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In solid-state physics, pressure is an essential parameter, which can continuously change the interaction between electrons. The combination of terahertz electron spin resonance (THz ESR) with pressure is a powerful approach to clarify the spin state under pressure from the microscopic point of view, especially for quantum magnets. The most important feature of our high-pressure THz ESR system is that our pressure cell ensures the high pressure and wide frequency range using ceramics in internal parts, which are well balanced in terms of toughness and transmittance of electromagnetic waves. This approach allowed to very accurately describe the spin state under pressure. In this review, high-pressure ESR systems are reviewed with a focus on those in the high-frequency region. In addition, we show differences between other systems and our system. Moreover, we show the usefulness of our system by showing application examples of several magnetic materials of interest.

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.

We systematically studied the zero-field splitting (ZFS) parameters of Fe(III) protoporphyrin IX chloride, or hemin, using the terahertz electron paramagnetic resonance (EPR) spectroscopy technique at ambient and high pressures. Although hemin is known as a model substance of hemoproteins, the pressure effect on the electronic structure has not yet been explored owing to the large ZFS. In this study, high-field and high-frequency EPR measurements were carried out in the frequency range up to 700 GHz and at hydrostatic pressures up to 2 GPa. At ambient pressure, multiple EPR branches were clearly observed, and the axial and rhombic components of ZFS were determined as $$D = 6.90\pm 0.01\hbox { cm}^{-1}$$ and $$E = 0.055 \pm 0.005\hbox { cm}^{-1}$$ , respectively. Upon pressure application, we observed a systematic shift of the resonance field, indicating a monotonous increase of the axial component from D = 6.9 to 7.9 $$\hbox {cm}^{-1}$$ at 2 GPa. The origin of this unusually large shift was discussed from a microscopic viewpoint of the electronic structure of iron under pressure.

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.

Recent wide-band (65–850 GHz) electron magnetic resonance (EMR) study determined the zero field splitting (ZFS) parameters (ZFSPs) for Fe2+ in natural and synthetic forsterite (Mg2SiO4) based on the fictitious spin S′ = 1 approach. So-obtained ZFSPs (D′, E′) are incompatible with those available in literature for 3d6 (Fe2+) and 3d4 (Fe4+, Mn3+, Cr2+) ions with the electronic spin S = 2, which predominantly pertain to the effective spin S ˜ = 2. Background for the effective spin S ˜ versus the fictitious ‘spin’ S′ (J′) approaches is provided with focus on application to Fe2+ ions. To enable comparison of the S′ = 1 ZFSPs with the S ˜ = 2 ones methodology for conversions has been worked out and appropriate conversion relations derived for various combinations of the possible energy level schemes for the spin S ˜ = 2 and S′ = 1. The second-rank S′ = 1 ZFSPs (D′, E′) measured by high-frequency EMR for Fe2+ in Mg2SiO4 are converted to the S ˜ = 2 ZFSPs (D, E) and compared with literature data. Suitability of Fe2+:Mg2SiO4 systems for application as high-pressure probes for high-magnetic field and high-frequency EMR (HMF-EMR) is then considered. The results of this study may be applied to other cases of 3d6 and 3d4 (S = 2) ions in various hosts.

We report the thermally detected electron spin resonance (ESR) spectroscopy in the frequency range of millimeter and submillimeter waves. Under high vacuum conditions, a cantilever-shaped device detects ESR absorption of a mounted sample as a temperature difference in its beam direction. Despite the simple experimental setup, the spin sensitivity of the order of 10(12) spins/G was achieved at 10 K. The developed sample stage is small enough to be used in a 10 T split-pair superconducting magnet with a bore of 25 mm, enabling precise field-angle-dependent ESR measurements at multi-frequencies above 500 GHz. We demonstrate its usefulness by studying the field-angle dependence of the excitation energy of the dimer triplet state in the Shastry–Sutherland magnet SrCu2(BO3)2.

