Development of molecular conducting magnets oriented to π-d interaction control

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1. The electrical resistivities and magnetoresistances (MR) of a radical ion salt (EDO-TTFBr_2)_2MCl_4 (M=Fe,Ga) are investigated under pressures. The resistivity of the FeCl4 salt shows two anomalies due to the successive transitions of the π-electron layer, and another anomaly coming from the long-range magnetic ordering of the FeCl_4 layer. For the GaCl_4 salt negative MR is observed in the low-field region, suggesting a decrease of the SDW gap by applying the magnetic field. The MR of the FeCl_4 salt shows two abrupt increases, which proves the presence of field-induced successive transitions in the composite π-d spin system. The field dependence of the MR of this salt can be qualitatively explained based on the frustrated magnetic structure of π- and d-electron spins interacting with each other.2. The magnetic properties of weak ferromagnets (BDH-TTP)[M(isoq)_2(NCS)_4] (M = Cr, Fe ; isoq = isoquinoline) is investigated. These salts form two-dimensional magnetic sheets, where ferri … More magnetic chains of donor cation radical and anion are antiferromagnetically connected by S…S contacts. Under ambient pressure, both the Cr and Fe salts undergo a weak ferromagnetic transition at T_N = 7.6 K. The application of pressure elevates the transition temperatures up to 16.6 K and 11.6 K at 9 kbar for M = Cr and Fe, respectively. The pressure dependence of the remanent magnetization indicates that the spin canting angle of the Cr salt is reduced due to the increase of antiferromagnetic interaction by applied pressure, in contrast to the Fe salt where the Fe spins are more canted due to the increase of the Dzyaloshinsky-Moriya interaction. The mean-field analysis reveals that the intermolecular exchange interaction increases as the pressure increases, among which the inter-chain anion-anion interaction has the highest pressure sensitivity. This result is consistent with the temperature dependence of the crystal structure showing the thermal contraction in the distances of inter-chain anion-anion S…S contacts is the most remarkable in intra-chain S…S contacts. Less