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The authors report on anisotropic thermal transport in the spin-1/2 triangular-lattice antiferromagnet Cs${}_{2}$CuCl${}_{4}$ close to the transition into the three-dimensional long-range-ordered state. This behavior is related to an additional heat-transport channel through magnetic excitations, that can best propagate along the direction of the largest exchange interaction.

High-resolution time-of-flight powder neutron diffraction and high-field magnetization were measured to investigate the magnetic structure and existence of a field-induced magnetic phase transition in the distorted kagome antiferromagnet ${\mathrm{Cs}}_{2}{\mathrm{Cu}}_{3}{\mathrm{SnF}}_{12}$. Upon cooling from room temperature, the compound undergoes a structural phase transition at ${T}_{\mathrm{t}}=185\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ from the rhombohedral space-group $R\overline{3}m$ with the perfect kagome spin network to the monoclinic space-group $P{2}_{1}/n$ with the distorted kagome planes. The distortion results in three inequivalent exchange interactions among the $S=1/2\phantom{\rule{0.28em}{0ex}}{\mathrm{Cu}}^{2+}$ spins that magnetically order below ${T}_{\mathrm{N}}=20.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Magnetization measured with a magnetic field applied within the kagome plane reveals small in-plane ferromagnetism resulting from spin canting. On the other hand, the out-of-plane magnetization does not show a clear hysteresis loop of the ferromagnetic component nor a prominent anomaly up to 170 T with the exception of the subtle kneelike bend around 90 T, which could indicate the 1/3 magnetization plateau. The combined analysis using the irreducible representations of the magnetic space groups and magnetic structure refinement on the neutron powder-diffraction data suggests that the magnetic moments order in the magnetic space-group $P{2}_{1}^{\ensuremath{'}}/{n}^{\ensuremath{'}}$ with the all-in--all-out spin structure, which by symmetry allows for the in-plane canting, consistent with the in-plane ferromagnetism observed in the magnetization.

We have conducted magnetization measurements of the $S = 1$ spin-ladder compound BIP-TENO using the single-turn coil method which generates micro-second pulsed magnetic field. We observed three magnetization plateaus in the magnetization process up to 100 T. We have also found that the magnetization process depends on the sweep speed of the magnetic fields. Possible origins of this phenomenon are (1) lowering temperature of the samples due to magnetocaloric effect with adiabatic condition realized in the fast (micro-second) pulsed magnetic field, and (2) spin-lattice separation due to a slow response of the magnetostriction whose response time is longer than microsecond range.

We carried out inelastic neutron scattering experiments on a buckled honeycomb lattice antiferromagnet Ba$_{2}$NiTeO$_{6}$ exhibiting a stripe structure at a low temperature. Magnetic excitations are observed in the energy range of $\hbar \omega \lesssim 10$ meV having an anisotropy gap of 2 meV at 2 K. We perform spin-wave calculations to identify the spin model. The obtained microscopic parameters are consistent with the location of the stripe structure in the classical phase diagram. Furthermore, the Weiss temperature independently estimated from a bulk magnetic susceptibility is consistent with the microscopic parameters. The results reveal that a competition between the NN and NNN interactions that together with a relatively large single ion magnetic anisotropy stabilize the stripe magnetic structure., Comment: 6 pages, 5 figures

We successfully synthesized single crystals of the verdazyl radical $2\text{\ensuremath{-}}\mathrm{Cl}\text{\ensuremath{-}}3,6\text{\ensuremath{-}}{\mathrm{F}}_{2}\text{\ensuremath{-}}\mathrm{V}$ [=3-(2-chloro-3,6-difluorophenyl)-1,5-diphenylverdazyl], which is a rare model compound with an $S=\frac{1}{2}$ Heisenberg antiferromagnetic (HAF) honeycomb lattice. Ab initio molecular orbital calculations indicate two dominant AF interactions, forming a slightly distorted honeycomb lattice. We explain the magnetic susceptibility and the magnetization curve up to the saturation field based on the expected spin model using the quantum Monte Carlo method. In the low-temperature regions, we found a phase transition to an AF ordered state at about 0.77 K for the zero field and obtained the magnetic field-temperature phase diagram from the magnetic susceptibility and the specific heat for various magnetic fields. Through the analysis considering the effect of lattice distortion on magnetic behavior, we confirm that the lattice distortion of the present model is small enough that it does not affect the intrinsic behavior of the uniform $S=\frac{1}{2}$ HAF honeycomb lattice.

