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Samarium (Sm) has antiferromagnetic (AFM) ordering at the hexagonal site at ${T}_{\mathrm{N}}(\text{hex})=106$ K, and at the cubic site at ${T}_{\mathrm{N}}(\text{cub})=14$ K at ambient pressure. The structural transition from the so-called Sm-type structure to the double hexagonal close-packed (dhcp) structure occurs at approximately 6 GPa. According to electrical resistance measurements, the shift of ${T}_{\mathrm{N}}$(hex) toward the low-temperature side occurs simultaneously with the shifting of ${T}_{\mathrm{N}}$(cub) toward the high-temperature side with increasing pressure. We conducted dc magnetic measurements on Sm at high pressure up to 30 GPa using a SQUID-based vibrating-coil-magnetometer to pursue ${T}_{\mathrm{N}}$(cub) at high pressure: The magnetic measurements revealed stepwise anomalies below 2 GPa. A ferromagnetic (FM) anomaly was observed in the dhcp structure, suggesting that spins at both structural sites formed the FM magnetization. In the face-centered cubic (fcc) structure at above 12 GPa, a reduction in the magnetic signal occurred. In the distorted fcc structure at above 20 GPa, sufficient suppression of the FM moment was observed and afterward the diamagnetic signal suggesting the possibility of superconductivity was observed.

In fifth-group element superconductors V, Nb, and Ta, the increase in superconducting transition temperature (Tc) was attempted by using both high-pressure torsion (HPT) and additional hydrostatic pressure (HP) compression. The former brings about the grain refinement and strain accumulation in the unit-cell level. The additional compression for severely strained superconductors triggers strengthening intergrain-contact and/or structural deformation in the unit-cell level. The manner of the appearance of the above two effects depends on the kind of elements: First, in V, there is no prominent effect of HPT, comparing to the hydrostatic compression effects on its non-strained material. Next, in Ta, the effect of strengthening intergrain-contact appears at small hydrostatic compression, resulting in temporal increase in Tc. Finally, Nb exhibits prominent increase in Tc by both effects and, in particular, the structural deformation in the unit-cell level promotes the increase in Tc. Thus, the accumulation of residual strain in the level of starting material can be a promising work to manipulate Tc under HP compression.

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations.

We investigated transport and magnetic properties of single crystal ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ under hydrostatic pressures up to 50 GPa. The ambient pressure superconductivity at ${T}_{c}\ensuremath{\sim}$ 45.4 K was fully suppressed at ${P}_{c}\ensuremath{\sim}21$ GPa. Upon a further increase of pressure, ferromagnetism associated with the order of the rare-earth subsystem was induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M(H$) loops and the strong increase in the field cooled data $M(T$) toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi liquid to non-Fermi liquid to Fermi liquid. The Hall measurements suggest that the multiband electronic structures have changed with pressure, which should also affect the resistivity behavior. These results give access to the high-pressure side of the superconducting phase diagram in the 1111 type of materials.

The magnetic memory phenomena in a ceramic YBCO superconductor are studied by zero-field-cooled and thermoremanent magnetization after specific cooling protocols. The ceramic YBCO is considered as random Josephson-coupled networks of 0 and π junctions and shows successive phase transitions. The magnetic glass behavior, similar to those of spin-glasses, is observed below the lower transition temperature. The memory effect occurs strongly in an intermediate stop temperature and suppressed with an increased probing field. The results can be understood by the screening effect of the ceramic superconductor with a finite self- inductance.

The hydrostatic pressure response of TC of the bisdiselenazolyl radical ferromagnet 1 up to 5 GPa was investigated by AC magnetic susceptibility measurements using a SQUID magnetometer and a miniature diamond anvil cell. It was found that the ambient pressure value of TC = 17 K could be raised to 21 K at a pressure of 0.9 GPa. The experimental technique is described in detail and the pressure response is compared to that observed in related systems.

