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High-pressure and high-field electron spin resonance (ESR) measurements have been performed on a single crystal of the orthogonal-dimer spin system SrCu$_{2}$(BO$_{3}$)$_{2}$. With frequencies below 1 THz, ESR signals associated with transitions from the singlet ground state to the one-triplet excited states and the two-triplet bound state were observed at pressures up to 2.1 GPa. We obtained directly the pressure dependence of the gap energies, finding a clear first-order phase transition at $P_{c}=1.85\pm0.05$ GPa. By comparing this pressure dependence with the calculated excitation energies obtained from an exact diagonalization, we determined the precise pressure dependence for inter- ($J'$) and intra-dimer ($J$) exchange interactions considering the Dzyaloshinski-Moriya interaction. Thus this system undergoes a first-order quantum phase transition from the dimer singlet phase to a plaquette singlet phase above the ratio $(J'/J)_{c}=0.660\pm0.003$.

High field magnetization processes were measured for the single crystal of an S = 1/2 ferro-antiferromagnetic delta-chain system [Cu(bpy)(H 2 O)][Cu(bpy)(mal)–(H 2 O)](ClO 4 ) 2 . At low temperatures, the observed magnetization curve shows a rapid increase to half of the saturation moment below about 1 T and then followed by a gradual increase to fully polarized state below about 7 T. Concerning this characteristic magnetization process, we discuss the ground state of the system qualitatively based on a non-interacting two sublattice model, where a delta-chain is divided into apical free spins and remaining basal spins regarded as quasi one dimensional antiferromagnetic chain. Essential effect of ferromagnetic interaction, which connects an apical spin and basal spins, is pointed out from the comparison with theoretical curve for the non-interacting two sublattice model as well as with the numerically calculated curve. Presence of the Dzyaloshinskii–Moriya interaction is also suggested from the arguments ...

Intensive high-field ESR studies have been performed to investigate the peculiar ground state of a quasi-one dimensional cupric oxide Ca 1- x CuO 2 ( x =0.164), to which a two-sublattice model (chain-dimer model) composed of a Heisenberg antiferromagnetic chain and dimer state with low spin density is proposed from the results of magnetic susceptibility and specific heat measurements. Using the powder sample and high frequency sources from 60 to 490 GHz, we observed antiferromagnetic resonance mode with a typical frequency–field relation of the easy-axis type at 1.8 K. Moreover, weak ESR signals are also observed in the ordered state. From the analysis of the integrated intensity for those weak signals, it is found that the dimer model reproduces the temperature dependent behaviors quite well. These results support the chain-dimer model from the microscopic point of view.

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).

We have developed a low-temperature ESR system under pressure, which has the frequency region above 70 GHz, the pulsed magnetic field up to 16 T and the pressure region up to about 3.5 kbar. A suitable pressure-calibration method using the change of the ESR of ruby under pressure has been also developed. We can determine the pressure and observe ESR signals of a sample at the same time. The high-field ESR measurements with the pulsed magnetic field up to 16 T have been performed on triangular antiferromagnet CsCuCl 3 for H||c around 6 K at 1 bar and at 3.1 ± 0.6 kbar in the frequency region from 120 GHz to 370 GHz. We observed the increase of the phase transition field from the umbrella-type configuration to the coplanar configuration due to the quantum fluctuation and the increase of the antiferromagnetic gap under pressure. The possible origin of this increase of the phase transition field was discussed in connection with the recent magnetization measurements under pressure.

We give an overview of our studies on the electron spin resonance (ESR) in the hexagonal triangular antiferromagnets of ABX 3 type using millimeter and submillimeter waves and static and pulsed high magnetic fields. Novel ESR modes, which cannot be understood by the conventional theory of the antiferromagnetic resonance, were observed. Examples of this include that there exists the gapless excitation even for the easy-axis anisotropy. The ESR modes can be classified by the signs of the exchange interaction J 0 along the c-axis and the anisotropy. The ESR modes were calculated within the framework of the mean-field approximation using three-sublattice and six-sublattice models for the ferromagnetic and antiferromagnetic J 0 interaction, respectively. The experimental results were well described by the present calculations.

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.

X-band ESR measurements of BIP-TENO (3,3 ′ ,5,5 ′ -tetrakis( N -tert-butylaminoxyl)biphenyl) single crystal, which is an S =1 spin ladder model substance, have been performed in the temperature reg...

