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A multiextreme (high-field, low-temperature, high-pressure and nanoscale) electron spin resonance (ESR) measurement system is under development in Kobe. In this connection, our recent development is introduced and two applications of our high-frequency high-field ESR are described. High-frequency high-field ESR measurements of dioptase (Cu6Si6O18·6H2O), which has an interesting antiferromagnetic Cu2+ network, have been performed using a pulsed magnetic field of up to 55 T. Antiferromagnetic resonances (AFMR) are clearly observed at 4.2 K with the light sources of up to 1017 GHz. However, a deviation from the conventional two-sublattice AFMR theory is observed in the high field. Temperature dependence of the X-band and high-frequency ESR has been also observed in the triangular lattice antiferromagnet EtMe3P[Pd(dmit)2]2 which shows the spin-Peierls-like transition below Tc = 25 K. The preliminary field dependence of the spin gap estimated from the analyses of our ESR results has been shown in connection with the previous magnetic susceptibility results.

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.

We have investigated the antiferromagnetic resonance modes of the kagome antiferromagnet Cs2Cu3SnF12 both theoretically and experimentally. High-field electron spin resonance experiments on single crystals were conducted in the ordered phase at different frequencies and temperatures with the external magnetic field parallel to the c axis. Considering three sublattices, we calculated the resonance modes for the motions of the sublattice magnetizations within the framework of the mean field approximation. It was found that the frequency-field diagram is in good agreement with the experimental results.

We have developed high-pressure, high-field and multi-frequency ESR apparatus. The pressure is available up to 14 kbar by using the clamped-type piston-cylinder pressure cell. All inner parts of the pressure cell are made of zirconia which has good transmittance of the electromagnetic wave in the frequency region from 70 to 700 GHz. The magnetic field region is up to 55 T and the temperature region is from 1.6 to 4.2 K. We present the results of quantum spin systems SrCu2(BO3)2 and CsCuCl3 as the application of this multi-extreme condition ESR technique and they will demonstrate its usefulness for studying the quantum spin system.

High-field and high-pressure ESR measurements of SrCu2(BO3)2 have been performed at 1.6 K in the field region up to 30 T, the frequency region from 70 to 360 GHz and the pressure region up to 12 kbar. The direct ESR transition mode between the singlet ground state and the triplet excited states was observed at various pressures. We have succeeded in observing the decrease of the spin gap directly upon applying the pressure. The origin of the decrease of the gap energy is discussed.

X-band and high field electron spin resonance (ESR) measurements on S = 1/2 kagome lattice antiferromagnet Cu3V2O7(OH)2 2H2O (volborthite) have been performed. Although the kagome lattice is slightly deformed in the ab plane, no sign of magnetic order is observed down to 3.8 K, which is consistent with the magnetic susceptibility and the speciflc heat results, because the X-band ESR result shows no g-shift and the broadening of the linewidth down to 3.8 K. However, high field ESR results above 40 GHz show continuous g-shift below 20 K, which coincides with the broad maximum in the magnetic susceptibility. These results suggest the existence of the short range order and the field dependence in the system, and they are discussed in connection with the recent magnetization result.

We report the spin dynamics of the quantum kagome lattice ZnCu3(OH)6Cl2 observed by our high field ESR. The spin susceptibility deduced from our ESR result follows the Curie-Weiss-dependence, which is identical with the magnetic susceptibility. No resonance shift is observed down to 1.8 K in our measurement. This means that the internal field does not develop down to very low temperature. Moreover, the linewidth turns out to take the constant value from room temperature to 1.8 K. It suggests that the spin-spin correlation does not develop down to very low temperature of 0.009J. Our results suggest that ZnCu3(OH)6Cl2 looks like an ideal S = 1/2 kagome antiferromagnet from the ESR point of view.

Multi-frequency ESR measurements of S = 1/2 phyrochlore system ZnxCu4-x(OH)6Cl2(x = 0 ~ 0.5) have been performed using the pulse magnetic field up to 16 T and in the frequency region from 30 GHz to 360 GHz. Temperature dependence ESR measurement of the distorted pyrochlore system (x = 0) from 1.76 K to 265 K shows the linewidth broadening below 20 K, and the coexistence of AFMR modes and the peculiar mode is suggested below 6 K. Moreover, the x dependence measurements at 1.8 K suggest the existence of phase boundaries around x = 0.3 and 0.5. Possible model to interpret the result will be presented.