We report the hydrothermal synthesis, X-ray crystal structures, and thermal, optical, and magnetic properties of two isostructural layered coordination polymers, [M(II)(5)(OH)(2)(chtc)(2)(H(2)O)(10)]·2H(2)O where M = Co (1Co) or Ni (2Ni) and chtc = cis,cis,cis-1,2,4,5-cyclohexanetetracarboxylate. The structures consist of segregated layers where each layer is formed of M(3)(OH)(2) diamond chains, involving edge- and corner-sharing octahedra, bridged by M(chtc)(2). In the space created by the chains and the bridges exists channels, which house the coordinated and noncoordinated water molecules, all hydrogen bonded to form a dodecamer, with a central cyclohexane chair shape ring. Interestingly, the water molecules of 2Ni are more difficult to remove than those of 1Co, but pyrolysis of the latter takes place at higher temperature. The magnetic properties are characterized by Néel transition to fully compensated antiferromagnets at 5.8 K (1Co) and 3.0 K (2Ni). The moments are easily reversed in an applied field of 150 Oe (1Co) and 300 Oe (2Ni) to a ferrimagnet with one uncompensated moment followed by a nonlinear increase to a saturation corresponding to a ferrimagnet with three uncompensated moments. Further, point charge calculations to estimate the weak bonding energies between the various types of interaction are reported.

The magnetic property and intraparticle structure of the γ phase of Fe2O3 (maghemite) nanoparticles with a diameter (D) of 5.1±0.5 nm were investigated through AC and DC magnetic measurements and powder X-ray diffraction (XRD) measurements at pressures (P) up to 27.7 kbar. Maghemite originally exhibits ferrimagnetic ordering below 918 K, and has an inverse-spinel structure with vacancies. Maghemitenanoparticles studied here consist of acore with structural periodicity and a disordered shell without the periodicity, and core shows superparamagnetism. The DC and AC susceptibilities reveal that the anisotropy energy barrier (ΔE/kB) and the effective value of the core moment decrease against the initial pressure (P≤3.8 kbar), recovering at P≥3.8 kbar. The change of ΔE/kB with P is qualitatively identical with that of the core moment, suggesting a down-and-up fluctuation of the number of Fe3+ ions constituting the core at the pressure threshold of about 4 kbar. This phenomenon was confirmed by the analysis of the XRD measurement using Scherrer’s formula. The core volume decreased for P≤2.5 kbar, whereas at higher pressure the core was restructured. For 2.5≤P≤10.7 kbar, the volume shrinkage of particle hardly occurs. There, ΔE/kB is approximately proportional to the volume associated to the ordered fraction of the nanoparticles as seen from XRD, Vcore. From this dependence it is possible to separate the core/shell contribution to ΔE/kB and estimate core and surface anisotropy constants. As for the structural experiments, similar experimental data have been obtained for D=12.8±3.2 nm as well., This work in Japan was supported by the CREST project of the Japan Science and Technology Agency (JST) and a Grant-in-Aidfor Young Scientists (B) (19750118) from the MEXT of Japan. The work in Zaragoza has been supported by the research grants MAT2007-61621 and CSD2007-00010 from the Ministry of Education. The Aveiro-Zaragoza collaboration has been supported by the Integrated Spanish-Portuguese Action PT2009-0131. N.J.O. Silva acknowledges CSIC for an I3Pcontract and FCT for a Ciencia 2008 contract.

We physisorbed nitric oxide (NO) molecules with spin ( S ) of 1/2 into the nano-sized pores of the meso-porous medium MCM-41, which allows molecule adsorption of the multi-layer form. The adsorption and desorption isotherms were observed at 121.5 K, and the effects of adsorption efficacy (i.e., the filling ratio, n ) on the magnetic properties were investigated. Changes in the condensation state in the temperature region of 100–120 K were observed through the magnetic measurements, and the resultant phase diagram against n resembled that obtained for the physisorption of oxygen (O 2 ) molecules with S = 1. For T 90 K, two NO molecules dimerized independently of n and diamagnetic dinitrogen dioxide (N 2 O 2 ) was formed. The two-dimensional magnetic state, seen for O 2 physisorption, was not observed in the present system, probably due to the prominent electric polarity of the NO molecule.

The Gd–Ho series of lanthanide ferromagnets, which includes gadolinium (Gd), terbium (Tb), dysprosium (Dy), and holmium (Ho), undergoes similar structural transitions, e.g., the hcp→Sm-type→dhcp→fcc transitions, under pressure. Through high-field DC magnetic measurements and structural analyses, we found that the ferromagnetic moments disappeared at a specified critical pressure, which resulted in volume shrinkage of 16.7±1.7% for each ferromagnet. The results of the present study suggest that the disappearance of the ferromagnetic moments of Gd–Ho under pressure could be understood within the framework of a band picture related to volume shrinkage.

