Junichi Nishijo

Chromium acetylide complex based ferrimagnet and weak ferromagnet.

The crystal structures and magnetic properties of new molecule-based magnets, [CrCyclam(C[triple bond]C-3-thiophene)(2)][Ni(mdt)(2)] (1) and [CrCyclam(C[triple bond]C-Ph)(2)][Ni(mdt)(2)](H(2)O) (2) (Cyclam = 1,4,8,11-tetraazacyclotetradecane, mdt = 1,3-dithiole-4,5-dithiolate), are reported. The crystal structures of both compounds are characterized by ferrimagnetic chains of alternately stacked [CrCyclam(C[triple bond]C-R)](+) cations and [Ni(mdt)(2)](-) anions with intrachain exchange interactions of 2J = -6.1 K in 1 and -5.7 K in 2 (H = -2J Sigma(i) S(i) x S(i+1)). The material 1 exhibits ferrimagnetic transition at 2.3 K owing to weak interchain antiferromagnetic interactions between cations and anions. In the case of 2, cations in adjacent ferrimagnetic chains are bridged by a water molecule, resulting in an interchain antiferromagnetic coupling. Despite a centrosymmetry of a whole crystal of 2, one bridging water molecule occupies only one of the two centrosymmetric sites and breaks a local centrosymmetry between adjacent cations. The interchain antiferromagnetic interaction and Dzyaloshinsky-Moriya interaction originated from the local symmetry breakdown of 2 bring a weak-ferromagnetic transition at 3.7 K with a coercive force of less than 0.8 mT, followed by the second magnetic phase transition at 2.9 K. Below this temperature, the coercive force rapidly increases from 1 to 11.8 mT as the temperature decreases from 2.9 to 1.8 K, while the remanent magnetization monotonically increases from 0.008 mu(B) at 3.6 K to 0.12 mu(B) at 1.8 K.

Increased Electric Conductance through Physisorbed Oxygen on Copper Nanocables Sheathed in Carbon

Adsorption of molecules onto solid surfaces can be classified into physisorption and chemisorption. Physisorbed molecules are so weakly bound to surfaces that adsorption and desorption can proceed reversibly even at room temperature. By contrast, chemisorption is accompanied by chemical bond formation, and higher temperatures are necessary for desorption. Solid gas sensors are normally based on chemisorption for modification of the electronic band conduction. We found that copper nanocables sheathed in carbon can detect physisorbed oxygen at room temperature by just measuring electric resistance. The sensing principle based on hopping conduction is specific to nanomaterials and enables detection of physisorbed oxygen.

π–d Interaction-based molecular magnets

Abstract The crystal structures and physical properties of molecular magnets developed in our group are reviewed. (1) (DMET) 2 FeBr 4 and its analogues are composed of alternating stacks of quasi-one-dimensional donor sheets and square lattice magnetic anion sheets. These salts undergo an SDW transition of the donor layer and an antiferromagnetic transition of Fe 3+ spins on the anion layer. The one-to-one correspondence of the anomalies appearing on the magnetization curves and the magnetoresistance supports the presence of the π–d interaction. On applying pressure, a large negative magnetoresistance is observed for the all-sulfur compound (EDTDM) 2 FeBr 4 in the marginal region of the SDW and metallic ground states. (2) (BDH-TTP)[M(isoq) 2 (NCS) 4 ] (M=Cr, Fe) show bulk weak ferromagnetism at 7.6 K. The donor cation radicals ( S =1/2) and anions ( S =3/2 (Cr), 5/2 (Fe)) form ferrimagnetic chains with close intermolecular S⋯S contacts, which are then antiferromagnetically coupled through the π – π overlap of the ligands and inter-chain S⋯S contacts of the donors. The non-collinear alignment of the molecular axes of adjacent anions is responsible for the canted spin structure.

π-d interaction-based molecular magnets in TTF-type salts

Structure, transport properties and magnetism of the following π- d systems based on TTF-type salts with magnetic ions are investigated. i) C 1 TET-TTF·FeBr 4 : The magnetic anions form zigzag chains, whose property is described in terms of triangular-based ladder systems accompanied with weak ferromagnetism. Its magnetic properties are affected by the sulfur-to-selenium or bromine-to-chlorine substitution. ii) (EDO-TTFI 2 ) 2 M(mnt) 2 (M=Ni, Pt): Both donor and anion molecules form segregated one-dimensional columns, which are connected via short -CN···I- contacts. These salts show the coexistence of metallic conductivity on the donors and ferromagnetic interaction between localized moments on the anions. iii) (DMET) 2 FeBr 4 : The donor molecules form one-dimensional column with metallic conductivity, whereas the magnetic anions form square-lattice layers and show antiferromagnetic transition. The coincidence of the anomalies on magnetization curves and magnetoresistance supports the presence of π- d interaction between these two layers.