The spin Hamiltonian (SH) parameters experimentally determined by EMR (EPR) may be corroborated or otherwise using various theoretical modeling approaches. To this end semiempirical modeling is carried out for high-spin (S=2) manganese (III) 3d4 ions in complex of tetraphenylporphyrinato manganese (III) chloride (MnTPPCl). This modeling utilizes the microscopic spin Hamiltonians (MSH) approach developed for the 3d4 and 3d6 ions with spin S=2 at orthorhombic and tetragonal symmetry sites in crystals, which exhibit an orbital singlet ground state. Calculations of the zero-field splitting (ZFS) parameters and the Zeeman electronic (Ze) factors (g||=gz, g⊥=gx=gy) are carried out for wide ranges of values of the microscopic parameters using the MSH/VBA package. This enables to examine the dependence of the theoretically determined ZFS parameters bkq (in the Stevens notation) and the Zeeman factors gi on the spin-orbit (λ), spin-spin (ρ) coupling constant, and the ligand-field energy levels (Δi) within the 5D multiplet. The results are presented in suitable tables and graphs. The values of λ, ρ, and Δi best describing Mn(III) ions in MnTPPCl are determined by matching the theoretical second-rank ZFSP b20(D) parameter and the experimental one. The fourth-rank ZFS parameters (b40, b44) and the ρ (spin-spin)-related contributions, which have been omitted in previous studies, are considered for the first time here and are found important. Semiempirical modeling results are compared with those obtained recently by the density functional theory (DFT) and/or ab initio methods.

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.

We have developed a high-pressure electron spin resonance probe and successfully installed into the world’s highest-field cryogen-free superconducting magnet having a maximum central field of 24.6 T. The high pressure of 2.5 GPa is achieved by the specially designed piston-cylinder pressure cell using THz-wave-transparent components. In the first application of this high-pressure high-field ESR system, we observed that the orthogonal dimer spin system SrCu2(BO3)2 undergoes a quantum phase transition from the dimer singlet ground to the plaquette singlet ground states.

We experimentally studied the pressure dependence of the zero-field splitting (ZFS) parameter of hemin (iron(III) protoporphyrin IX chloride), which is a model complex of hemoproteins, via high-frequency and high-field electron paramagnetic resonance (HFEPR) under pressure. Owing to the large ZFS, the pressure effect on the electronic structure of iron-porphyrin complexes has not yet been explored using EPR. Therefore, we systematically studied this effect using our newly developed sub-terahertz EPR spectroscopy system in the frequency range of 80-515 GHz, under magnetic fields up to 10 T and pressure up to 2 GPa. We observed a systematic shift of the resonance fields of hemin upon pressure application, from which the axial component of the ZFS parameter was found to increase from D = 6.9 to 7.9 cm-1 at 2 GPa. In contrast to the previous methods used to study proteins under pressure, which mainly focused on conformational changes, our HFEPR technique can obtain more microscopic insights into the electronic structures of metal ions under pressure. In this sense, our technique provides novel opportunities to study the pressure effects on biofunctional active centers of versatile metalloproteins.

We have developed a high-pressure and multi-frequency electron spin resonance (ESR) system in the submillimeter wave region. The pressure is generated by the piston–cylinder pressure cell whose inner parts are all made of ceramics. The requirements for the inner parts of the pressure cell are toughness and transmission in the submillimeter wave region and they enable us to observe the transmitted light through a sample subjected to high pressure. It was found that both properties can be achieved by the combination of the inner parts made of the ZrO2-based ceramic and the Al2O3 ceramic. For the Shastry-Sutherland compound SrCu2(BO3)2, we observed the direct ESR transition from the singlet ground state to the first excited triplet sates at 1.5 GPa successfully in the frequency region from 300 to 700 GHz.