The structures (nuclear and magnetic), magnetic properties (2-300 K, 1-10(4) bar), and heat capacity of the layered ferromagnet Co5(OH)6(SO4)2(H2O)4 are reported. The crystal structure consists of brucite-like M(II)-OH layers of edge-sharing octahedra, but having two different Co sites, which are pillared by ...O3SO-Co(H2O)4-OSO3.... The absorption spectrum confirms the presence of divalent Co, and by comparison of the two isotopic materials, the assignment of the vibrational spectra is proposed. The magnetic properties are those of a ferromagnet with a Curie temperature of 14 K. Temperature and field dependence magnetization data taken on an aligned sample suggest an easy-plane magnet. The Curie temperature increases linearly with pressure at a rate of +0.12 K/kbar, suggesting small progressive and uniform modifications of the Co-Co exchange interactions. Rietveld refinement of the neutron powder diffraction data and consideration of a group analysis reveal the direction of the moments of the Co within the layer to be along the b-axis, with a maximum moment of 3.33 micro(B) per cobalt. Those of the pillars remain random. Estimation of the entropy from the heat capacity data accounts for the presence of four ordered moments of Co with spin 1/2 at the long-range ordering temperature, while the moment of the pillaring Co contributes only at lower temperature due to the increase of the internal field as the temperature is lowered. The purely 2D-magnetic ordering in an easy-plane magnet, evidenced by neutron diffraction and heat capacity, challenges the existing theories and is a rare example of a single-layer magnet.

Ice-rule breaking spin-flip in a pyrochlore spin ice compound Dy2Ti2O7 has been studied in a field applied near the [112] direction, by means of angle-resolved magnetization measurements. We obtained strong evidence that the magnetization component perpendicular to the kagom? plane undergoes a first-order transition below ~0.26?K. This transition is driven by a ferromagnetic ordering in the field-decoupled spins on the triangular lattice in Dy2Ti2O7, as predicted theoretically.

A radical cation salt, BBDTA (= benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole) · GaCl 4 , has three polymorphs, labeled α, β and γ. The γ-phase shows a ferromagnetic ordering at 7.0 K, and this Curie temperature ( T C ) is eminently high, in contrast to those of the other organic radical ferromagnets. We have investigated the pressure effects on the γ-phase of BBDTA · GaCl 4 through ac magnetic susceptibility measurements under hydrostatic pressure up to 16.2 kbar. The value of T C increases with linear fashion against pressure and exceeds 14 K at 16.2 kbar. By applying further pressure, it may exceed 16 K of the organic ferromagnet, TDAE-C 60 , which has the highest T C in organic materials . However, the real component of the ac susceptibility below T C is remarkably suppressed with increasing pressure and, at around 2 kbar, it decreases down to 3.5% of that at ambient pressure. Those pressure effects might originate from the pressure-induced structural transformation from the ferromagnetic γ-phase to the diamagnetic α- and/or β-phases and the enhancement of the intermolecular interactions in the surviving γ-phase.

We have investigated pressure effects on a dimetallic ferrimagnet [Mn(en)] 3 [Cr(CN) 6 ] 2 · 4H 2 O (en; ethylenediamine) through the magnetic measurements using a diamond anvil cell in the pressure region up to P = 4.7 GPa. This ferrimagnetic compound has an eminent high transition temperature ( T c) of 69 K at ambient pressure in the structurally characterized molecule-based magnet system. Under hydrostatic pressure, T c linearly increases against pressure, and exceeds 130 K at P = 4.7 GPa. The amount of the saturated moment hardly changes in the considered pressure region. This pressure experiment might become a prototype of artificial material control for the high- T c molecule-based magnet.

Angular and temperature dependence of the ESR spectrum of a single crystal of a salt, MEM–[TCNQ] 2 , were measured under uniaxial pressure up to 1 kbar by using a hand-made pressure cell and a conventional ESR instrument in the temperature range of 1.5–300 K. The ESR spectrum is deformed with increasing pressure and the deformed ESR spectrum significantly depends on the temperature and orientation of the crystal. The ESR spectrum at low temperatures is well separated into three absorptions. One of the absorptions seems to result from free spins and the other two absorptions seem to result from spin-pairs.

