Kengo Enomoto

Magnetic and electronic properties of (DMET)2FeBr4

(DMET) 2 FeBr 4 is an organic conductor whose structure consists of an alternate stacking of ID chain-based donor layers and magnetic Fe 3+ (S=5/2) square lattices. The electronic structure of the π-electrons is metallic with a Ml transition at T MI ∼40K, while Fe 3+ spins undergo an antiferromagnetic transition at T N =3.7K. The magnetization curves which show field-direction-dependent anomalies in addition to a spin-flop transition arc demonstrated to have a participation of donor π-electron spins in the magnetization processes. The magnetoresistances snow a strong correlation with the detailed behaviour of the magnetizations, suggesting an important role of the interaction between π- and d-electrons existing on adjacent layers.

Electronic and Magnetic Properties of π-d Interaction System: (EDTDM) 2 FeBr 4

(EDTDM) 2 FeBr 4 based on π-d interaction system consists of quasi one-dimensional donor columns and square-lattice magnetic anion sheets (Fe 3+ (s=5/2)). It behaves metallic at high temperatures and undergoes a metal-insulator transition at T MI ∼15-2OK, where the low temperature insulating phase is considered to be magnetic ordered state. The magnetic susceptibility, which is governed by Fe 3+ d -spins, obeys the Curie-Weiss law in the high-temperature regime. Below 50K, the susceptibility deviates from the Curie-Weiss law and shows an antiferromagnetic transition at T N ∼3K. The anomalous magnetization curves indicate an important role of π- d interaction in the magnetic behavior.

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 π-d interaction system (DMET)2FeCl4

The structure of (DMET) 2 FeCl 4 is featured with an alternate stacking of quasi-1D chain-based donor sheets and magnetic Fe 3+ (S=5/2) sheets. At ambient pressure, the salt shows metallic behavior down to ∼100K with the presence of a resistivity anomaly around 7K. at which the susceptibility has a broad maximum of magnetic short-range ordering in the Fe 3+ magnetic sheet of d-electrons. An antiferromagnetic transition takes place at T N =2.8K. The large Weiss temperature Θ = -11K and the field dependent anomalies in the magnetoresistance, which indicate the presence of a strong antiferromagnetic interaction, suggest that the donor π-electrons mediate the interlayer interaction between the Fe 3+ magnetic layers.

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

Submillimeter and millimeter wave ESR measurements of (DMET)2FeBr4 below TN

Submillimeter and millimeter wave ESR measurements of (DMET) 2 FeBr 4 , which has π-d interaction between quasi 1D π-electrons and localized d-electrons, have been performed at 2.1K. Frequency-field relation below the saturation field B=7T agrees with antiferromagnetic resonance (AFMR) modes with the uniaxial anisotropy. However, AFMR modes above B,=7T for B//a and B//c do not follow the AFMR modes with the same uniaxial anisotropy. As the anomalies in the field dependence of g-value coincide with the previous magnetoresistance measurement, the ESR result also suggests the existence of strong π-d interaction.

Millimeter Wave ESR Measurements of (DMET) 2 FeBr 4

We have performed millimeter wave ESR measurements of (DMET) 2 FeBr 4 single crystals, which consist of electric quasi 1D chain-based donor layers and magnetic Fe 3+ square lattices, in the temperature region from 1.8 K to 70 K. We observed the splitting of absorption line at 130 GHz, and the estimate the exchange interaction between d -electrons and ~ -electrons. The temperature dependence of ESR at low temperature is discussed in connection with the conventional antiferromagnetic resonance (AFMR). DMET ~ - d system high field ESR

π- d Interaction-Based Molecular Magnets: Role of Sulfur-to-Selenium Substitution

Abstract The crystal structure and physical properties of the three conducting molecular magnets are discussed. (DMET)2FeBr4 is composed of alternating stacks of quasi-one-dimensional donor sheets and square lattice magnetic anion sheets. This salt undergoes an spin density wave (SDW) transition of the donor layer at 40 K and an antiferromagnetic transition of Fe3+ spins on the anion layer at 3.7 K. The one-to-one correspondence of the anomalies appearing on the magnetization curves with those on the magnetoresistance supports the presence of the π-d interaction. The all-sulfur analog (EDTDM)2FeBr4 shows, besides similar behaviors as the DMET salt, insulator-to-metal transition of the ground state by applying the pressure, accompanied with a large negative magnetoresistance. (EDS-TTF)2FeBr4 shows little π-d interaction despite the presence of close Se–Br contacts, showing the importance of the intermolecular orbital overlap between the π- and d-components.

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.