Masafumi Tamura

Pressure effect on organic radicals

Abstract The magnetic properties of galvinoxyl (4-[3,5-bis(1,1-dimethylethyl)-4-OXO-2,5-cyclohexadiene-1-ylidenemethyl]-2,6-bis (1,1-dimethyl-ethyl)-phenoxy) and F5PNN (2-pentafluorophenyl-4,4,5,5-tetramethyl-4,5-dihydro-lH-imidazol-1-oxyl 3-oxide) under pressure are presented. We have found that the structural changes of both compounds can be suppressed by pressurization. The phase transition to a diamagnetic state of galvinoxyl can be suppressed by applied pressure. It has been found that under 7 kbar, the transition is sufficiently suppressed and the ferromagnetic interactions are preserved down to low temperature. The temperature dependence of the paramagnetic susceptibility under 7 kbar obeys the ferromagnetic chain model with 2J/kB = 25 K. The room-temperature crystal structure of F5PNN involves a uniform chain, while the low-temperature magnetism can be interpreted by the alternating antiferromagnetic chain model with 2J/K B = -5.6 K and α = 0.4. We found that under 5 kbar, the low-temperature magn...

Novel BEDT-TTF salts containing rare earth ions, (ET)4Ln(NCS)6·CH2Cl2

Abstract We have prepared single crystals of a novel BEDT-TTF salts. (ET)4Ln(NCS)6·CH2Cl2, where ET is an abbreviation of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene and En is a heavy rare earth element, Ho, Er, Yb and Y. They are semiconductors below room temperature. From the crystal structure and the magnetism of the Y compound, it is concluded that the π electrons form a low dimensional spin system, as a result of charge separation and Mott localization. Characteristics of the 4ƒ electrons appears as remarkable anisotropy and temperature dependence of the magnetic susceptibility of the Ho, Er and Yb compounds.

Possible Superhigh-Spin Cluster in the Crystals of a Novel Nitronyl Nitroxide Radical as Studied by FT Pulsed ESR

Abstract Syntheses and magnetic properties of novel nitronyl nitroxide derivatives are reported. In the derivatives under study, the bulky tetramethylethylene group of the conventional nitronyl nitroxide has been replaced by aromatic rings, giving planar π-conjugation systems. The first half of the paper is devoted to the syntheses and characterization of new derivatives of such radical, 2-phenylbenzimidazol-l-yl-N,N′-dioxide (PBIDO). Chemical stability, π-spin density distribution, and solid state magnetic properties have been examined from solution ESR and static susceptibility measurements. In the latter half, we discuss an anomaly of paramagnetic relaxation found in crystals of PBIDO by pulsed ESR spectroscopy. The origin of the anomaly is attributed to a possible formation of paramagnetic species with huge S, or “super-high-spin cluster” at lattice defects in the crystal.

Transport study of an organic conductor, θ-(BEDT-TTF)2I3

Abstract Pressurization of θ-(BEDT-TTF) 2 I 3 has been shown to expand the cross-sectional area of small three-dimensional Fermi surface viewed along the b m -axis. At room temperature, θ-(BEDT-TTF) 2 I 3 has been found to undergo a transition at about 5 kbar to a new phase ( θ h -phase), whose electric behavior is similar to that of the α-phase. The temperature dependence of the Hall coefficient of θ-(BEDT-TTF) 2 I 3 has been interpreted by taking account of the thermal effect on large two-dimensional Fermi surface.

Theoretical Studies of Magnetic Interactions in P-Cyanophenyl Nitronyl Nitroxide Crystal

Abstract Magnetic interactions in crystal of p-cyanophenyl nitronyl nitroxide (p-CNPNN) were theoretically investigated. Semiempirical and ab initio molecular orbital calculations were performed for pair models of p-CNPNN molecules in the crystal. Effective exchange integrals (Jab ) are positive for the pairs of neighboring molecules in a sheet parallel to the ac-plane of the crystal, suggesting pseudo-two-dimensional ferromagnetic interaction in the ac-plane. Close contact between p-CN-phenyl group and NO group of the adjacent molecules plays a dominant role for the ferromagnetic interaction. Inter-sheet interaction is weak and the sign of Jab depends on the pairs.

Preparation And Electrical Properties Of Radical Cation Salts Based On 1,3-Dithiane Derivatives

Abstract Preparation of radical cation salts based on 1,3-dithiane derivatives (1,2 and 5–8) was investigated. Although attempts to prepare stable radical cation salts derived from 1 and 2 were unsuccessful, various radical cation salts composed of TTF (tetrathiafulvalene) derivatives fused with a 2-(1,3-dithiol-2-ylidene)-1,3-dithiane (DTDT) unit (5–8) were formed with ease by electrochemical oxidation. Several radical cation salts of 6–8 exhibited metallic conducting behaviors on a single crystal or compressed pellets.

Correlation Between the Thermal and Magnetic Properties of (DCNQI)2Cu

Abstract Low temperature electronic specific heat, C, was measured for three iodine-containing (DCNQI)2Cu. The temperature coefficient γ has been found to correlate with the temperature dependence of magnetic susceptibility, χ. This supports the interpretation of the anomalous magnetism of these salts on the basis of itinerant electron magnetism.

Pressure effects of a genuine organic ferromagnet p-NPNN

Pressure-induced ferro-antiferromagnetic transition in genuine organic compounds was observed for the first time in the β-phase p-NPNN crystal. This phenomenon is understood by the charge-transfer mechanism, in which some of the ferromagnetic exchange integral comes to compete with the antiferromagnetic one being enhanced via molecular orbitals under the hydrostatic pressure.

Temperature independent conductivity in α-(BEDT-TTF)2I3 : A novel mechanism

Abstract The electric conductivity of α-(BEDT-TTF) 2 I 3 under a high pressure is almost independent of temperature. On the other hand, the Hall coefficient ( R H) increases with lowering temperature; R H at 4 K is 10 4 times as large as that at room temperature, indicating an extraordinary low carrier density at low temperatures. We have found that the carrier density plays a key role in the physics behind this peculiar behavior. The carrier density at 4 K is estimated to be 10 −4 holes per unit cell, whereas that at room temperature amounts to 1–2 holes per unit cell. The situation is entirely modified, when the thermal effect is taken into account. The carrier can spread in the k -space over the region with energy of ∼ e F ± κ B T /2. Thus, at high temperatures the hole and electron pockets are expected to fuse into an obscured hole-like orbit, which is large enough to afford 1–2 holes per unit cell. This crossover from a semi-metal with low carrier density and high mobility to a metal with an obscured “Fermi surface” is novel and unique.