The pressure effects on an S = 1 Haldane compound Ni(C 5 H 14 N 2 ) 2 N 3 (PF 6 ) (NDMAP) have been investigated through the magnetic susceptibility (χ) measurements using single crystalline sample...

Ceramic high-T c superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7-y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T c1 and furthermore among the grains at T c2 (< T c1 ). The increase in T c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T c2 also increases with increase of pressure, however, the sharpness of the transition at T c2 does not change. In spite of difference in T c2 's, the increasing rates of t c2 by pressure are nearly the same.

Static-pressure dependence of the susceptibility and the magnetization of an organic spin-Peierls substance, MEM-[TCNQ] 2 , was measured by using a SQUID susceptometer with a hand-made pressure cell in the temperature range of 2-150K and at the pressure up to 8kbar. We have found that the susceptibility of the substance changes significantly with increasing static-pressure. As the pressure increases, the susceptibility at low temperatures increases as if free magnetic spins are produced and it overwhelms the spin-Peierls transition. Above lkbar, there are no marks of spin-Peierls transition and the susceptibility at low temperatures follows the Curie-Weiss law.

Magnetic measurements have been performed to investigate random-bond effects on a spin-ladder system. We synthesize a random mixture compound Cu 2 (C 5 H 12 N 2 ) 2 (Cl 1− x Br x ) 4 which is expected to form a spin ladder with random-exchange legs and rungs. The magnetic properties of the compound are rather different from those of the pure systems. Paramagnetic divergence is significantly observed for the susceptibility at low temperatures and the magnetization appears even at low fields. The results suggest that the ground state of the random-bond ladder is a Griffiths phase.

Bimetallic ferrimagnet [Cr(CN) 6 ][Mn( R )-pnH(H 2 O)]H 2 O with crystallographical chirality of space group P 2 1 2 1 2 1 exhibits a giant third-order harmonic susceptibility at around 38 K. This anomaly appears as precursory magnetic phenomenon of the magnetic ordering. In order to investigate the effect of lattice distortion on this giant nonlinear magnetic response (NLMR) and finally elucidate the correlation between the structural chirality and the giant NLMR, we performed the ac susceptibility measurement for the single crystal in the situation of bringing about lattice distortion along the direction parallel to the a -axis. This giant NLMR was drastically suppressed even by slight strain, whereas the traces could be confirmed in the region up to 3.7 kbar. We conclude that the appearance of the giant NLMR requires crystal chirality well-regulated over the crystal.

Since the observation of porosity in organic–inorganic hybrids there has been a major development in the study of gas sorption and catalysis as they provide novel surfaces in contrast to metals and oxides including silicates. Of major interest is the possibility of trapping fuel gases and toxic fumes. In addition, their applications as sensors have been demonstrated by use of the change in magnetic or optical properties upon sorption and desorption. The latter experiments have been performed ex-situ. Thus, there is a need to understand the properties in-situ, in particular to follow the performance of the materials continuously. Here, the development of an in-situ apparatus to study the magnetism is presented and applied for the partially dehydrated Ni 2 (H 2 O) 4 PM · 2H 2 O, PM = benzene-1,2,4,5-tetracarboxylate with oxygen (O 2 ) and nitric oxide (NO) in the cavities.

By the measurement of magnetic susceptibility under pressures ( P ) up to 8 kbar, we investigated pressure effects of the S = 1 Heisenberg antiferromagnetic bond alternating chain compound [{Ni(333...

We investigated the magnetization of the organic weak-ferromagnet p -NC.C 6 F 4 .CNSSN in the pressure P region up to 16 kbar. The transition temperature T c at ambient pressure is 35.5 K. T c is enhanced with increasing pressure, and reaches 64.5 K at P =16 kbar, which is the highest T c for a genuine organic weak-ferromagnet. On the other hand, the net moment is suppressed by the pressure, and this can be explained with the Dzyaloshinsky–Moriya model.