High-field electron spin resonance measurements of perovskite compound YCrO3 powder have been performed to obtain the magnetic anisotropy. Isotropic absorption lines of electron paramagnetic resonance, which can be described as single Lorentzian shape, are observed. The g value of paramagnetic state is estimated to be 1.983 at 265 K. Magnitude of D vector of Dzyaloshinskii–Moriya interaction is estimated to be 0.36 K from Moriya’s formula using the obtained g value. Powder patterns of antiferromagnetic resonance (AFMR) with large antiferromagnetic gaps are observed below T N = 140 K. Two antiferromagnetic gaps at zero field have been estimated to be 181 GHz (6.5 T) and 338 GHz (12.1 T), and a spin flop transition field have been estimated to be 3.5 T from widely observed frequency–field diagram at 1.9 K. Although obtained AFMR modes seem to be typical AFMR modes of two sublattice antiferromagnet with triaxial magnetic anisotropy, the obtained zero-field gap is twice as large as the obtained spin flop transition field which suggests the unconventional antiferromagnetic order.

We have developed the high-pressure electron spin resonance (ESR) system using a micro-coil in the frequency region up to around 2 GHz and potentially 10 GHz. The hybrid-type piston-cylinder pressure cell whose maximum pressure reaches 4 GPa was used. In this study, we obtained ESR spectra at 2.3 GPa successfully, which can never be obtained by the single-layer piston-cylinder pressure cell. The minimum detectable spin number was estimated to be the order of 1012 spins/G. Moreover, it is shown that the sensitivity can be improved by two orders of magnitude using the field modulation technique. This high-pressure ESR technique is a promising one to achieve the sensitivity and the high pressure simultaneously.

YSrCrO4 is first synthesized and found to be a hettotype of the K2NiF4 structure, although the combination of Y and Sr ions in K2NiF4-type oxides is very rare. The space group of the compound is determined to be orthorhombic Pccn by the electron diffraction and the powder X-ray diffraction. Magnetic and dielectric properties of the compound, together with LaSrCrO4, YCaCrO4 and A2MnO4 (A = Sr and Ca), are investigated. YSrCrO4 shows two-dimensional (2D) spin correlations and a canted antiferromagnetic (AF) ordering. With increasing distortion of the crystal structure, 2D spin correlations and the AF transition temperature decrease, while spontaneous magnetization increases. From the multi-frequency electron spin resonance (ESR) measurements, the g-values of the paramagnetic state are estimated to be 1.976, 1.978 and 1.976 for YSrCrO4, LaSrCrO4 and YCaCrO4, respectively. Evidence of AF ordering of the Cr oxides is obtained microscopically from ESR. The dielectric measurements suggest the existence of in-gap states, while no magneto-dielectric coupling was observed in the above compounds.

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.

To develop a new spin-crossover functional material, a magnetic hybrid compound [Fe(qsal)(2)][Ni(mnt)(2)] was designed and synthesized (Hqsal = N-(8-quinolyl)salicylaldimine, mnt = maleonitriledithiolate). The temperature dependence of magnetic susceptibility suggested the coexistence of the high-spin (HS) Fe(III) cation and pi-radical anion at room temperature and a magnetic transition below 100 K. The thermal variation of crystal structures revealed that strong pi-stacking interaction between the pi-ligand in the [Fe(qsal)(2)] cation and [Ni(mnt)(2)] anion induced the distortion of an Fe(III) coordination structure and the suppression of a dimerization of the [Ni(mnt)(2)] anion. Transfer integral calculations indicated that the magnetic transition below 100 K originated from a spin-singlet formation transformation in the [Ni(mnt)(2)] dimer. The magnetic relaxation of Mossbauer spectra and large thermal variation of a g-value in electron paramagnetic resonance spectra below the magnetic transition temperature implied the existence of a magnetic correlation between d-spin and pi-spin.

Various light sources for plant production have been developed to establish a completely controlled factory. We propose a pulse light source using Xe discharge. For the rapid growth of the plant, an intense light covering the absorption spectral region of chlorophyll is demanded, and periodic irradiation is also required because the photosynthetic reaction has a certain period. In this study, Sr3MgSi2O8:Eu,Mn phosphor powders with various Eu and Mn content, which show two emission peaks in the absorption region of photosynthetic pigment and phytochrome, were synthesized, and were analyzed using X-ray diffraction, photoluminescence and electron spin resonance spectroscopy. It was found that the exchange-coupled Mn-Mn and Eu-Mn pairs lead the rapid decay of Mn emission and efficient energy transfer from Eu and Mn ions, respectively. A plasma tube array panel using the phosphor was also fabricated and the similar emission spectrum of Sr3MgSi2O8:Eu,Mn is obtained under vacuum-UV excitation in comparison to the UV excitation.

