Pressure-tuned quantum criticality in the large-D antiferromagnet DTN.

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl2 ⋅ 4SC(NH2)2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

Field-angle-dependent multi-frequency electron spin resonance spectroscopy in submillimeter wave range based on thermal detection.

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 1012 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.

Spin-Crossover-Triggered Linkage Isomerization by the Pedal-like Motion of the Azobenzene Ligand in a Neutral Heteroleptic Iron(III) Complex.

The temperature dependence of magnetic susceptibility of [FeIII(azp)(qsal-Me)]·0.5CH3OH [Hqsal-Me = 5-methyl-N-(8-quinoyl)salicylaldimine, H2azp = 2,2'-azobisphenol] demonstrated that the spin-crossover (SCO) transition behavior changed from an abrupt transition to consecutive gradual conversions, and moreover, the initial abrupt transition was recovered, keeping the complex at room temperature. The variable-temperature crystal structures revealed that an SCO-triggered linkage isomerization of the azobenzene ligand from one orientation to two disordered orientations and the relaxation from the disordered orientations to the original orientation occurred. The high-spin to low-spin relaxation kinetics and theoretical calculation indicate that the pedal-like motion of the azobenzene ligand can be on in the high-spin state whereas off in the low-spin state.

Continuous control of classical-quantum crossover by external high pressure in the coupled chain compound CsCuCl3.

In solid materials, the parameters relevant to quantum effects, such as the spin quantum number, are basically determined and fixed at the chemical synthesis, which makes it challenging to control the amount of quantum correlations. We propose and demonstrate a method for active control of the classical-quantum crossover in magnetic insulators by applying external pressure. As a concrete example, we perform high-field, high-pressure measurements on CsCuCl3, which has the structure of weakly-coupled spin chains. The magnetization process experiences a continuous evolution from the semi-classical realm to the highly-quantum regime with increasing pressure. Based on the idea of "squashing" the spin chains onto a plane, we characterize the change in the quantum correlations by the change in the value of the local spin quantum number of an effective two-dimensional model. This opens a way to access the tunable classical-quantum crossover of two-dimensional spin systems by using alternative systems of coupled-chain compounds.

The Impact of the Next-Nearest Neighbor Dispersion Interactions on Spin Crossover Transition Enthalpy Evidenced by Experimental and Computational Analyses of Neutral π-Extended Heteroleptic Fe(III) Complexes.

A neutral heteroleptic Fe(III) complex 1 derived from a π-extension of the parent complex 2 was prepared and characterized. Complex 1 exhibited an abrupt spin crossover (SCO) transition exactly at room temperature (TSCO = 298 K). A crystal structure analysis of 1 revealed that the Fe(III) complex molecules formed a three-dimensional π-stacking interaction network. To thermodynamically clarify the mechanism of the SCO transition, the thermodynamic parameters of the SCO transitions for 1 and 2 were deduced from the temperature dependence of the magnetic susceptibility in the solid and solution states using the regular solution model. A comparison of the SCO enthalpy difference between the solid and molecule for 1 and 2 revealed that the lattice enthalpy difference would largely contribute to the SCO transition enthalpy difference. A computational evaluation of intermolecular interactions and lattice energies before and after the SCO transitions in 1 and 2 disclosed the significant contribution of the next-nearest neighbor dispersion interactions to the lattice enthalpy differences. This finding indicates that not only conventional nearest neighbor intermolecular interactions but also next-nearest neighbor dispersion interactions should be taken into account to understand the fundamental mechanism of a phase transition in molecular solids.

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn_{3}P.

Antiferromagnet Mn_{3}P with Neel temperature T_{N}=30 K is composed of Mn tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at ∼1.6 GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn_{3}P, which is most likely effected by the underlying frustration effect.

Force-detected high-frequency electron spin resonance spectroscopy using magnet-mounted nanomembrane: Robust detection of thermal magnetization modulation.