Positive muons can be implanted into organic and molecular magnets in order to study their internal magnetic field distribution and any associated dynamics. The muon behaves essentially as a “microscopic magnetometer”, sensitive to local magnetic order and magnetic fluctuations. We describe some recent experiments using this technique which were performed on a variety of organic systems, including nitronyl nitroxide magnets and materials with spin-Peierls ground states, MEM(TCNQ)2 and DEM(TCNQ)2, and demonstrate how the technique can give information concerning their ground states.

The measurements of the magnetic susceptibility, electron-spin-resonance (ESR) and heat capacity on the title substance have revealed that the substance is a typical S=1/2 Heisenberg antiferromagnetic chain. The intrachain exchange interaction energy is estimated to be J/k=-55K. No three-dimensional long range ordering due to interchain interaction has been observed down to 1.1K. Furthermore, the critical behavior of the magnetic susceptibility at low temperatures, which has been predicted theoretically by Eggert et al. (Phys. Rev. Lett. 73 (1994) 332), is successfully examined. The ESR linewidth of a single crystal, which strongly depends on the direction of the external magnetic filed, is well explained on the basis of the spin-diffusion theory for one-dimensional magnets.

The continuous variation of magnetic intermolecular interactions has been studied in the pressurized genuine organic ferromagnet, β -phase para -nitrophenyl nitronyl nitroxide ( T c = 0.61 K), in which a pressure-induced ferro- to antiferromagnetic transition was observed under pressure around 6.5 kbar at 0.45 K. The rotation of the radical group C 6 H 4 NO 2 around its symmetrical axis in crystallized molecules is pointed out to be essential to change magnitude and a sign of the dominant exchange interactions, rather than the shrinkage of the lattice parameters under pressure.

We investigated elastic properties of the filled skutterudite compound with heavy lanthanide TbRu 4 P 12 by means of ultrasonic measurements for the first time. TbRu 4 P 12 undergoes a two-successive phase transition from a paramagnetic to an antiferro-magnetically ordered phase at T N ∼20 K, then to another phase transition at T 1 ∼10 K. We found that a clear elastic anomaly was observed at the two successive phase transition. We argue the elastic behavior and the possible interpretation in each ordering phase. The data of the present study indicates great importance of the interplay between orbitally and magnetically degrees of freedom.

Magnetic measurements have been performed to investigate nonmagnetic impurity effects on a Zn-doped ladder system (Cu 1- x Zn x ) 2 (C 5 H 12 N 2 ) 2 Cl 4 . The paramagnetic contribution in the sus...

Ni 2 (H 2 O) 4 PM·2H 2 O, where PM is benzene-1,2,4,5-tetracarboxylate, forms square-grid layers with Ni—OCO–Ni bridges that are connected to one another by the benzene moiety. It consists of ferromagnetic layers coupled antiferromagnetically to their neighbors with a long-range Neel ordering at 3 K. A critical field of 1300 Oe suffices to reverse all the moments to a ferromagnetic state. On dehydration to Ni 2 (H 2 O) 2 PM the structure is transformed irreversibly to chains connected by benzene and the magnetic behavior is that of a paramagnet with very weak coupling. By the rehydration of the latter to Ni 2 (H 2 O) 2 PM·2H 2 O, the structure hardly changes but the magnetism recovers showing T N at 3 K.

We have investigated the magnetic properties of two-dimensional networked single-molecule magnet, [Mn4(hmp)4Br2(OMe)2(dcn)2]·0.5H2O·2THF (Hmp=2-hydroxymethylpyridine, dcn=dicyanamide, THF=tetrahydrofuran), through the AC susceptibility measurements up to P=15.3 kbar. This compound exhibits superparamagnetic relaxation phenomenon below 3 K, and the blocking temperature (TB) in low frequency limit is estimated to be 2.00 K at ambient pressure. As pressure increases, the TB once decreases and, at around P=3 kbar, it turns to increase. At P=15.3 kbar, the TB reaches 2.27 K. By analyzing the frequency dependence of TB, it has been revealed that a dynamical critical exponent of the power-law fitting shows the change of up and down, and it approaches 2.0 at last. This result suggests that the intercluster magnetic network changes by pressurization and finally a conventional phase transition may be realized.

