Takeo Otsuka

Anionic NaCl-type frameworks of [MnII(HCOO)3−], templated by alkylammonium, exhibit weak ferromagnetism

We present the synthesis, characterization by IR, TGA, single crystal X-ray structure and magnetic properties of a novel series of NaCl-type frameworks of [AmineH(+)][Mn(HCOO)(3)(-)], templated by alkylammonium. The anionic NaCl-framework of [Mn(HCOO)(3)(-)] is counter-balanced by the alkylammonium cations located in the cavities of the framework to which they are hydrogen-bonded. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. All the compounds exhibit long-range antiferromagnetism below 9 K with a slight non-collinear arrangement of the moments. The canting, likely due to second-order spin-orbit coupling, is via a Dzyaloshinski-Moriya antisymmetric exchange mechanism. A spin-flop is observed in each case at fairly low fields. An orthorhombic to monoclinic transformation was observed for the protonated cyclotrimethyleneamine that is accompanied by localization of the cations into two positions below 240 K from the rapid dynamic flipping of the ring observed at room temperature.

Crystal structures and magnetic properties of acid–base molecular complexes, (p-pyridyl nitronylnitroxide)2X (X=hydroquinone, fumaric acid and squaric acid)

The reactions of 2-(4-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide (or p-pyridyl nitronylnitroxide, abbreviated as p-PYNN) with the three dibasic organic acids, X [=fumaric acid (FA), squaric acid (SA) and hydroquinone (HQ)], result in the formation of hydrogen-bonding complexes of ( p-PYNN)2 X composition. In their crystals, the organic acids make selective hydrogen bonds to the two kinds of hydrogen-bond accepting sites in p-PYNN; (a) the oxygen atom in the NO group and (b) the nitrogen atom of the pyridyl ring. p-PYNN2HQ crystallizes in the monoclinic P21 /n space group. The HQ molecule bridges two p-PYNNs, and selects site (a) in p-PYNN as the hydrogen bond acceptor [i.e. ( p-PYNN)NOHO(HQ)OHON( p-PYNN)]. p-PYNN2 FA crystallizes in the monoclinic P21 /n space group. The FA molecule connects two p-PYNN molecules with an intermolecular hydrogen bond to site (b) [( p-PYNN)NHO(FA)OHN( p-PYNN)]. The 2:1 compound of p-PYNN and SA crystallizes with the crystal solvent, 1,4-dioxane (abbreviated as diox), in the formula for p-PYNN2 SAdiox. The crystal belongs to the triclinic P space group. The SA molecule occupies the space between two p-PYNNs, making contact with site (b), as FA does in the p-PYNN2 FA crystal. However the structure of SA indicates that it is a dianion in which the two protons are missing and, thus, the hydrogen bond is ionic [( p-PYNN)NH+ O–(SA)O– H+N( p-PYNN)]. The selectivity and features of the hydrogen bonds can be qualitatively understood in terms of competition between the electrostatic and charge-transfer terms in the hydrogen-bonding energy, which is governed by the acidity of the organic acids and the proton accepting abilities of the two sites in p-PYNN. The three molecular compounds exhibited different antiferromagnetic properties, which depend on the mutual arrangement of p-PYNN in the crystals. The intermolecular interactions were interpreted based on the McConnell’s spin polarization mechanism.

The pressure effect on the antiferromagnetic and superconducting transitions of κ-(BETS)2FeBr4

The temperature–pressure phase diagram of the first antiferromagnetic organic superconductor κ-(BETS)2FeBr4 shows that the Neel temperature increases with pressure, while the superconducting transition temperature decreases rapidly around 3 kbar.

Antiferromagnetism and superconductivity of BETS conductors with Fe3+ ions

The SQUID measurements revealed an antiferromagnetic transition of κ-BETS 2 FeBr 4 around 2.5 K (BETS=bis(ethylenedithio)tetra-selenafulvalene). Resistivity measurements showed an anomaly at T N which gave a clear evidence for the interaction between n metal electrons and localized magnetic moments in organic metal system for the first time. A superconducting transition was observed at 1.1 K. The specific heat indicates the coexistence of antiferromagnetism and superconductivity below T c . Similar successive antiferromagnetic and superconducting transitions were observed in κ-BETS 2 FeCl 4 . A superconducting transition was observed also in λ-BETS 2 FeCl 4 at high pressure.

Pressure Effect on Haldane Gap System

Abstract The [Ni(dmpn)2N3]ClO4(dmpn=2,2-dimethyl-1,3-propylenediamine) is a Haldane gap compound with Eg=21.6 K and J=−70.6 K. The pressure effect on the magnetic susceptibilities has revealed that with increase of the pressure the absolute J values increase. This is due to the increase of the orbital overlapping between the Ni2+ ions and the N atoms of bridging azido-ligands.