A hybrid-type piston-cylinder pressure cell for the electron spin resonance (ESR) measurement has been developed. The cylinder of this pressure cell consists of a NiCrAl inner cylinder and a CuBe outer sleeve, and all inner parts are made of zirconium oxide which has good transmittance to the millimeter and submillimeter waves. We confirmed that the pressure reaches 2.1 GPa. We have also developed a transmission-type high-field ESR system having two different modulation methods for this pressure cell. A test measurement without pressure cell for the two-dimensional orthogonal-dimer spin system of SrCu2(BO3)2 has been done successfully in the wide frequency region. The combination of this electromagnetic wave transmission-type pressure cell and this high-field ESR system is a promising tool for the study of the pressure-induced phase transition of SrCu2(BO3)2.

A single crystal S=3/2 perfect kagome lattice antiferromagnet $\mathrm{KCr_3(OH)_6(SO_4)_2}$ (Cr-jarosite) has been studied by X-band and high-frequency electron spin resonance (ESR). The g-values perpendicular to the kagome plane (c-axis) and in the plane are determined to be $g_c=1.9704 \pm 0.0002$ and $g_\xi=1.9720 \pm 0.0003$, respectively, by high-frequency ESR observed at 265 K. Antiferromagnetic resonances (AFMR) with the antiferromagnetic gap of 120 GHz are observed at 1.9 K, which is below $T_N$=4.5 K. The analysis of AFMR modes by the conventional molecular field theory shows $d_p=0.27$ K and $d_z=0.07$ K, where $d_p$ and $d_z$ are in-plane and out-of-plane components of DM d vector, respectively. From these results and the estimated exchange interaction J=6.15 K by Okuta et al., the ground state of Cr-jarosite is discussed in connection with the Monte Carlo simulations result with classical Heisenberg spins on the kagome lattice by Elhajal et al. Finally, the angular dependence of linewidth and the lineshape observed at 296 K by X-band ESR show typical behavior of a two-dimensional Heisenberg antiferromagnet, suggesting a good two-dimensionality of Cr-jarosite.

Recent developments of “multi-extreme” high magnetic field electron spin resonance (ESR) in Kobe will be reviewed. Our high magnetic field ESR covers the frequency region between 0.03 and 7 THz and the temperature region between 1.8 and 300 K. With this high magnetic field ESR system we can apply the magnetic field up to 55 T using a Cu-Ag pulsed magnet and a 300 kJ (10 kV) capacitor bank. Under this high magnetic field we can also apply the high pressure up to 1.4 GPa. As we can make the measurement under low temperature, high magnetic field and high pressure simultaneously, we name it as “multi-extreme” ESR. Moreover, in order to gain the sensitivity of our high magnetic field ESR, we have developed a micro-cantilever ESR system using a torque method, which enables the ESR measurement of micrometer size single crystal at low temperature. At the moment we are in the process of extending the magnetic field region of micro-cantilever ESR. Recently we have succeeded in making the measurement up to 369 GHz and the achieved sensitivity is about 1010 spins/G, which is much higher than that using the conventional transmission method. Finally our development of magnetization detected ESR using SQUID magnetometer (SQUID ESR) will be also presented.

High-frequency electron spin resonance (ESR) measurements of S = 1/2 one-dimensional Heisenberg antiferromagnetic zig-zag chain substance (VO)(SO4)(2,2-bpy) were performed in the temperature region from 1.8 to 265 K. ESR measurements at 265 K show typical powder pattern spectra of the V4+ ion and g || = 1.925 ± 0.001 and g ⊥ = 1.976 ± 0.001 were obtained. Although the magnetic susceptibility shows a maximum at 3.1 K suggesting the existence of a short-range order, no g-shift and line width broadening were observed down to 1.8 K. We suggest that these temperature-independent features of ESR can be attributed to the result of frustration in (VO)(SO4)(2,2-bpy).