Superconducting quantum interference device (SQUID) is a powerful tool, which can detect small magnetic signal with high sensitivity, but it is not easy to make use of the SQUID. Especially in the case of the pressure experiment, it is more difficult to utilize the SQUID due to the space limitation of the coil arrangement. We are trying to establish the vibrating coil magnetometer (VCM) using the SQUID, and our attempt started from the imitating of Ishizuka type of VCM. In the development process, we have developed the original method of coil vibration control and real-time phase-detection system. As the performance test, the superconducting transition of lead was observed in some conditions, and the present sensitivity was confirmed to be about 10 −8 emu at earth field.

The effect of pressure on two-dimensional Heisenberg antiferromagnet Cu(HCOO)2·4H2O has been investigated through X-ray diffraction, susceptibility and magnetization curve measurements. The pressurization induces the paramagnetic behavior above 1.4 kbar. The paramagnetic component increases continuously with increasing pressure, and the antiferromagnetic ordering and field-induced phase transition vanish above 2.8 kbar. The X-ray diffraction pattern at 6.6 kbar is not that of Cu(HCOO)2·4H2O but Cu(HCOO)2·2H2O. Therefore, the pressurization induces the crystal structural change from Cu(HCOO)2·4H2O to Cu(HCOO)2·2H2O with dehydration.

From the measurements of magnetic susceptibility, heat capacity, magnetization and EPR, the magnetism of the organic radical crystal of 1,3,5-triphenyl-6-oxoverdazyl is revealed to be descrived by a two-dimensional quantum Heisenberg antiferromagnet with the effective intra-plane exchange interaction J/Kb= − 4.3 K. The appearance of weak ferromagnetic moment is observed below 5.4 K. The magnetic susceptibility, however, shows its broad maximum around 2 K and decrease toward the low temperatures. These results are shown to be described by a four-sublattice model with two Dzyaloshinsky-Moriya vectors of opposite sign on the ac-plane. A proposal is made for crystal symmetry in which weak ferromagnetic moment is stabilized without direct ferromagnetic interactions.

A genuine organic radical crystal 1,3,5-triphenyl-6-oxoverdazyl (TOV) has been revealed to be an S =1/2 two-dimensional (2D) weak-ferromagnetic Heisenberg antiferromagnet with the exchange interaction 2 J / k B =-9.0 K by the experiments of magnetic susceptibility, heat capacity, magnetization and electron paramagnetic resonance. The anomalous temperature dependence of the weak ferromagnetism at low temperatures is qualitatively explained by a four-sublattice model with two kinds of Dzyaloshinsky-Moriya (D-M) vectors of opposite sign on the a c -plane. The staggered susceptibility, which is a fictitious but intrinsic quantity for antiferromagnets, is estimated in TOV by making use of the canted weak ferromagnetic moments which dominate below 6 K. Especially the rapid growth of the magnetic susceptibility below 6 K is explained by the staggered susceptibility which makes the crossover effect from 2D Heisenberg to 2D Ising system, triggered by above the D-M anisotropy.

We report the first muon-spin relaxation study of an organic spin - Peierls system, the linear-chain compound MEM(TCNQ). Our results show a crossover from a Gaussian relaxation to an exponential relaxation as the temperature is lowered below the spin - Peierls transition. We associate this behaviour with the slowing down of the electronic fluctuations resulting from the opening of a gap in the magnetic excitation spectrum.

Heterojunction diodes, which comprise boron-doped p-type ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films prepared by coaxial arc plasma deposition and n-type Si substrates, were electrically studied. The current–voltage characteristics showed a typical rectification action. An ideality factor of 3.7 in the forward-current implies that carrier transport is accompanied by some processes such as tunneling in addition to the generation–recombination process. From the capacitance–voltage measurements, the built-in potential was estimated to be approximately 0.6 eV, which is in agreement with that in a band diagram prepared on the assumption that carriers are transported in an a-C:H matrix in UNCD/a-C:H. Photodetection for 254 nm monochromatic light, which is predominantly attributable to photocurrents generated in UNCD grains, was evidently confirmed in heterojunctions. Since dangling bonds are detectable by electron spin resonance spectroscopy, their control might be an important key for improving the rectifying action and photodetection performance.