We have succeeded in synthesizing a new organic biradical F4BIPBNN [= 2,2′-(3,3′,5,5′-tetrafluorobiphenyl-4,4′-diyl)bis(4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide)] which forms an S = 1=2 Heisenberg three-dimensional honeycomb antiferromagnet. Each site of a honeycomb layer alternately couples with upper or lower layers, which results in the unique three-dimensional honeycomb network with four nearest neighbors. At zero magnetic field, antiferromagnetic long-range order has been observed below TN = 2.7 K. Magnetic susceptibility in both paramagnetic and antiferromagnetic states and the magnetization curves are well reproduced by quantum Monte Carlo calculations with three antiferromagnetic interactions in the range of 4.3 to 6.6 K. From the concave shape of the magnetization curve, the shrinkage of spin due to spin fluctuations is evaluated to approximately 30% with respect to its classical value. The phase diagram of magnetic field versus temperature was determined by heat capacity and magnetization. In the field region below 3 T, a slight increase of TN was observed, which reflects the effect of spin fluctuations., Dr. A. Arauzo from General Services for Research of University of Zaragoza is acknowledged. JC and FP acknowledge grant number MAT2015-68200-C2-2-P. This work was partly supported by JSPS KAKENHI Grant Number JP15H03695 for YH. This work was performed in part under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University, and in Institute for Molecular Science, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

We present the first experimental realization of an $S=2$ ferromagnetic-antiferromagnetic (F-AF) alternating chain in a new Mn-verdazyl complex [Mn(hfac)$_2$]$\cdot$($o$-Py-V) [hfac=1,1,1,5,5,5-hexafluoroacetylacetonate; $o$-Py-V=3-(2-pyridyl)-1,5-diphenylverdazyl]. Through the $ab$ $initio$ molecular orbital calculation, magnetization, and ESR measurements, this compound is confirmed to form an $S=2$ F-AF alternating chain with Ising anisotropy below about 100 K. Furthermore, we find an anomalous change in magnetization at 1/4 of the saturation value, which is probably a manifestation of the quantum nature of the system.

Single crystals of a two-dimensional quantum spin system with geometric frustration lead to the observation of a ‘pinwheel’ valence-bond ground state. In this case, the distortion of the ideal kagome lattice structure helps to stabilize the quantum spin state. Determining ground states of correlated electron systems is fundamental to understanding unusual phenomena in condensed-matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states1, potentially prompting the emergence of quantum spin states, such as resonating valence bond2,3,4,5,6,7 and valence-bond solid8,9,10,11 (VBS). Although theoretically proposed to exist in a kagome lattice—one of the most highly frustrated lattices in two dimensions being comprised of corner-sharing triangles—such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the ‘pinwheel’ VBS ground state in the S=1/2 deformed kagome lattice antiferromagnet Rb2Cu3SnF12 (refs 12, 13). In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.

In magnetic field, KCuGaF6 can be mapped onto the quantum sine-Gordon model, for which low-energy elementary excitations are solitons, antisolitons and their bound states breathers. Using high-frequency, high-field ESR measurements, we observed these unusual magnetic excitations.

We investigate the magnetic order of the buckled honeycomb lattice antiferromagnet ${\mathrm{Ba}}_{2}{\mathrm{NiTeO}}_{6}$ and its related antiferromagnet ${\mathrm{Ba}}_{3}{\mathrm{NiTa}}_{2}{\mathrm{O}}_{9}$ by neutron diffraction measurements. We observe magnetic Bragg peaks below the transition temperatures, and identify propagation vectors for these oxides. A combination of representation analysis and Rietveld refinement leads to a collinear magnetic order for ${\mathrm{Ba}}_{2}{\mathrm{NiTeO}}_{6}$ and a ${120}^{\ensuremath{\circ}}$ structure for ${\mathrm{Ba}}_{3}{\mathrm{NiTa}}_{2}{\mathrm{O}}_{9}$. We find that the spin model of the bilayer triangular lattice is equivalent to that of the two-dimensional buckled honeycomb lattice having magnetic frustration. We discuss the magnetic interactions and single-ion anisotropy of $\mathrm{Ni}^{2+}$ ions for ${\mathrm{Ba}}_{2}{\mathrm{NiTeO}}_{6}$ in order to clarify the origin of the collinear magnetic structures. Our calculation suggests that the collinear magnetic order of ${\mathrm{Ba}}_{2}{\mathrm{NiTeO}}_{6}$ is induced by the magnetic frustration and easy-axis anisotropy.