Highly oriented pyrolytic graphite (HOPG) samples were irradiated by highly charged ions Ar11+ and Xe26+. Scanning electron microscopy (SEM), electron spin resonance (ESR), a superconducting quantum interference device (SQUID), and Raman spectroscopy were used to investigate the effect of this irradiation. SEM observations revealed that the contrast between irradiated and unirradiated regions became discernible at a fluence of about 1014cm -2. On the other hand, for samples irradiated by Ar+ ions, a fluence of the order of 1015cm -2 was necessary to obtain a similar contrast to that obtained by Ar11+ ion irradiation. This demonstrates that highly charged ions effectively enhance modification of solid surfaces. Furthermore, the ESR and SQUID measurements revealed the formation of defects and magnetization of the irradiated surface. Raman measurements were also performed to investigate the structural transformation at different fluences. © 2011 The Surface Science Society of Japan.

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 $(J'/J)_{c}=0.660\pm0.003$.

Development of multi-extreme electron spin resonance (ESR) measurement system is in progress in Kobe. We have installed the pulsed magnetic field up to 55 T and our high field ESR system can cover the frequency region from 30 GHz to 7 THz. Our aim is to extend our high field ESR system to the multi-extreme ESR measurement system by combining low temperature, high pressure and nano techniques, and the combination with the high pressure is focused in the paper. High pressure can be achieved up to 1.4 GPa by using the transmission type piston cylinder pressure cell. We have succeeded in combining the high field and the high pressure. As an example, the pressure dependence of the spin gap in SrCu2(BO3)2, S=1/2 Shustally-Sutherland dimer system, observed by our multi-extreme ESR measurement is presented. This measurement is the direct and precise measurement of the spin gap, and the spin gap decreased from 723 GHz (34.7 K) to 581 GHz (27.9 K) at 1.2 GPa.

The microscopic magnetic properties of high-quality GdN thin films have been investigated by electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements. Detailed temperature dependence ESR measurements have shown the existence of two ferromagnetic components at lower temperatures, which was not clear from the previous magnetization measurements. The temperature, where the resonance shift occurs for the major ferromagnetic component, seems to be consistent with the Curie temperature obtained from the previous magnetization measurement. On the other hand, the divergence of line width is observed around 57 K for the minor ferromagnetic component. The magnetic anisotropies of GdN thin films have been obtained by the analysis of FMR angular dependence observed at 4.2 K. Combining the X-ray diffraction results, the correlation between the magnetic anisotropies and the lattice constants is discussed.

High-field ESR technique has been applied to the novel frustrated spin chain system KCu5V3O13, to reveal the peculiar ground state proposed by the previous results of the magnetic susceptibility and the magnetization measurements. We successfully observed the antiferromagnetic resonance with the easy axis type anisotropy below 7 K . In addition, small shifts of the resonance field probably due to the re-arrangements of spin configuration in the ordered phase were also observed at about 4 K in the detailed temperature dependence measurements. This result confirmed the observation of double peaks at 3.9 and 7 K in the specific heat measurements performed in advance to the high-field ESR measurements. We will discuss about the ground state of KCu5V3O13 based on these results together with those obtained in the KMgCu4V3O13, where no antiferromagnetic resonance was observed.

Recent developments of the high-field facility in Kobe University are presented. A far infrared radiation (FIR) laser system, which can generate electromagnetic waves from 500 GHz to 3 THz has been installed. Now the frequency region for high-field ESR is extended from 30 GHz to 3 THz using Gunn oscillators, BWOs and a FIR laser. Recent high-field ESR results of quantum spin systems CsFeBr3 and Cu3(CO3)2(OH)2 using these light sources are presented. CsFeBr3 is known as a field-induced antiferromagnetic ordering substance, and we found new ESR modes in the high-field phase of CsFeBr3 for B//c at 2.0 K . Cu3(CO3)2(OH)2 is the model substance for the S= 1 2 quantum diamond chain, and we observed the direct transition in the ESR measurement at 1.9 K for B//c.

