E. Ogura

Novel TTF-based molecular magnets

Abstract Magnetic properties are discussed for several TTF-based molecular magnets from the viewpoints of π-electron-based low-dimensional magnets and π-d interaction systems. These are summarized with a Mott-Hubbard system with an interplay of magnetism and electron transport (BEDT-TTT) 2 Br·CH 2 (OH)CH 2 OH, a triangular-lattice spin frustration system (C 1 TET-TTF) 2 Br, a π-electron-mediated superexchange magnet BMT-TTFI 2 ·FeX 4 (X=Cl,Br), a 2D π-d interaction system (BEDT-TTF) 3 CuBr 4 , and a metallic magnet (DMET) 2 FeBr 4

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.

Conducting charge-transfer and radical ion salts based on bitetrathiafulvalenes; an approach to organic metals using stoichiometry control

Charge-transfer and radical ion salts of 4,5,4′″,5′″-tetramethylthio-, 4,5,4′″,5′″-bis(ethylenedithio)-, and 4,5,4″′,5″′-bis(ethylenedioxy)-4′,4″-bitetrathiafulvalenes showed fairly high electric conductivities, reflecting the effect of stoichiometry control.

Novel Molecular Magnets Based on Organic Complexes

Abstract The crystal structure and the physical properties of the radical ion salts BMT-TTFI2·FeX4 (X ˭ Br, Cl) and (DMET)2FeBr4 are investigated. The structures of BMT-TTFI2·FeX4 are characterized as the sheets made of dimerized donor molecules and one-dimensional anion chains. These salts show three-dimensional Curie-Weiss behavior accompanied with antiferromagnetic transitions, with the highest Neel temperature (T N= 15 K) among the π-d interaction based magnets for the FeBr4 salt. Within a crystal of (DMET)2FeBr4, the donor molecules form one-dimensional columns, between which magnetic anion sheets are inserted. This salt is metallic down to T MI = 200 K, above which the Curie-Weiss type magnetism is described in terms of donor and anion spins, in spite of the metallic condaction of the salt. Below T MI the magnetism is governed by anion spins, and an antiferromagnetic phase transition takes place at T N=3.5 K.

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.

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.