TlCuCl 3 is composed of spin dimers and has a gapped singlet ground state. This system undergoes antiferromagnetic orderings under both external magnetic field and hydrostatic pressure, which are described as Bose–Einstein condensation and softening of triplet excitations. An overview of the magnetic quantum phase transitions in TlCuCl 3 is given.

Elementary excitations of the $S=\frac{1}{2}$ one-dimensional antiferromagnet ${\mathrm{KCuGaF}}_{6}$ were investigated by inelastic neutron scattering in zero and finite magnetic fields perpendicular to the $(1,1,0)$ plane combined with specific heat measurements. ${\mathrm{KCuGaF}}_{6}$ exhibits no long-range magnetic ordering down to 50 mK despite the large exchange interaction $J/{k}_{\mathrm{B}}=103$ K. At zero magnetic field, well-defined spinon excitations were observed. The energy of the des Cloizeaux and Pearson [J. des Cloizeaux and J. J. Pearson, Phys. Rev. 128, 2131 (1962)] mode of the spinon excitations is somewhat larger than that calculated with the above exchange constant. This discrepancy is mostly ascribed to the effective $XY$ anisotropy arising from the large Dzyaloshinsky-Moriya [T. Moriya, Phys. Rev. 120, 91 (1960)] interaction with an alternating $\mathbit{D}$ vector. ${\mathrm{KCuGaF}}_{6}$ in a magnetic field is represented by the quantum sine-Gordon model, for which low-energy elementary excitations are composed of solitons and antisolitons and their bound states called breathers. Unlike the theoretical prediction, it was found that the energy of a soliton is smaller than that of the first breather, although the energy of the first breather coincides with that observed in a previous electron spin resonance measurement.

Geometric frustration, in which competing interactions give rise to degenerate ground states, potentially induces various exotic quantum phenomena in magnetic materials. Minimal models comprising triangular units, such as triangular and Kagome lattices, have been investigated for decades to realize novel quantum phases, such as quantum spin liquid. A pentagon is the second-minimal elementary unit for geometric frustration. The realization of such systems is expected to provide a distinct platform for studying frustrated magnetism. Here, we present a spin-1/2 quantum pentagonal lattice in the new organic radical crystal α-2,6-Cl2-V [=α-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl]. Its unique molecular arrangement allows the formation of a partially corner-shared pentagonal lattice (PCPL). We find a clear 1/3 magnetization plateau and an anomalous change in magnetization in the vicinity of the saturation field, which originate from frustrated interactions in the PCPL.

The layered compound $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is composed of a honeycomb lattice of magnetic ${\mathrm{Ru}}^{3+}$ ions with the $4{d}^{5}$ electronic state. We have investigated the magnetic properties of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ via magnetization and specific heat measurements using single crystals. It was observed that $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ undergoes a structural phase transition at ${T}_{t}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}150$ K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related ${\mathrm{CrCl}}_{3}$. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic $g$ factors, i.e., ${g}_{ab}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}2.5$ and ${g}_{c}\phantom{\rule{0.16em}{0ex}}\ensuremath{\simeq}\phantom{\rule{0.16em}{0ex}}0.4$ for magnetic fields parallel and perpendicular to the $ab$ plane, respectively. These $g$ factors and the obtained entropy indicate that the effective spin of ${\mathrm{Ru}}^{3+}$ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}ab$ is obtained. We discuss the strongly anisotropic $g$ factors in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ and deduce that the exchange interaction is strongly XY-like. $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.