High-field ESR measurements of S= 1 2 triangular antiferromagnet CsCuCl3 have been performed at 6 K under pressure using the pulsed magnetic field up to 16 T in the frequency region from 120 to 360 GHz . We found that both the phase transition field and the antiferromagnetic gap increased under pressure. The origin of the increase of the antiferromagnetic gap was concluded to be due to the increase of the easy-plane type anisotropy as well as the increase of the interchain interaction of this system. The increase of the easy-plane type anisotropy was consistent with the increase of the phase transition field under pressure as suggested by Nikuni and Shiba's theory.

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.

Due to the existence of strong spin frustration in a system, the spin dynamics of S= 1/2 kagome lattice antiferromagnet at low temperature has attracted much interest. High-field ESR has been measured on its model substances, Cu 3 V 2 O 7 (OH) 2 · 2H 2 O (volborthite) and BaCu 3 V 2 O 3 (OH) 2 (vesignieite), down to 1.8 K using pulsed magnetic fields up to 16 T. The measurements are performed for 160 and 315 GHz using polycrystalline samples. Although both samples showed the g-shift and the change of linewidth at low temperature, volborthite showed a small gap excitation of the order of 40 GHz (1.9 K) while vesignieite showed a paramagnetic behavior down to 1.9 K. Observed difference will be discussed in connection with the crystal structure, and the possible spin liquid state in vesignieite will be discussed.

Temperature-dependent electron spin resonance (ESR) measurements of two low-dimensional quantum spin systems, theS = 1/2 spin-ladder system Cu2(1,4-diazacycloheptane)2Cl4 (Cu2(C5H12N2)2Cl4) and the gaplessS = 1 bond-alternating one-dimensional antiferromagnettrans-Ni(333-tet)(N3)(ClO4), have been performed. X-band electron spin resonance (ESR) measurements of single-crystal Cu2(C5H12N2)2Cl4 show the increase of linewidth and anisotropicg-shifts below 8 K similar to those known for one-dimensional antiferromagnet. On the other hand, Ni(333-tet)(N3)(ClO4) has broad line-width and ESR has been observed for the first time by our high-field ESR. Its linewidth increases as the temperature is decreased, while theg-shift seems to be isotropic and theg-value decreases as the temperature is decreased. Thisg-shift can be connected to the quantum fluctuation of the system.

X-band ESR measurements have been performed on Na 0.5 CoO 2 in the temperature region between 4.5 K and room temperature. The anisotropic g values were observed at room temperature. A large g shift and an increase of the linewidth were also observed as temperature was decreased above T c 1 = 87 K . In addition, no clear signal was observed below T c 1 in a range of the measured magnetic field and this behavior supports the long range antiferromagnetic order at T c 1 . The origin of the large g shift with decreasing temperature will be discussed.

High field ESR measurements have been performed on newly synthesized layered manganese oxides A 4 Mn 3 O 10 (A=Ba, Sr) using a pulsed magnetic field up to 16 T. From the temperature and the frequency dependence measurements, it was confirmed that both ground states at low temperature are the antiferromagnetic ordered state. The typical AFMR absorption lines were observed for both compounds and both were well explained by the AFMR theory for a two-sublattice antiferromagnet with orthorhombic magnetic anisotropy. Moreover, we found that the magnetic anisotropy energies in these compounds are very small as compared with their exchange interactions.

The magnetic ground state of Sr2CrO4 with a distorted honeycomb lattice was investigated by means of measurements of the magnetic susceptibility, high-field magnetization process, and electron spin resonance (ESR). Antiferromagnetic ordering was observed clearly below TN?=?3.2?K, while the magnetic ground state had been thought to be a spin-singlet state. A two-sublattice model with biaxial anisotropy was applicable for the observed ESR modes. Plateau-like behavior and a sharp kink were detected in the magnetization curve.