In this study, we report a conceptually novel broadband high-frequency electron spin resonance (HFESR) spectroscopic technique. In contrast to the ordinary force-detected electron spin resonance (ESR) technique, which detects the magnetization change due to the saturation effect, this method measures the magnetization change due to the change of the sample temperature at resonance. To demonstrate its principle, we developed a silicon nitride nanomembrane-based force-detected ESR spectrometer, which can be stably operated even at high magnetic fields. Test measurements were performed for samples with different spin relaxation times. We succeeded in obtaining a seamless ESR spectrum in magnetic fields of 15 T and frequencies of 636 GHz without significant spectral distortion. A high spin sensitivity of 1012 spins/G s was obtained, which was independent of the spin relaxation time. These results show that this technique can be used as a practical method in research fields where the HFESR technique is applicable.

Pressure Effect on Zero-Field Splitting Parameter of Hemin: Model Case of Hemoproteins under Pressure.

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.

Note: Force- and torque-detection of high frequency electron spin resonance using a membrane-type surface-stress sensor.

We developed a practical useful method for force- and torque-detected electron spin resonance (FDESR/TDESR) spectroscopy in the millimeter wave frequency region. This method uses a commercially available membrane-type surface-stress (MSS) sensor. The MSS is composed of a silicon membrane supported by four beams in which piezoresistive paths are integrated for detecting the deformation of the membrane. Although this device has a lower spin sensitivity than a microcantilever, it offers several distinct advantages, including mechanical strength, ease of use, and versatility. These advantages make this device suitable for practical applications that require FDESR/TDESR.

Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged FeII Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine.

A series of [FeII(L)2](BF4)2 compounds were structurally and physically characterized (L = 2,6-bis(2-methylthiazol-4-yl)pyridine). A crystal structure phase transformation from dihydrate compound 1 to anhydrous compound 3 through partially hydrated compounds 2 and 2' upon dehydration was found. Compounds 1 and 3 exhibited a gradual spin crossover (SCO) conversion, whereas compounds 2 and 2' demonstrated two-step and one-step abrupt SCO transitions, respectively. An X-ray single-crystal structural analysis revealed that one-dimensional and two-dimensional Fe cation networks linked by π stacking and sulfur-sulfur interactions were formed in 1 and 3, respectively. A thermodynamic analysis of the magnetic susceptibility for 1, 2', and 3 suggests that the enthalpy differences may govern SCO transition behaviors in the polymorphic compounds 2' and 3. A structural comparison between 1 and 3 indicates that the SCO behavior variations and crystal structure transformation in the present [FeII(L)2](BF4)2 compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF4 anions as in typical ionic crystals.

Paramagnetic ionic plastic crystals containing the octamethylferrocenium cation: counteranion dependence of phase transitions and crystal structures.

In recent years, ionic plastic crystals have attracted much attention. Many metallocenium salts exhibit plastic phases, but factors affecting their phase transitions are yet to be elucidated. To investigate these factors, we synthesized octamethylferrocenium salts with various counteranions [Fe(C5Me4H)2]X ([1]X; X- = B(CN)4-, C(CN)3-, N(CN)2-, FSA (= (SO2F)2N-), FeCl4-, GaCl4- and CPFSA (= CF2(SO2CF2)2N-)) and elucidated their crystal structures and phase behavior. Correlations between the crystal structures and phase sequences, and the shapes and volumes of the anions are discussed. Except for [1][CPFSA], these salts exhibit phase transitions to plastic phases at or above room temperature (TC = 298-386 K), and the plastic phases exhibit either NaCl- or anti-NiAs-type structures. X-ray crystal structure analyses of these salts at 100 K revealed that they have structures in which cations and anions are alternately arranged, with the exception of [1][CPFSA]. [1][CPFSA] exhibits a structure in which anions and cations are separately stacked to form columns. [1][N(CN)2] exhibits a polar crystal structure that undergoes a monotropic phase transition to a centrosymmetric structure. The magnetic susceptibilities of room-temperature plastic crystals [1][GaCl4] and [1][FeCl4] were investigated; the latter exhibits a small ferromagnetic interaction at low temperatures.