Magnetization, specific heat and X-ray measurements have been carried out on Cs 2 Cu 3 ZrF 12 which is a candidate of a two-dimensional S = 1/2 Heisenberg Kagome antiferromagnet. A structural phase transition was observed at T t ≃ 210 K. The hexagonal unit cell is enlarged to 2a, 2a, c below T t . A jump was observed in the susceptibility at T t . The magnetic susceptibility obeys the Curie-Weiss law with the Weiss constants Θ = -355 K and Θ = -300 K above and below T,, respectively. Weak ferromagnetic behavior was observed below T wf ≃ 30 K. However, specific heat exhibits no anomaly at T wf .

Quantum antiferromagnet NH 4 CuCl 3 shows magnetization plateaus at 1/4 and 3/4 of the saturation magnetization. Specific heat measurements at ambient pressure and magnetization measurements under hydrostatic pressure were performed on NH 4 CuCl 3 in magnetic fields. From specific heat measurements, we obtained the detailed magnetic phase diagram below 9 T, which includes the lower edge field H c1 of 1/4 plateau. The phase boundary just below H c1 can be described by a power low, H c1 - H N (T) T Φ with Φ = 2, where H N (T) is the transition field at temperature T. Magnetic susceptibility measurements under pressure revealed reentrant quantum phase transitions, such that the gapless ground state at zero field changes into gapped state with increasing pressure, and further into gapless state again.

An attempt to substitute Co 2+ , Mn 2+ for Pd 2+ was performed to yield dilute kagome lattice magnetism of S = 1/2 on a ternary transition metal chalocogenide Rb 2 Pd 3 S 4 . This compound takes a crystal structure, Pd sites of which constitute kagome lattice nets. Because the divalent Pd 2+ (4d) 8 's on the kagome lattice are in a low-spin state of spin-less owing to specific characteristics of the crystal field influencing on the Pd sites. Magnetic properties of specimens thus obtained were examined through magnetic susceptibility, ESR and specific heat measurements. Several data were obtained for confirming the syntheses of S = 1/2 systems in spite of dilute magnetic ones with substitutional ratio x's below the site percolation threshold x p = 0.62 of the kagome lattice.

Magnetization measurements under hydrostatic pressure were performed on S = 1/2 coupled dimer systems TlCuCl 3 and KCuCl 3 under magnetic field H parallel to the [2,0,1] direction. With increasing applied pressure P, the gap in TlCuCl 3 decreases and closes completely at P c = 0.42 ± 0.05 kbar. TlCuCl 3 undergoes pressure-induced quantum phase transition. The gap and Neel temperature are presented as functions of pressure. KCuCl 3 also undergoes pressure-induced quantum phase transition at P c ∼ 8.8 kbar. Pressure dependence of intra- and interdimer interactions in KCuCl 3 are presented.

The field induced magnetic phase transitions of Cs 2 CuBr 4 were investigated by means of magnetization process and neutron scattering experiments. Cs 2 CuBr 4 should be characterized as S = 1/2 two-dimensional triangular antiferromagnet. Below the ordering temperature T N = 1.4 K, the Spin structure is the helical incommensurate structure almost within the triangular lattice plane. In the field direction within the triangular lattice plane, Cs 2 CuBr 4 exhibits the magnetization plateaux at one-third and two-thirds of the saturation magnetization. The spin structure in the one-third plateau phase is found to be almost collinear up-up-down structure which should be stabilized by quantum fluctuation as predicted by the theoretical studies.

Magnetic measurements under high pressures have been performed on the hexagonal ABX 3 type antiferromagnets, CsFeCl 3 , CsNiCl 3 and RbNiCl 3 to investigate the pressure effect on the magnetic phase transitions. CsFeCl 3 has the singlet ground state due to the large single-ion anisotropy of the easy-plane type at ambient pressure. Above the critical pressure P c ≃ 0.9 GPa, CsFeCl 3 exhibits the long range magnetic ordering at zero field as the result of the collapse of the energy gap between the singlet state and the lowest excited doublet. CsNiCl 3 and RbNiCl 3 having the easy-axis type magnetic anisotropy undergo the successive phase transitions in the temperature variations and exhibit the spin-flop phase transition in the field parallel to the c-axis. With increasing pressure, the temperature range of the collinear intermediate phase and the spin-flop field H SF are enhanced. This fact indicates that the easy-axis anisotropies of CsNiCl 3 and RbNiCl 3 are enhanced by the pressure as compared with the increase of the intra- and inter-chain interactions.

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.