High-field electron spin resonance (ESR) measurements of ${\mathrm{Bi}}_{3}{\mathrm{Mn}}_{4}{\mathrm{O}}_{12}({\mathrm{NO}}_{3})$, which has been suggested recently as a new honeycomb lattice antiferromagnet with spin frustration, have been performed in a field region up to 20 T. An ESR measurement at 265 K shows an isotropic broad resonance, and the obtained $g$ value is $1.974\ifmmode\pm\else\textpm\fi{}0.016$, which is consistent with the ${\mathrm{Mn}}^{4+}$ ion in a crystal field with low symmetry. At 1.8 K, resonances, whose frequency-field relation corresponds to antiferromagnetic resonance (AFMR) with an easy-plane type anisotropy and a Dzyaloshinsky-Moriya (DM) interaction, appear above 6 T. This result is consistent with the field-induced magnetic order (FIMO) phase suggested by a previous neutron measurement. From the analysis of our AFMR result, the $D$ term of the DM interaction is estimated as 1.3 K, and the direction of the $D$ term is suggested to be along the $c$ axis, which is consistent with the crystal symmetry. Using this $D$ and the exchange interaction, the canted angle is estimated to be 2.42${}^{\ensuremath{\circ}}$, which can interpret the magnetization jump at the metamagnetic transition qualitatively. The possible origin of the FIMO phase is discussed with recent theoretical results within the ${J}_{1}\ensuremath{-}{J}_{2}$ model.

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.

Dolerophanite Cu2O(SO4) is one of model substances for S = 1/2 kagome like antiferromagnet. Although many X-ray diffraction studies have been performed on natural mineral Cu2O(SO4), little is known about its magnetic property. Therefore, the multi-frequency electron spin resonance (ESR) measurements of dolerophanite powder sample have been performed using pulsed magnetic fields up to 16 T in the frequency range from 50 GHz to 315 GHz in the temperature range from 1.8 K to 86 K Antiferromagnetic resonance (AFMR) has been observed below TN = 20 K, which is consistent with the previous magnetic susceptibility result by Asano et al. Moreover, the frequency-field relation at 1.8 K suggests typical easy-plane type AFMR with Dzyaloshinsky-Moriya (DM) interaction, which is also consistent with the weak ferromagnetism below TN suggested by Asano et al. As the estimation of DM interaction is very important to discuss the quantum phase transition in kagome antiferromagnet as suggested by C?pas et al. [2008, Phys. Rev. B 78 140405(R)], we have estimated the D term of DM interaction as 6.98 ? 0.01 K from our AFMR results.

High field ESR measurements have been performed on (Dl-DCNQI) 2 Ag single crystals at 1.9 K under I bar and 2.4 kbar in the frequency region from 70 to 160 GHz using the pulsed magnetic field up to 8 T. This compound undergoes the antiferromagnetic transition at T N = 5.5 K and the antiferromagnetic resonance (AFMR) was observed under 1 bar. The AFMR was also observed under 2.4 kbar and the zero-field antiferromagnetic gap is found to be reduced under pressure. The origin of this result is discussed.

Highly oriented pyrolytic graphite (HOPG) samples were irradiated with a highly charged ion (HCI) Ar11+ at a fluence of 1014 cm−2. After irradiation, scanning electron microscopy (SEM) was used to observe the irradiated trace. During SEM measurement, an electron acceleration voltage of 0.5, 1 and 5 kV was applied to measure the samples, respectively. The irradiated contrast can be observed in the Si surface at 0.5 kV, which could not be found at 1 kV or higher. The irradiated area becomes brighter than that of the unirradiated area. When Ar1+ was used to bombard the solid surface, the fluence should become of the order of 1015 cm−2. In this case, the irradiated contrast could be observed. It means HCI is able to enhance more effectively the surface modification. Furthermore, electron spin resonance was used to measure the surface of highly oriented pyrolytic graphite (HOPG). A clear resonant peak appears in the irradiated sample, which cannot be found in the pristine sample. The phenomenon could be related to defects induced by the HCI impact.

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.

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.