Mechanically detected terahertz electron spin resonance using SOI-based thin piezoresistive microcantilevers.

We developed piezoresistive microcantilevers for mechanically detected electron spin resonance (ESR) in the millimeter-wave region. In this article, fabrication process and device characterization of our self-sensing microcantilevers are presented. High-frequency ESR measurements of a microcrystal of paramagnetic sample is also demonstrated at multiple frequencies up to 160 GHz at liquid helium temperature. Our fabrication is based on relatively simplified processes with silicon-on-insulator (SOI) wafers and spin-on diffusion doping, thus enabling cost-effective and time-saving cantilever fabrication.

Electron magnetic resonance data on high-spin Mn(III; S=2) ions in porphyrinic and salen complexes modeled by microscopic spin Hamiltonian approach.

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.

Cooperative spin-crossover transition from three-dimensional purely π-stacking interactions in a neutral heteroleptic azobisphenolate FeIII complex with a N3O3 coordination sphere.

A novel neutral heteroleptic FeIII complex from two kinds of π-extended tridentate ligands was designed and prepared. The π-ligands formed a three-dimensional purely π-stacking interaction network. The present complex proved to be the first neutral spin-crossover (SCO) FeIII complex with a N3O3 coordination sphere exhibiting an abrupt SCO transition with a thermal hysteresis of 10 K and the light-induced excited spin-state trapping effect.

Thermochromic Magnetic Ionic Liquids from Cationic Nickel(II) Complexes Exhibiting Intramolecular Coordination Equilibrium.

Among the various thermochromic materials, liquid thermochromic materials are comparatively rare. To produce functional thermochromic liquids, we have designed ionic liquids based on cationic nickel complexes with ether side chains, [Ni(acac)(Me2 NC2 H4 NR1 R2 )]Tf2 N ([1]Tf2 N: R1 =C3 H6 OEt, R2 =Me; [2]Tf2 N: R1 =C3 H6 OMe, R2 =Me; [3]Tf2 N: R1 =R2 =C3 H6 OMe), where acac=acetylacetonate and Tf2 N=(F3 CSO2 )2 N- . The side chains (R1 , R2 ) can moderately coordinate to the metal center, enabling temperature-dependent coordination equilibria in the liquid state. [1]Tf2 N is a liquid at room temperature. [2]Tf2 N is obtained as a solid (Tm =352.7 K) but remains liquid at room temperature after melting. [3]Tf2 N is a solid with a high melting point (Tm =422.3 K). These salts display thermochromism in the liquid state, appearing red at high temperatures and orange, light-blue, or bluish-green at lower temperatures, and exhibiting concomitant changes in their magnetic properties. This phenomenon is based on temperature-dependent equilibrium between a square-planar diamagnetic species and a paramagnetic species with intramolecular ether coordination.

Cantilever detected ferromagnetic resonance in thin Fe50Ni50, Co2FeAl0.5Si0.5 and Sr2FeMoO6 films using a double modulation technique.

In this work we introduce a new method, which employs commercial piezo-cantilevers, for a ferromagnetic resonance (FMR) detection from thin, nm-size, films. Our setup has an option to rotate the sample in the magnetic field and it operates up to the high microwave frequencies of 160GHz. Using our cantilever based FMR spectrometer we have investigated a set of samples, namely quasi-bulk and 84nm film Co2FeAl0.5Si0.5 samples, 16nm Fe50Ni50 film and 150nm Sr2FeMoO6 film. Low frequency and room temperature test of our setup using 84nm Co2FeAl0.5Si0.5 film yielded a result identical to a standard X-Band spectrometer, namely a single line with quite small linewidth. Our measurements at low temperatures and high frequencies revealed a quite strong FMR response detected in all samples. The FMR spectra share common features, such as the emergence of the second line with an opposite angular dependence, and a drastic increase of the linewidths with increasing microwave frequency. We believe that these findings are results of the complicated dynamics of the magnetization at low temperatures and high frequencies, which we were able to probe using our cantilever based FMR setup.