We have measured the magnetization of the organic compound BIP-BNO (3,5'-bis(N-tert-butylaminoxyl)-3',5-dibromobiphenyl) up to 76 T where the magnetization is saturated. The S = 1/2 antiferromagnetic Heisenberg two-leg spin-ladder model accounts for the obtained experimental data regarding the magnetization curve, which is clarified using the quantum Monte Carlo method. The exchange constants on the rung and the side rail of the ladder are estimated to be J(rung)/kB = 65.7 K and J(leg)/kB = 14.1 K, respectively, deeply in the strong coupling region: J(rung)/J(leg) > 1., 8 pages, 2 figures

An antiferromagnetic system on a 2-D triangular lattice leads to geometric topological frustration. This ideal system has been the subject of theoretical investigations. One experimental realization of this system is the compound Cs2CuCl4. Various magnetization, heat capacity, neutron scattering and NMR studies have identified several magnetic transitions when the magnetic field is applied along one of the three principal axes. The current work investigates the evolution of these phases at intermediate angles as the crystal is rotated relative to the magnetic field. These phases were investigated using a novel rotating calorimeter allowing complete coverage of the experimental parameter space. New magnetic phases only existing at intermediate angles have been found.

We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung S=1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T1. 19F-NMR spectra for higher- and lower-temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings., 12 pages, 3 figures

Elastic neutron scattering experiments were carried out to investigate the magnetic structure and the critical behaviour of the mixed triangular antiferromagnetic system CsMn(Br0.19I0.81)3, for which our previous magnetic measurement revealed the macroscopic anisotropy to be negligible. It is found that in the ordered state below TN = 8.29 K, the spin planes in which the spins lie with the 120° structure are tilted from the basal plane, and that the average of the tilting angle is evaluated as cos2 = 0.362, i.e. = 53°. This result suggests that the present system is almost at the Heisenberg limit for the interaction. The critical exponent for the sublattice magnetization is obtained as = 0.28±0.02, which is in agreement with the value predicted for the chiral Heisenberg universality class.

Magnetic phase transitions under hydrostatic pressures in spin gap systems TlCu 0.988 Mg 0.012 Cl 3 and KCu 0.973 Mg 0.027 Cl 3 were investigated by magnetization measurements. The present doped systems exhibit impurity-induced magnetic orderings. With increasing pressure, ordering temperature T N increases. With a further increase in pressure, the present systems undergo phase transitions to uniform antiferromagnetic phases due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at P ≃ 1.3 kbar for TlCu 0.988 Mg 0.012 Cl 3 .

Magnetization measurements were performed on TlCuCl 3 with gapped ground state. The critical density and the magnetic phase diagram were obtained. The interacting constant was obtained as U / k B = 313 K . The experimental phase boundary for T 5 K agrees perfectly with the magnon BEC theory based on the Hartree–Fock approximation with realistic dispersion relations and U / k B = 320 K . The exponent φ obtained with all the data points for T 5 K is φ = 1.99 , which is somewhat larger than theoretical exponent φ BEC = 3 2 . However, it was found that the exponent converges at φ BEC = 3 2 with decreasing fitting window.

TlCuCl 3 is a spin- 1 2 magnetic insulator with a spin gap Δ / 2 μ B = 5.5 T . Its magnetic properties are described by a three-dimensionally coupled spin dimer model. Tl ( Cu 1 - x Mg x ) Cl 3 doped with nonmagnetic Mg 2 + undergoes impurity-induced magnetic ordering. Because a finite gap for triplet excitations still remains, this doped system can also undergo magnetic-field-induced magnetic ordering. In order to investigate the phase diagram of TlCu 1 - x Mg x Cl 3 with x ∼ 0.01 , we performed specific heat measurements and neutron scattering experiments in a magnetic field. It was found that impurity- and field-induced ordered phases are the same, and that the gapped spin liquid state observed in pure TlCuCl 3 is completely wiped out by such a weak doping.