X-band and millimeter wave ESR measurements have been performed on (DI-DCNQI) 2 Ag single crystals. X-band ESR measurements were carried out with the static magnetic field up to 1 T. Electron param...

Transitions between the ground singlet state to the excited triplet state has been observed in CuGeO3 by means of submillimeter wave electron spin resonance. The strong absorption intensity shows the break down of the selection rule. 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 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., 6 pages, 8figures submitted to J. Phys. Soc. Jpn

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.

High-frequency ESR results on the S=1/2 Heisenberg hexagonal antiferromagnet InCu2/3V1/3O3 are reported. This compound appears to be a rare model substance for the honeycomb lattice antiferromagnet with very weak interlayer couplings. The high-temperature magnetic susceptibility can be interpreted by the S=1/2 honeycomb lattice antiferromagnet, and it shows a magnetic-order-like anomaly at TN=38 K. Although, the resonance field of our high-frequency ESR shows the typical behavior of the antiferromagnetic resonance, the linewidth of our high-frequency ESR continues to increase below TN, while it tends to decrease as the temperature in a conventional three-dimensional antiferromagnet decreases. In general, a honeycomb lattice antiferromagnet is expected to show a simple antiferromagnetic order similar to that of a square lattice antiferromagnet theoretically because both antiferromagnets are bipartite lattices. However, we suggest that the observed anomalous spin dynamics below TN is the peculiar feature of the honeycomb lattice antiferromagnet that is not observed in the square lattice antiferromagnet., Comment: 5 pages, 5 figures

High-field electron spin resonance (ESR) measurements have been performed on vesignieite BaCu$_3$V$_2$O$_8$(OH)$_2$, which is considered as a nearly ideal model substance of $S$=1/2 kagome antiferromagnet, in the temperature region from 1.9 to 265 K. The frequency region is from 60 to 360 GHz and the applied pulsed magnetic field is up to 16 T. Observed g-value and linewidth show the increase below 20 K, which suggest the development of the short range order. Moreover, a gapless spin liquid ground state is suggested from the frequency-field relation at 1.9 K., Comment: 5 pages, 6 figures, jpsj2 class file, to be published in J. Phys. Soc. Jpn

High-field electron spin resonance (ESR) measurements of single crystals of (CH 3 ) 2 NH 2 CuCl 3 (DMACuCl 3 ) have been performed in the frequency range of 50 to 550 GHz in the paramagnetic region. In addition to the electron paramagnetic resonance, we have observed four ESR modes ω 1 , ω 2 , ω 3 , and ω 4 . The modes ω 2 and ω 3 have a frequency gap of 325 GHz (15.6 K), while the mode ω 1 has a gap of 420 GHz (20 K) at 0 T. The modes ω 2 and ω 3 are attributed to the transitions between the singlet and the triplet of the antiferromagnetic (AF) dimer. We found that the large spin-gap for the AF dimer still remains at the critical field B c2 = 3.5 T determined by magnetization and specific heat measurements.

We have performed high-frequency ESR measurements on powder and aligned samples of one-dimensional quantum ferrimagnets, A 3 Cu 3 (PO 4 ) 4 , with A=Sr and Ca. Large shifts of paramagnetic g -values were observed. We explain the observed g -shifts by means of an antiferromagnetic trimer model. Moreover, gradual increases of the linewidth of the ESR spectra are observed, indicating that a short-range correlation between the effective spins on the trimers begins to develop with decreasing temperature below 25 K. Analysis of the ESR signal, taking the local structures around Cu 2+ ions into account, indicates quantum nature of A 3 Cu 3 (PO 4 ) 4 compounds.

High field ESR measurements of a single crystal of the spin frustrated spinel compound MgCr 2 O 4 have been performed in the temperature range from 1.7 to 80 K in the frequency range from 60 to 370 GHz. We found that MgCr 2 O 4 is an easy-plane type antiferromagnet with the hard axis almost along the direction below the Neel temperature T N . In order to obtain more detailed information, we have performed high field ESR measurements in an attempt to align the domains by applying a weak biaxial or uniaxial pressure. These results indicate that the structural phase transition is accompanied by a contraction along one of the directions and the contracted direction becomes the magnetic hard axis below T N .

Higher Order Effects due to both the electromagnetic and the SU(3)-violating interactions have been investigated for masses of particles belonging to the octet and decouplet representations of the unitary symmetry.