High-frequency electron paramagnetic resonance of metal-containing porphyrin compounds using a microcantilever.

In this article, we report a novel technique of high-frequency electron paramagnetic resonance (HFEPR) using a microcantilever. In this method, a sample is mounted on a cantilever, and the field-gradient force associated with EPR absorption is detected as a cantilever bending. By using a micrometer-sized cantilever, this technique can be applied to a very tiny sample on the order of µg. In addition, the use of a piezoresistive cantilever makes the experimental setup easy and compact. In this study, we applied this technique to multi-frequency HFEPR measurements of metal-containing porphyrin compounds, which are an important composing element of metal-containing proteins and coenzymes such as hemoglobin and cyanocobalamin.

A New Family of Anionic Fe(III) Spin Crossover Complexes Featuring a Weak-Field N2O4 Coordination Octahedron.

Unprecedented anionic Fe(III) spin crossover (SCO) complexes involving a weak-field O,N,O-tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO-1 of a ligand might contribute to the enhancement of a ligand-field splitting energy. The present anionic SCO complex also exhibited the light- induced excited-spin-state trapping effect.

Neurological and neuropsychological functions in adults with a history of developmental arsenic poisoning from contaminated milk powder.

During the summer of 1955, mass arsenic poisoning of bottle-fed infants occurred in the western part of Japan due to contaminated milk powder, and more than 100 died; some childhood victims were later found to suffer from neurological sequelae in adolescence. This unique incident enabled us to explore infancy as a critical period of arsenic exposure in regard to developmental neurotoxicity and its possible persistence through adulthood. The purpose of this work is to evaluate the association between developmental arsenic exposure and the neurological outcomes more than 50 years later. We conducted a retrospective cohort study during the period from April 2012 to February 2013 in two hospitals in Okayama Prefecture, Japan. The study sample consisted of 50 individuals: 27 known poisoning victims from Okayama Prefecture, and 23 non-exposed local controls of similar age. In addition to neurological examination, we adapted a battery of neurophysiological and neuropsychological tests to identify the types of brain functions affected by early-life arsenic exposure. While limited abnormalities were found in the neurophysiological tests, neuropsychological deficits were observed. Except for Finger tapping, all test scores in the exposed group--Vocabulary and Block Design from Wechsler Adults Intelligent Scale III, Design memory subtest from Wide Range Assessment of Memory and Learning 2, and Grooved pegboard test--were substantially below those obtained by the unexposed. The exposed group showed average performance at least 1.2 standard deviations below the average for the controls. Exposed participants performed less well than controls, even after exclusion of subjects with recognized disabilities or those with a high level of education. Adults who had suffered arsenic poisoning during infancy revealed neuropsychological dysfunctions, even among those subjects not recognized as having disabilities. Developmental neurotoxicity due to arsenic likely results in permanent changes in brain functions.

Frequency Extension to the THz Range in the High Pressure ESR System and Its Application to the Shastry-Sutherland Model Compound SrCu2(BO3)2.

We have made a survey of ceramics for the inner parts of the transmission-type pressure cell to achieve the high pressure and the high transmission in the THz range. By using the optimal combination of ZrO2-based ceramic and Al2O3 ceramic, we have succeeded in obtaining a pressure up to 1.5 GPa and a frequency region up to 700 GHz simultaneously. We show the high-pressure ESR results of the Shastry-Sutherland compound SrCu2(BO3)2 as an application. We observed the direct ESR transition modes between the singlet ground state and the triplet excited states up to a pressure of 1.51 GPa successfully, and obtained the precise pressure dependence of the gap energy. The gap energy is directly proved to be suppressed by the pressure. Moreover, we found that the system approaches the quantum critical point with pressure by comparing the obtained data with the theory. This result also shows the usefulness of high-pressure ESR measurement in the THz region to study quantum spin systems.