Electronic properties of BETS superconductors with magnetic anions (BETS=bis(ethylenedithio)tetraselenafulvalene)☆

The physical properties of BETS (=bis(ethylenedithio)tetraselenafulvalene) conductors with tetrahalide magnetic anions and related systems were investigated. We have recently succeeded to find the superconducting transition of the crystal of K-type T1C1 4 - salt coated with epoxy resin (T c (onset) = 2.5 K). The specific heat measurements on the antiferromagnetic organic superconductor, k(BETS) 2 FeCl 4 was performed down to 60 mK, which gave sharp peak at 0.45 K (T N ). The peak trails a tail at T > T N , suggesting low-dimensionality of the spin system. Resistivity measurements under magnetic field of λ-(BETS) 2 Fe x Ga 1-x Cl 4 x 0.4) exhibiting superconductor-to-insulating (SC-I) transition, I-SC phase transition was observed around 4 T at T < 3 K. The low-field superconducting region between metallic and insulating phases around zero magnetic field is connected with the high-field superconducting region.

Hydrogen-Bonded Acid-Base Molecular Complexes of Nitronylnitroxides

Abstract Intermolecular hydrogen-bonded complexes of P-nitronylnitroxide benzoic acid (p-NNBA-H) with m- andp-pyridylnitronylnitroxides (m- and p-PYNN, respectively) were prepared. The m-PYNN·p-NNBA-H complex crystallizes into the monoclinic P21 /c space group [a=6.685(3) A, b=17.657(2) A, c=22.169(l) A, β=92.90(1)°, V=2613(1) A3, Z=4], while the crystal of p-PYNN·p-NNBA-H belongs to the monoclinic P21 In space group[a=12.199(3) A, b=23.265(4) A, c=9.522(2) A, β=101.84(2)° V=2644.9(9) A3, Z=4]. In their crystals, there is an intermolecular hydrogen bond between the nitrogen atom on the pyridyl ring of PYNN and one of the oxygen atoms of the carboxyl group of p-NNBA-H. The intermolecular N…O distances are 2.642(5) A in m-PYNN·p-NNBA-H and 2.613(6) A in p-PYNN·p-NNBA-H, indicating rather intense hydrogen bonds. The magnetic properties of m-PYNN·p-NNBA-H and p-PYNN·p-NNBA-H are well interpreted in terms of the singlet-triplet model with 2J AF /k B =-16.0 K and the Curie-Weiss law with θ =-0.68 K, respectively.

Interplay of magnetism and superconductivity in BETS conductors (BETS= bis(ethylenedithio)tetraselenafulvalene)

Electromagnetic properties of organic superconductors based on BETS [=bis(ethylenedithio)tetraselenafulvalene)] and MX 4 -(M= Fe, Ga; X= Cl, Br) were examined. Based on the results of resistivity measurements, the temperature-pressure-composition phase diagram of λ-(BETS) 2 Fe x Ga 1-x Br y Cl 4-y was determined. Due to the existence of Fe 3+ ions, the crystal of λ-(BETS) 2 Fe x Ga 1-x Cl 4 (x=0.4) exhibiting a superconductor-to-insulator transition showed an unprecedented resistivity behavior under magnetic field. λ-(BETS) 2 Fe 0.4 Ga 0.6 Cl 4 exhibited the successive insulator→superconductor→metal→(field-induced) superconducting transitions with increasing the magnetic field parallel to the conduction plane up to 15 T and the insulator→superconductor→metal transitions for the field Perpendicular to the conduction plane. The magnetoresistance experiments on the first antiferromagnetic organic superconductor, κ-(BFTS) 2 FeBr 4 down to 0.58 K disclosed an onset of field-induced superconducting transition around 12.5 T.

Magnetic Organic Superconductors Based on BETS Molecules--Interplay of Conductivity and Magnetism

Among six BETS (=bis(ethylenedithio)tetraselenafulvalene) superconductors, the systems with FeX 4 m (X=Cl, Br) anions exhibit salient electronic properties due to the coupling between ~ and d electron systems. u -BETS 2 FeCl 4 undergoes successive transitions as, antiferromagnetic insulating phase M metallic phase with ferromagnetically oriented Fe 3+ spins M superconducting phase ( H //conduction plane) M metallic phase, with increasing magnetic field. s -BETS 2 FeX 4 exhibits successive antiferromagnetic and superconducting transitions with decreasing temperature. Resistivity anomaly observed at antiferromagnetic transition temperature indicates an evidence for the coupling between ~ and d electron systems. antiferromagnetic organic superconductor BETS magnetic organic superconductor organic superconductor

Magnetic Conversion Induced by Non-Magnetic Impurities in a Molecular Spin Ladder System, p-EPYNN·[Ni(dmit)2]

We studied the impurity effects on the molecular spin ladder, ( p - N -ethylpyridinium nitronylnitroxide cation (= p -EPYNN; S =1/2))·(bis(4,5-dithiolato-1,3-dithiole-2-thione) nickelate anion (=[Ni(dmit) 2 ]; S =1/2)), that involved a two-leg spin ladder of [Ni(dmit) 2 ] sandwiched by ferromagnetic chains of p -EPYNN, by making a solid solution system p -EPYNN · [Ni(dmit) 2 ] 1- x [Au(dmit) 2 ] x with x ≦0.5. Doping of the non-magnetic [Au(dmit) 2 ] increases Curie defects in the ladder and results in antiferromagnetic behavior of p -EPYNN. This can be explained by the antiferromagnetic domains on the p -EPYNN chain induced by the spin polarization around the impurity site on the [Ni(dmit) 2 ] spin ladder.