High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $

We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3- (3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [= 3-(3-bromo-4-fluorophenyl)-1,5- diphenylverdazyl]. Their crystal strucutures are found to be isomorphous to that of previously reported spin ladder 3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements and magnetic properties, we confirm that these materials form ferromagnetic chain-based spin ladders with slightly modulated magnetic interactions. These results present the first quantitative demonstration of the fine-tuning of magnetic interactions in the molecular- based materials., Comment: 11 pages, 4 figures, to appear in J. Phys. Soc. Jpn

We have succeeded in synthesizing single crystals of an organic radical $p$-Br-V [3-(4-bromophenyl)-1,5-diphenylverdazyl]. The dominant intermolecular interactions forming a one-dimensional spin chain are deduced from the molecular packing in the crystal, and the ab initio molecular orbital calculation also indicates such an exchange network. We analyzed the magnetic susceptibility, magnetization curve, and magnetic specific heat through the quantum Monte Carlo method and successfully explained them as being attributed to an $S=\frac{1}{2}$ Heisenberg antiferromagnetic chain. Furthermore, we have found an unexpected successive phase transition in the specific heats, which indicates the formation of a noncollinear magnetic structure induced by frustration.

We have succeeded in synthesizing single crystals of the verdazyl radical $\ensuremath{\beta}$-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl. The $ab$ $initio$ molecular orbital calculation indicates the formation of an $S=1/2$ Heisenberg antiferromagnetic chain with fourfold magnetic periodicity consisting of three types of exchange interactions. We successfully explain the magnetic and thermodynamic properties based on the expected spin model by using the quantum Monte Carlo method. Furthermore, we reveal that the alternating and unique Ising ferromagnetic chains become effective in the specific field regions and observe a cooperative phenomenon caused by the magnetic order and quantum fluctuations. These results demonstrate that the verdazyl radical could form an unconventional spin model with interesting quantum behavior and provide a way to study a variety of quantum spin systems.

Magnetic excitations in the spin-$\frac{1}{2}$ distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$, which has an ordered ground state owing to the strong Dzyaloshinskii-Moriya interaction, were studied using inelastic neutron scattering. Although the spin-wave dispersion can be qualitatively understood in terms of linear spin-wave theory (LSWT), the excitation energies are renormalized by a factor of approximately 0.6 from those calculated by LSWT, almost irrespective of the momentum transfer. This inadequacy of LSWT, which is attributed to quantum fluctuations, provides evidence of negative quantum renormalization in the spin-$\frac{1}{2}$ kagome lattice antiferromagnet., 8 pages, 6 figures, including Supplemental Materials

We have succeeded in synthesizing single crystals of a new organic radical 3-Cl-4-F-V [3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl]. Through the ab initio molecular orbital calculation and the analysis of the magnetic properties, this compound was confirmed to be the first experimental realization of an $S=1/2$ spin-ladder system with ferromagnetic leg interactions. The field-temperature phase diagram indicated that the ground state is situated very close to the quantum critical point. Furthermore, we found an unexpected field-induced successive phase transition, which possibly originates from the interplay of low dimensionality and frustration.

We successfully synthesized verdazyl biradical crystals of $p$-BIP-V${}_{2}$ [4,4${}^{\ensuremath{'}}$-bis(1,5-diphenylverdazyl-3-yl)biphenyl]. Two types of intermolecular interactions are deduced from the spin-density distribution and molecular packing in the crystal, and these interactions are considered to result in the formation of a honeycomb lattice. The $\mathit{ab}$ $\mathit{initio}$ molecular orbital calculation also indicated such an exchange network of intermolecular interactions. The intramolecular interaction acts as a diagonal bridge across the honeycomb lattice to form a square lattice. We evaluated the intramolecular antiferromagnetic (AF) interaction ${J}_{1}$ to be about 8.8 K from an analysis of the magnetic susceptibility of isolated $p$-BIP-V${}_{2}$ molecules in terms of an isolated $S$ $=$ 1/2 AF dimer. We analyzed the magnetic susceptibility of the crystals using the quantum Monte Carlo method. Consequently, two types of intermolecular AF interactions, ${J}_{2}$ and ${J}_{3}$, were found to be almost comparable and about 4.5 times larger than the intramolecular interaction ${J}_{1}$. Thus, we confirmed that $p$-BIP-V${}_{2}$ has a slightly distorted $S=1/2$ honeycomb-like lattice.