Susceptibility and magnetization measurements on the diluted Haldane system (CH 3 ) 4 NNi 1- x - Zn x (NO 2 ) 3 (Zn-doped TMNIN) have been carried out using a SQUID magnetometer. The contribution of edge moments in Ni 2+ chains is extracted from the experimental data using the formula employed in the quantum Monte Carlo calculation by Yamamoto and Miyashita (Phys. Rev. B 50 (1994) 6277). In the temperature range 5 K< T

Magnetic susceptibility of an organic ion-radical salt, [3,3 ′ -dimethyl-2,2 ′ -thiazolinocyanine]-TCNQ, is measured in the temperature range of 1.4–300 K. Above 10 K, it is quantitatively explained in terms of a S =1/2 Heisenberg antiferromagnetic chain with J / k B =-32.5 K based on the Bonner-Fisher curve. Below 10 K, however, the measured susceptibility seems to show the characteristic slope reported by Eggert et al. down to the ordering temperature T c =1.49 K. Below T c the heat capacity of this salt displays the temperature dependence expected for a gapless spin-wave excitation, which rules out the spin-Peierls transition in this system.

We measured the magnetic heat capacity, susceptibility and magnetization of (CH 3 ) 4 NCo(NO 2 ) 3 (TMCON) which is the iso-structural compound with the Haldane compound (CH 3 ) 4 NNi(NO 2 ) 3 (TMNIN). From the analysis of these thermodynamic properties, we found that TMCON is a one-dimensional S =1/2 Ising system where the ferromagnetic nearest neighbor interaction J 1 and the antiferromagnetic next-nearest neighbor interaction J 2 are competing. The value of J 1 is estimated to be J 1 / k B =28.0±4.0 K. It is confirmed that the results can be comprehensively explained only when we take J 2

The magnetic properties of honeybees have been investigated by measuring their susceptibility, magnetization and electron-spin-resonance (ESR) absorption. The abdomen of adult worker honeybees shows typical paramagnetism with small magnetic remanence down to 4.2 K, while the other parts of the honeybees are apparently diamagnetic. The paramagnetism of the abdomen seems to develop not in the pupal stage but after coming out of comb. Drones show no paramagnetism, but a queen bee seems to have some paramagnetic species in her abdomen.

Electron spin resonance (ESR) and magnetization measurements have been performed on the diluted Haldane system (CH 3 ) 4 NNi 1- x Zn x (NO 2 ) 3 (Zn 2+ :TMNIN). We observed ESR absorption from the S =1/2 moment at the end of the Ni 2+ chains at low temperatures. The dependence of the ESR absorption intensity on temperature and the concentration of Zn 2+ are compared to the theory derived from the valence bond solid (VBS) model by Mitra, Halperin and Affleck (Phys. Rev. B 45 (1992) 5299). The paramagnetic contribution to the magnetization of Zn 2+ :TMNIN increases with Zn 2+ concentration. We find that on average, one S =1/2 moment appears on each segment of finite chains at low temperatures.

We have performed AC susceptibility and DC magnetic relaxation measurements on the spin ice system Dy$_2$Ti$_2$O$_7$ down to 0.08 K. The relaxation time of the magnetization has been estimated below 2 K down to 0.08 K. The spin dynamics of Dy$_2$Ti$_2$O$_7$ is well described by using two relaxation times ($\tau_{\rm S}$ (short time) and $\tau_{\rm L}$ (long time)). Both $\tau_{\rm S}$ and $\tau_{\rm L}$ increase on cooling. Assuming the Arrhenius law in the temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K, both $\tau_{\rm S}$ and $\tau_{\rm L}$ show a clear deviation from the thermal activated dynamics toward temperature independent relaxation, suggesting a quantum dynamics., Comment: 4 pages

An organic ion-radical salt, 3,3'-diethyl-4,4'-dimethyl-2,2'-thiazolocyanine-[TCNQ] 2 , is synthesized and its crystal structure and electronic properties are investigated. The salt forms triclinic crystal in the space group P \bar1 with a =13.153(4), b =18.314(5), c =7.792(2) A, α=107.21(3), β=103.60(4), γ=77.75(3)° and Z =2. The electric conduction of single crystals is anisotropic and shows semiconductive temperature dependence. The intrinsic spin-susceptibility of the salt shows a broad maximum around 265 K and decreases to zero below 15 K. A single ESR absorption is observed for a single crystal and its linewidth strongly depends on the temperature and the direction of external magnetic field. These electronic properties are discussed on the basis of the peculiar crystal structure of the salt.

