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As THz electron spin resonance (ESR) measurement has several advantages of high spectral resolution, ESR measurement beyond the zero-field splitting or the magnetic phase transition, it is a powerful means to study novel magnetic systems. Focusing on these advantages, we have been developing this THz ESR, and we are now extending to the multi-extreme THz ESR. Here the multi-extreme corresponds to the high magnetic field, the high pressure, the low temperature and the micro-mechanically detected highly sensitive THz ESR. Recent new developments and applications, especially focusing on the high pressure THz ESR, will be shown.

THz electron spin resonance (ESR) measurement has been a powerful means to study novel magnetic systems because of the advantages of high spectral resolution, ESR measurement beyond the zero-field splitting or the magnetic phase transition. Therefore, we have been developing this THz ESR system since 1988, and we are now extending to the multi-extreme THz ESR. Here the multi-extreme corresponds to the high magnetic field, the high pressure, the low temperature and the micro-mechanically detected ESR. Recent new developments, especially focusing on the high pressure THz ESR, will be shown.

Due to the advantages of high spectral resolution, ESR measurement beyond the zero field splitting or the magnetic phase transition, THz electron spin resonance (ESR) measurement has been a powerful means to study novel magnetic systems. We have been developing this THz ESR system since 1988, and we are now extending to the multi-extreme THz ESR. Here the multi-extreme corresponds to the high magnetic field, the high pressure, the low temperature and the micro-mechanically detected ESR. Recent new developments, and the application of the high pressure THz ESR will be shown.

Due to the advantages of high spectral resolution, ESR measurement beyond the zero field splitting or the magnetic phase transition, THz electron spin resonance (ESR) measurement has been a powerful means to study novel magnetic systems. We have been developing this THz ESR system since 1988, and we are now extending to the multi-extreme THz ESR. Here the multi-extreme corresponds to the high magnetic field, the high pressure, the low temperature and the micro-cantilever ESR. Recent new developments and the application to the study of multiferroic system will be shown.

Multi-extreme THz ESR measurements have been performed on Cr-jarosite, which is an ideal model substance of highly frustrated kagome antiferromagnet. The analysis of antiferromagnetic resonances observed at 1.9K revealed the Dzyaloshinsky-Moriya interaction, which plays an important role on the ground state.

Developments of our highly sensitive THz ESR systems are presented. One method is the ESR detection using a micro-cantilever, and we achieved the sensitivity of about 1011 spins/G up to 315 GHz. Another method is the ESR detection up to 315 GHz using a commercial SQUID magnetometer.

Spin dynamics of S=2 triangluar lattice antiferromagnet have been investigated by the submillimeter and the millimeter wave ESR measurements of polycrystalline samples of InMnO 3 . Single absorption line is observed in whole temperature range from 2.0 K to 265 K. The resonance field shifts due to the development of internal field are observed below 40 K. The spin dynamics of InMnO 3 will be discussed in connection with other triangluar lattice antiferromagnets.

Cu3(OH)4SO4, which was obtained by the hydorothermal synthesis, was studied by our high frequency electron spin resonance (ESR) measurements. It has an interesting magnetic structure which consists of triple chains of Cu2+ spins (s=1/2). It undergoes the 3D-transition below 5.3 K. In the ordered state, the magnetic moment of the central chain becomes zero. This interesting behavior is called as "idle spin". We have carried out the high frequency high field ESR measurements in the millimeter wave region to obtain the information about the 3D-ordering and the magnetic anisotropy of Cu3(OH)4SO4. We have observed antiferromagnetic modes at 1.8 K, which suggests the existence of the 3D-transition. The energy gap, which has been analyzed by the conventional 2- sublattice AFMR theory, is estimated to be 2.6 K. We have also succeeded in observing the change of the AF modes in the high magnetic field region. The critical Held is about 5T.

We have developed the submillimeter wave ESR measurement system under pressure up to 3.5 kbar using the transmission technique. This system has the advantages that the pulsed magnetic field up to 16 T and the wide frequency region up to 670 GHz are available. The pressure is generated by the clamped-type pressure cell whose inner parts are all made by the sapphire. We applied this system to the study of the quantum spin-gap system KCuCl/sub 3/ and obtained the direct evidence that the gap energy of this compound decreased under pressure.

The edge states in the finite S=1 Haldane chain are expected to have different energy structures depending on the chain length due to the interaction through the finite chain. Electron spin resonance (ESR) signals coming from the finite chains with various chain length have been observed separately in Y2BaNi0.96Mg0.04O5, which is a famous Haldane system doped with non-magnetic ions, due to the high spectral resolution of our submillimeter wave ESR. From the analysis, the information about the correlation length in the Haldane system at low temperature has been obtained directly by the experiment for the first time.

BaCu/sub 2/(Si/sub 1-x/Ge/sub x/)/sub 2/O/sub 7/ (x=0.65) is a one dimensional antiferromagnet (1D AF) with the staggered g-tensor and Dzyaloshinskii-Moriya (DM) interaction. Although the system is expected to have intrinsic randomness, our submillimeter wave ESR results show the typical behavior of the 1D AF and they are discussed along with the recent ESR theory of Oshikawa and Affleck (Phys. Rev. Lett. vol. 82, p. 5136, 1999).

Submillimeter and millimeter wave ESR measurements of the one-dimensional CuO/sub 2/ chain system, Ca/sub 2/Y/sub 2/Cu/sub 5/O/sub 10/, have been performed using a pulsed magnetic field up to 16 T. The g-shift is observed between T/sub N/=30 K and 50 K. The frequency-field diagram observed at 1.8 K shows AFMR modes of the easy axis type, The magnetic properties of the AF (antiferromagnetic) state are discussed.