The magnetic properties of the S =1/2 double spin chain system KCuCl 3 have been investigated by susceptibility measurements using single crystals. It is found that the susceptibilities for three different external field directions exhibit broad maxima at around 30 K and decrease exponentially to zero with decreasing temperature. This result indicates that the ground state of KCuCl 3 is nonmagnetic and there is a finite gap in its excitation spectrum. This behavior is attributable to the quantum effect characteristic of the double spin chain. The structure of the exchange couplings in the double chain is discussed. Applying the recent spin ladder theory, we estimate the magnitude of the excitation gap to be Δ/ k B =35 K.

The magnetic properties of Cs${}_{2}$Cu${}_{3}$CeF${}_{12}$ were investigated through magnetization and specific heat measurements. Cs${}_{2}$Cu${}_{3}$CeF${}_{12}$ is composed of a buckled kagome lattice of Cu${}^{2+}$, which is stacked along the $b$ axis. The exchange network in the buckled kagome lattice is strongly anisotropic. Consequently, Cs${}_{2}$Cu${}_{3}$CeF${}_{12}$ can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. The dangling spins couple with one another via effective exchange interactions, which are mediated by chain spins. The dangling spins are further divided into two subsystems, DS1 and DS2. The dangling spins in DS1 undergo three-dimensional ferromagnetic ordering at 3.14 K, while those in DS2 remain paramagnetic down to 0.35 K. The effective interaction between the DS1 spins is approximately expressed by the ferromagnetic $XXZ$ model with the $z$ direction parallel to the crystallographic $c$ axis. A magnetic phase diagram for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}c$ was obtained and was analyzed within the framework of the molecular field approximation. With increasing magnetic field, the dangling spins are polarized and the magnetization curve exhibits a wide plateau at one-third of the saturation magnetization.

Cs2CuBr4 and Ba3NiSb2O9 are magnetically described as quasi-two-dimensional triangular-lattice antiferromagnets with spin-½ and 1, respectively. We show that both systems exhibit a magnetization plateau at one-third of the saturation magnetization Ms due to the interplay of spin frustration and quantum fluctuation. In Cs2CuBr4 that has a spatially anisotropic triangular lattice, successive magnetic-field induced quantum phase transitions including a magnetization plateau were observed. For Ba3NiSb2O9, we performed exact diagonalization for rhombic spin clusters with up to 21-sites to analyze the magnetization process. The calculated results are in agreement with experimental observations.

Showing low-temperature specific heat and other experimental data and also on the basis of established physics, we argue against the comment made by Zheludev and H\"uvonnen critiquing our recent study on the magnetic-field-induced spin ordering and critical behavior in Tl${}_{1\ensuremath{-}x}$K${}_{x}$CuCl${}_{3}$, which is described as the Bose glass-condensate transition of triplons.

We report the results of magnetization and specific heat measurements on Ba$_3$NiSb$_2$O$_9$, which is a quasi-two-dimensional spin-1 triangular-lattice antiferromagnet. We observed a nonclassical magnetization plateau at one-third of the saturation magnetization that is driven by spin frustration and quantum fluctuation. Exact diagonalization for a 21-site rhombic cluster was performed to analyze the magnetization process. Experimental and calculated results agree well., published in Journal of the Physical Society of Japan 80 (2011) 093702

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0x1. Based on dc-magnetic-susceptibility data, combined with results of 59 Co nuclear magnetic resonance NMR and nuclear quadrupole resonance NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x=0.35 and 0.40, which separates the Pauli-paramagnetic and Curie-Weiss metals. In the Pauli-paramagnetic regime x0.35, the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x=0 end member is a noncorrelated metal in the whole temperature range studied. In the Curie-Weiss regime x0.40, on the other hand, various phase transitions are observed. For x=0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static antiferromagnetic order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt175 K in samples with x=0.50 =1/2 and 0.67 =2/3, while only a tiny kink appears at T210 K in the sample with an intermediate Li composition, x=0.60. Thus, the phase diagram of the LixCoO2 system is complex and the electronic properties are sensitively influenced by the Li content x.

We report the crystal structure and unconventional magnetic ordering of ${\text{Cs}}_{2}{\text{Cu}}_{3}{\text{CeF}}_{12}$, which is composed of buckled kagome lattice of ${\text{Cu}}^{2+}$ ions. The exchange network in the buckled kagome lattice is fairly anisotropic, so that the present spin system can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. Although the direct exchange interactions between neighboring spins were found to be all antiferromagnetic, ferromagnetic magnetic ordering of the dangling spins was observed. Magnetization exhibits a plateau at one-third of the saturation magnetization. These observations can be understood in terms of the indirect interaction between dangling spins mediated by the chain spin.

CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was widely believed to be a Mott insulator. We performed 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.

Hexagonal antiferromagnets ${\text{Cs}}_{2}{\text{Cu}}_{3}M{\text{F}}_{12}$ ($M=\text{Zr}$, Hf, and Sn) have uniform kagome lattices of ${\text{Cu}}^{2+}$ with $S=1/2$, whereas ${\text{Rb}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$ has a $2a\ifmmode\times\else\texttimes\fi{}2a$ enlarged cell as compared to the uniform kagome lattice. The crystal data of ${\text{Cs}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$ synthesized in the present work are reported. We performed magnetic-susceptibility measurements on this family of kagome antiferromagnet using single crystals. In the ${\text{Cs}}_{2}{\text{Cu}}_{3}M{\text{F}}_{12}$ systems, structural phase transitions were observed at ${T}_{t}=225$, 172, and 185 K for $M=\text{Zr}$, Hf, and Sn, respectively. The magnetic susceptibilities observed for $Tg{T}_{t}$ are almost perfectly described using theoretical results obtained by exact diagonalization for the 24-site kagome cluster with $J/{k}_{B}=244$, 266, and 240 K, respectively. Magnetic ordering accompanied by the weak ferromagnetic moment occurs at ${T}_{N}=23.5$, 24.5, and 20.0 K, respectively. The origins of the weak ferromagnetic moment should be ascribed to the lattice distortion that breaks the hexagonal symmetry of the exchange network for $Tl{T}_{t}$ and the Dzyaloshinsky-Moriya interaction. ${\text{Rb}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$ is magnetically described as a modified kagome antiferromagnet with four types of neighboring exchange interactions. Neither structural nor magnetic phase transition was observed in ${\text{Rb}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$. Its magnetic ground state was found to be a spin singlet with a triplet gap. Using exact diagonalization for a 12-site kagome cluster, we analyzed the magnetic susceptibility and evaluated individual exchange interactions. The causes leading to the different ground states in ${\text{Cs}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$ and ${\text{Rb}}_{2}{\text{Cu}}_{3}{\text{SnF}}_{12}$ are discussed.

Elementary excitations of the $S=\frac{1}{2}$ one-dimensional Heisenberg antiferromagnet ${\text{KCuGaF}}_{6}$ with exchange constant $J/{k}_{B}=103\text{ }\text{K}$ were investigated by high-frequency electron spin resonance (ESR) measurements combined with a pulsed high magnetic field. When an external magnetic field $H$ is applied in ${\text{KCuGaF}}_{6}$, a staggered magnetic field $h$ is induced perpendicular to $H$ owing to the staggered $\mathbit{g}$ tensor and the Dzyaloshinsky-Moriya (DM) interaction with an alternating $\mathbit{D}$ vector. Consequently, ${\text{KCuGaF}}_{6}$ in a magnetic field is represented by the quantum sine-Gordon (SG) model. We observed many resonance modes including a soliton resonance, breathers, interbreather transitions, and two-breather excitation. Their resonance conditions are beautifully described by the quantum SG field theory with one adjustable parameter ${c}_{s}=h/H$. To investigate the relationship between the Curie term due to the DM interaction and the proportionality coefficient ${c}_{s}$, magnetic susceptibility measurements were also performed varying the external field direction.

Natural mineral Dioptase (Cu6Si6O18 ⋅ 6H2O) is an S = 1/2 antiferromagnet with a unique spin network which consists of helical chains along the c axis connected by inter chain exchange interactions. In order to study the ground state of dioptase, we have performed the magnetization measurement for B//c (easy axis) at 1.5 K and have found the spin-flop transition at 13 T. Moreover, the pressure effect of the magnetic susceptibility is investigated up to 1.78 GPa using the SQUID magnetometer where TN increased while the broad maximum reflecting the low dimensionality of the system decreased by pressure. The results will be discussed in connection with our previous antiferromagnetic resonance (AFMR) measurement results and the phase diagram obtained by Gros et al.