We have performed ultrasonic measurements on the $\ensuremath{\alpha}$-pyrochlore spin compounds ${R}_{2}$Ti${}_{2}$O${}_{7}$, $R=$ Tb, Dy, and Ho. A distinct dip appears around 80 K, especially in transverse modes $({C}_{11}\ensuremath{-}{C}_{12})/2$ and $C$${}_{44}$ as a function of temperature in Dy${}_{2}$Ti${}_{2}$O${}_{7}$ and Ho${}_{2}$Ti${}_{2}$O${}_{7}$ compounds. This anomaly may correspond to a structural change detected by the temperature dependence of the permittivity and of the loss-angle tangent. No distinct anomaly was observed at lower temperatures in Dy${}_{2}$Ti${}_{2}$O${}_{7}$ and Ho${}_{2}$Ti${}_{2}$O${}_{7}$ compounds. On the other hand, a pronounced elastic softening toward low temperatures was observed in the temperature dependence of the principal elastic constants $C$${}_{11}$, $({C}_{11}\ensuremath{-}{C}_{12})/2$, and $C$${}_{44}$ in Tb${}_{2}$Ti${}_{2}$O${}_{7}$. A detailed discussion of the highly distinct behavior among three compounds is given in terms of the magnitude of a splitting energy between the ground state and the excited one formed by the crystal-field effect. It seems that geometrically structural frustration prohibits the formation of a long-range quadrupolar ordering as well as a magnetic one, even if the coupling constant between quadrupolar moments is rather large enough in Tb${}_{2}$Ti${}_{2}$O${}_{7}$ at low temperatures.

We report the pressure dependence of electrical resistance $R$($T$) of a frustrated pyrochlore oxide Nd${}_{2}$Ir${}_{2}$O${}_{7}$, which shows a second-order metal-insulator transition (MIT) at 36 K. This MIT is monotonically suppressed by the application of pressure. The insulating phase disappears at around 10 GPa. Then, in the pressure-induced metallic state above 10 GPa, a new pressure-induced phase transition emerges at around 3 K, which is characterized by a resistance drop. The new phase transition is due to a ferromagnetic (FM) ordering, which is suggested to be an ordered spin ice (two-in two-out) state of the Nd moment via the RKKY interaction. We succeeded in observing the phase competition between the MIT and the FM ordering by the application of high pressure.

In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd$_2$Ir$_2$O$_7$, which exhibits a metal-insulator transition at a temperature $T_{\rm MI}$ of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector $\vec{q}_{0}$ of (0,0,0) and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic Nd$^{3+}$ magnetic moments of magnitude $2.3\pm0.4$$\mu_{\rm B}$, where $\mu_{\rm B}$ is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd$^{3+}$ splits below $T_{\rm MI}$. This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir$^{4+}$ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal-insulator transition in terms of frustration., Comment: 6 pages, 1 table, 3 figures

Systematic studies of magnetic critical exponents in local moment molecular magnets can be made by combining three different types of μ SR measurement; zero-field precession, longitudinal relaxation and the frequency shift in transverse field. Systems have been studied with 1D, 2D and 3D structures, having both simple and non-collinear magnetic interactions and the measurements have included investigation of the effect of chirality on the critical spin fluctuations. In the case of the model S = frac(1, 2) Heisenberg antiferromagnetic 1D chain Heisenberg antiferromagnetic quantum critical fluctuations suppress magnetic ordering and TCNQ radical ion salts can provide some excellent examples of this model. In particular, DEOCC-TCNQF4 behaves as a nearly ideal case with no ordering seen down to 20 mK even though the intrachain exchange interaction is 110 K. In contrast the DMTzNC-TCNQ compound orders at 1.5 K, but still shows evidence for significant residual quantum fluctuations in its ordered state in the form of a large suppression of the magnitude of the ordered part of the magnetic moment. From analysis of the characteristic field dependent muon relaxation behaviour as a function of temperature above the transition, the nature of the spin fluctuations is found to change as the transition is approached, suggesting a crossover either to 2D diffusion or to 1D ballistic motion. © 2008 Elsevier B.V. All rights reserved.