Tadashi Sugano

Ferromagnetic intermolecular interaction in the galvinoxyl radical: Cooperation of spin polarization and charge-transfer interaction

Abstract The orbital energies and the intermolecular overlap integrals of the galvinoxyl radical are calculated. It is found that the ferromagnetic intermolecular interaction in galvinoxyl can be qualitatively interpreted by a combined effect of spin polarization within the radical and charge-transfer interaction between the radicals.

Ferromagnetic intermolecular interactions in a series of organic mixed crystals of galvinoxyl radical and its precursory closed shell compound

The magnetic properties of the 4:1, 6:1, 9:1, and 19:1 mixed crystals of galvinoxyl (4‐[[3,5‐bis(1,1‐dimethylethyl)‐4‐oxo‐2,5‐cyclohexadien‐1‐ylidene]methyl]‐2,6 ‐bis(1,1‐dimethylethyl) phenoxy) radical and its precursory closed shell compound, hydrogalvinoxyl, have been studied. From the measurements of the temperature dependence of the magnetic susceptibility, it is found that the ferromagnetic intermolecular interactions, which are lost below about 85 K in pure galvinoxyl because of the phase transition, are maintained down to 2 K in these mixed crystals, and that the number of galvinoxyl radicals keeping the ferromagnetically coupled structure at low temperature increases as the concentration of hydrogalvinoxyl increases. The magnetic behavior of the mixed crystals at low temperature depends on the thermal history of the sample and is well interpreted by assuming the presence of a glassy state into which the high‐temperature, ferromagnetically coupled phase is quenched. The magnetization curves show t...

Organic radical clusters with ferromagnetic intermolecular interactions

Abstract The magnetic properties of the mixed crystal of galvinoxyl radical (2,6-di- t -butyl-4-(3,5-di- t -butyl-4-oxocyclohexa-2,5-dienylidenemethyl) -phenoxyl) and its precursory compound, hydrogalvinoxyl, in 6:1 ratio have been studied. The magnetic susceptibility of the mixed crystal follows the Curie-Weiss law in the range of 2–300 K with a positive Weiss constant of 7 K. Thus the ferromagnetic interactions are maintained over the whole temperature range in the mixed crystal, in contrast to the pure galvinoxyl in which the paramagnetism disappears below the phase transition temperature of 85 K. The magnetization curves for the mixed crystal measured at 2 and 5 K seem to follow closely along the theoretical curve for S = 3. The ferromagnetic interactions thus extend over six radicals on an average. However, the observed magnetization could more reasonably be explained by assuming an assembly of segments consisting of galvinoxyl radicals ferromagnetically coupled with each other and having various spin multiplicities in proportion to the segment size.

Magnetic interactions among unpaired electrons in charge-transfer complexes of organic donors having a neutral radical

Abstract A new π-electron donor in which TTF is linked with tetramethylpiperidinoxyl radical is synthesized. Three donors consisted of dialkylaminophenyl and nitronyl nitroxide are also prepared. Magnetic properties of the donors and their charge-transfer complexes with the acceptors TCNQ and TCNQF4 are investigated by means of magnetization measurements below 5 K and magnetic susceptibility measurements between 2 and 300 K. Magnetic interactions among the unpaired electrons located on the radical and electron donating moieties of the donor molecule and on the acceptor molecule are discussed.

ESR evidence for the ferro- and antiferro-magnetic intermolecular interaction in pure and dilute mixed crystals of galvinoxyl

Abstract From the temperature dependence of ESR absorption intensity, it is shown that the intermolecular exchange interaction is ferromagnetic in the dilute mixed crystal and antiferromagnetic in the low-temperature phase of the pure crystals. The exchange energy is obtained as 2JF = 1.5 ± 0.7 meV and 2JAF = −45±2 meV on the basis of the singlet-triplet model.

X-ray evidence for structural changes in the organic conductors, α-(BEDT-TTF)2I3, α-(BEDT-TTF)2IBr2 and ß-(BEDT-TTF)2I3

Abstract Interrelations are investigated between the electrical and structural properties of organic conductors, BEDT-TTF (bis(ethylenedithiolo)tetrathiafulvalene) trihalides. In α-)BEDT-TTF)2I3 evidence is obtained by X-ray diffraction for BEDT-TTF molecular dimerization at the metal-insulator transition temperature of 135 K. Another BEDT-TTF molecular dimerization is found in α-(BEDT-TTF)2IBr2, which has semiconducting properties and an activation energy that increases with decreasing temperature. These dimerizations are considered to be responsible for the changes in the electrical properties. Superconducting s-(BEDT-TTF)2I3 is found to have a superstructure below 175 K, above which short-range fluctuations are observed up to about 200 K.

Polarized reflectance spectra of the organic conductors: α- and β-modifications of di{bis(ethylenedithiolo)tetrathiafulvalene}triiodide, (BEDT-TTF)2I3

Abstract The electronic structure of the organic conductors α- and β-modifications of (BEDT-TTF) 2 I 3 has been investigated by means of polarized reflectance measurements at room temperature over the spectral range from 4000 to 28000 cm −1 . From the analysis of plasma edges in reflectance, the direction with larger transfer integral is found to be parallel to the side-by-side contact direction of BEDT-TTF in the α-modification, whereas it is parallel to the face-to-face stack direction in the β-modification, reflecting the novel molecular characteristics of BEDT-TTF. The strong bands appearing at 20000 and 23500 cm −1 for the α- and β-modifications, respectively, are attributed to the σ g a σ u ∗ transition of the triiodide anion chain. These band positions indicate that the triiodide anions are electronically more isolated in the β-modification than in the α-modification.

Dependence of Magnetic Susceptibility of Ba2YCu3O7-x on the Oxygen Defect Concentration

We report the results of magnetic susceptibility measurements on Ba2YCu3O7-x with various oxygen content (0.1≤x≤0.82) in the temperature range between 78 and 500 K. No sign of long-range magnetic order was observed for any oxygen content. With increasing oxygen defect concentration, x, from 0.1 to 0.82, the Pauli susceptibility gradually decreases from 7×10-4 emu/mol f.u. to half, while a Curie-Weiss-like contribution increases significantly. We ascribe this contribution to the effect of electron localization caused by the random distribution of oxygen defects.

Ferromagnetic intermolecular interaction in an organic radical e-quinolyl nitronyl nitroxide (3-QNNN)

Abstract A new nitronyl nitroxide radical, 2-(3-quinolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide, is found to exhibit a ferromagnetic intermolecular interaction with a Weiss constant Θ=0.27 K. The crystal structure is described and the relationship with the intermolecular interaction is discussed.

μ+SR of the Organic Ferromagnet p-NPNN: Diamagnetic and Paramagnetic States

We use zero- and longitudinal-field muon spin rotation and relaxation to study the muon states in the organic molecular ferromagnet: β-phase para-nitrophenyl nitronyl nitroxide (p-NPNN). Below TC = 0.65 K, oscillations in the relaxation function are observed in zero applied field which follow the magnetic order parameter, implying that the muon is bonded in an electronic spin-singlet state. We also study the behaviour of the oscillations in an applied longitudinal field and interpret the results by considering the magnetisation process and the demagnetising field. A significant field-induced repolarisation is also observed which is ascribed to the presence of spin-triplet states formed by a fraction of muons.