-field ESR system using SQUID magnetometer

We have developed a high-pressure and high-field electron spin resonance (ESR) system using the combination of a commercially available superconducting quantum interference device (SQUID) magnetometer and a clamp-type piston cylinder pressure cell. The magnetic field range is up to 5 T, and the maximum pressure reaches 1.5 GPa. The most characteristic feature of this system is its easy handling as compared with other high-pressure ESR systems. Moreover, the macroscopic magnetization measurement can be performed simultaneously with the microscopic ESR measurement. In addition to these features, the well-established pressure calibration method utilizing the change of superconducting transition temperature of tin can be applied to this system. By using this system, we obtained pressure dependence of the single ion magnetic anisotropy parameter D of NiSnCl(6)·6H(2)O up to 1.5 GPa precisely, and the magnetization behavior of this material under pressure was explained well by its pressure dependence of the D value.

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.

Highly Sensitive Detection of Pulsed Field ESR Using a Cantilever at Low Temperature

We have succeeded in observing the high field ESR of 800 ng Co‐Tutton salt at 1.7 K in the millimeter wave region, using a cantilever and a pulsed magnetic field. This is a new high‐sensitivity ESR detection technique at low temperature in a pulsed magnetic field, with a sensitivity 104 times higher than that of the conventional transmission technique. Possible application to the terahertz ESR is discussed.

Recent developments of high field ESR systems in Kobe

The magnetic field of the high field ESR system in Kobe University has been extended to 55 T by using a new non-destructive pulse magnet and the 300 kJ (10kV) capacitor bank. The properties of new 55 T pulse magnet are reported. As an example of its application, the high field ESR measurement of a quantum spin system CsCuCl3 for H∥a will be shown.

High Field Phase Transitions of Quasi One-Dimensional S= 1/ 2 Heisenberg Antiferromagnet Cu 2(1,4-diazacycloheptane) 2Cl 4 Observed by Submillimeter Wave ESR

Millimeter and submillimeter wave ESR measurements on a single crystal of an S =1/2 Heisenberg ladder-like system Cu 2 (C 5 H 12 N 2 ) 2 Cl 4 have been performed using a pulsed magnetic field up to 16 T. The gap is estimated to be 9.7 K from the temperature dependence of ESR intensity. The detailed frequency dependence of ESR in the frequency region from 40 to 420 GHz at 1.8 K suggests magnetic transitions at H c1 =7.5 T, H c2 =10.1 T and H s =13.2 T. A transition at 10.1 T is suggested for the first time. These results are discussed with previous magnetization measurements and theoretical calculations, and our results appear to be difficult to interpret according to the simple ladder model.