Novel Magnetism of EDO-TTFX2 Salts (X=Br, I)

Charge transfer complexes of EDO-TTFX, (X=Br, I) form low-D structures featured with coordination-like-bond formation of X with counter anions, giving unconventional magnetic conductive systems with magnetic anions. (EDO-TTFI 2 ) 2 M(nmt) 2 (M=Pt. Ni) are 1D conductors interacting with ferromagnetic ID M(mnt) 2 chains. Applying pressure gives a unique ferromagnetic domain formation. 2D (EDO-TTFBr 2 ) 2 FeBr 4 is metallic, where ID FeBr 4 chains take an antiferromagnetic transition at a very high T N (=13.5K) with a short-range-order effect around the resistivity minimum (30K), suggesting the presence of strong π-d interaction.

Sintering of Soft Magnetic Material under Microwave Magnetic Field

We have developed a simple process for sintering of soft magnetization materials using microwave sintering. The saturated magnetization (Ms) of sintered magnetite was 85.6 emu/g, which was as high as 95% of magnetite before heating (90.4 emu/g). On the other hand, the averaged remanence (Mr) and coercivity (Hc) of the magnetite after heating were 0.17 emu/g and 1.12 Oe under measuring limit of SQUID, respectively. For the sintering process of soft magnetic materials, magnetic fields of microwave have been performed in nitrogen atmosphere. Therefore, a microwave single-mode system operating at a frequency of 2.45 GHz and with a maximum power level of 1.5 kW was used. We can sinter the good soft magnetic material in microwave magnetic field. The sample shrank to 82% theoretical density (TD) from 45%TD of green body. The sintered sample was observed the microstructure by TEM and the crystal size was estimated the approximate average size is 10 nm.

Magnetism in New Classes of TTF-Based Charge Transfer Complexes

Charge transfer (CT) complexes are promising for developing molecule-based magnets, where ~ -electrons play an important role. New classes of molecular magnets are presented in the TTF-based CT complexes from points of unconventional magnetism. ~ -d interaction in d-electron-spin-incorporated CT complexes give anomalous effect of magnetic transition on the electron transport. The coexistence of metallic conduction and localized spin feature appears in the system located in the metal-insulator boundary. 1D metal having ferromagnetic interaction and triangle-based spin ladder system are also obtained from CT complexes. organic metal molecular magnet ~ -d interaction metal-insulator transition spin ladder ferromagnet

Helical Nanostructure of Achiral Silver p-Tolylacetylide Molecules

Silver p-tolylacetylide is an achiral molecule; however, its nanostructure has been found to consist of twisted nanoribbons. The twisted ribbon is a helicoid that combines translation and perpendicular rotation along the ribbon axis. A helix, a typical chiral structure, can be created by the aggregation of achiral molecules, and the recrystallization conditions control the twist of the nanoribbons. Therefore, the recrystallization controls the chirality.

Unconventional Properties of TTF-Based Organic Magnetic Conductors

Unconventional magnetic properties of various TTF-based π-d interaction systems are presented. (D)2FeBr4 (D=DMET, EDTDM) are quasi-2D metals consisting of alternating stacking of donor π-electron conducting sheets and square lattice d-spins of FeBr4 - anion sheets. The magnetoresistance is strongly affected by changes of Fe3+ spin arrangement in the ordered state. Especially, for (EDTDM)2FeBr4, a large negative magnetoresistance appears in the vicinity of an MI transition. In (EDO-TTFBr2)2FeX4 (X=C1, Br) having similar sandwich structure to (D)2FeBr4, the presence of the Br atom of the donor bonded semicovalently to X atom of FeX4 - gives strong π-d interaction, which produces a strong correlation between the electron transport and magnetism in addition to a high Neel temperature with a complicated spin structure. (EDO- TTFI2)M(mnt)2 (M=Ni, Pt) are featured by a combination of 1D metal of EDO-TTFI2 columns and 1D ferromagnetic chain of M(mnt)2. For M=Pt, weak AF inter-chain interaction brings about a metamagnetic feature. The application of pressure enhances ferromagnetic features with an anomalous hysteretic hump.