An indication of magnetic-field-induced superconductivity in a bifunctional layered organic conductor,kappa-(BETS)(2)FeBr(4)

Hybrid systems consisting of the conducting layers of organic donor molecules and the magnetic layers of inorganic anions have been focused on as possible bifunctional materials, whose conducting properties can be tuned by controlling the magnetic state of the anion layers on an application of magnetic field. Here we report the magnetoresistance of the antiferromagnetic organic superconductor, kappa-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], consisting of the two-dimensional superconducting layers of the BETS semications and the insulating layers of the FeBr4- anions. Due to the metamagnetic nature of the Fe3+ spin system, characteristic resistivity decrease was observed just below the antiferromagnetic superconductor-to-ferromagnetic metal transition at 1.6 T. Furthermore, an indication of the onsets of the magnetic-field-induced superconductivity was discovered around 12.5 T.     

A metallic (EDT-DSDTFVSDS)2.FeBr4 salt: antiferromagnetic ordering of d spins of FeBr4- ions and anomalous magnetoresistance due to preferential pi-d interaction

The 2:1 salt of a new donor molecule, EDT-DSDTFVSDS with FeBr4- ion, (EDT-DSDTFVSDS)2.FeBr4 showed an essentially metallic behavior despite a small upturn in the electrical resistance below ca. 30 K (electrical conductivities at 290 and 4.2 K are 200 and 170 S cm-1, respectively). The Fe(III) d spins of the FeBr4- ions in this salt were subject to antiferromagnetic ordering at 3.3 K by virtue of a strong pi-d interaction (Jpid) which is comparable to that in a molecular metallic conductor, lambda-(BETS)2.FeCl4, and of a very weak d-d interaction (Jdd). This strong pi-d interaction was evidenced by a large and negative magnetoresistance effect (ca. 20% at 5 T) as well as by the appearance of a large dip in the resistance at the magnetic field (ca. 2.0 T) parallel to the easy axis for the spin-flop transition of the Fe(III) d spins.     

Pi-donor BETS based bifunctional superconductor with polymeric dicyanamidomanganate(II) anion layer: kappa-(BETS)2Mn[N(CN)2]3

Crystals of the bis(ethylenedithio)tetraselenafulvalene (BETS) radical cation salt with dicyanamidomanganate(II) anion, kappa-(BETS)2Mn[N(CN)2]3, were synthesized, which combine conducting and magnetic properties at ambient pressure and are superconducting (Tc approximately/= 5 K) at a moderate pressure of 0.3 kbar.     

Magnetic molecular conductors based on BETS molecules and divalent magnetic anions [BETS = bis(ethylenedithio)tetraselenafulvalene

Several conducting salts based on BETS [where BETS = bis(ethylenedithio)tetraselenafulvalene] molecules and divalent magnetic anions such as the (CoCl(4))(2-), (CoBr(4))(2-), and (MnBr(4))(2-) were prepared. Electrocrystallization by using the (CoCl(4))(2-) anion gave two kinds of crystals. Block-shaped crystals were cleared to be (BETS)(2)CoCl(4), which is an insulator with the high-spin state of cobalt 3d spin. On the other hand, the X-ray crystal structure analysis of a plate-shaped crystal of the (CoCl(4))(2-) salt revealed the system to be kappa-(BETS)(4)CoCl(4)(EtOH), which is metallic down to 0.7 K. The electronic band structure calculation gave a typical two-dimensional cylindrical Fermi surface. However, there is only very weak antiferromagnetic interaction between the S = 3/2 cobalt 3d spins because of its anion-solvent-intermingled layer structure. On the other hand, the electrocrystallization by using the (MnBr(4))(2-) anion yielded the plate-shaped black crystals of the (MnBr(4))(2-) salt. Crystal structure analysis of the (MnBr(4))(2-) salt showed that the salt is theta;-(BETS)(4)MnBr(4)(EtOH)(2) with alternating donor and anion-solvent mixed layers. The stacking direction in one donor layer is perpendicular to those of the neighboring layers. The electrical and magnetic properties of the theta;-(BETS)(4)MnBr(4)(EtOH)(2) salt showed the metallic behavior down to approximately 30 K and the paramagnetism of the high-spin manganese 3d spins. Band structure calculation of this salt gave an elliptical cylindrical Fermi surface. Because the Fermi surfaces of the adjacent donor layers are rotated to each other by 90 degrees, the theta-(BETS)(4)MnBr(4)(EtOH)(2) salt becomes a two-dimensionally isotropic metal.     

Hybrid organic-inorganic conductor with a magnetic chain anion: kappa-BETS2[Fe(III)(C2O4)Cl2] [BETS = bis(ethylenedithio)tetraselenafulvalene

The synthesis, crystal structure, and electrical, optical, and magnetic properties of kappa-BETS2[Fe(III)(C2O4)Cl2], where BETS is bis(ethylenedithio)tetraselenafulvalene, are reported. The black plate crystals consist of parallel donor layers, two per unit cell, displaying a kappa-type packing of BETS(0.5+) within the bc plane and anionic magnetic chains, [Fe(C2O4)Cl2-]n, running along the c axis. It displays metallic behavior down to 4.2 K, and analysis of the optical reflectivity data gives unscreened plasma energies of 0.69 eV (E parallel c) and 0.40 eV (E perpendicular c). The optical anisotropy is larger than that seen for other kappa phases and is described well by transfer integrals obtained from extended Hückel calculations. However, the transfer integrals need to be scaled down uniformly by a factor of 1.21 to reproduce the absolute experimental plasma frequencies. The band structure consists of a one-dimensional (1D) band and a hole pocket, characteristics of kappa phases. The magnetic properties were modeled by the sum of a 1D antiferromagnetic chain contribution from the d spins of Fe3+, a temperature-independent paramagnetic contribution, and a Curie impurity term. At 4.5 K, there is a signature of long-range magnetic ordering to a canted-antiferromagnetic state in the zero-field-cooled-field-cooled magnetizations, and at 2 K, a small hysteresis loop is observed.     

Dual-action molecular superconductors with magnetic anions

Dual-action organic superconductors, whose conducting properties can be sharply controlled by an external magnetic field, have been discovered in systems consisting of organic conduction layers based on bis(ethylenedithio)tetraselenafulvalene (BETS) molecules and magnetic anions. Owing to the metamagnetic nature of the anion layers, the superconducting state of kappa-BETS2FeBr4 can be switched on or off by applying the external field. In lambda-BETS2Fe0.4Ga0.6Cl4, exhibiting a field-induced superconducting transition for the field parallel to the conduction plane, the insulating, metallic, and superconducting states can be realized in a stepwise manner by slightly tuning the external magnetic field.     

Ferrimagnetic ordering due to Fe(III) d and donor pi spins in (ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide)2.FeBr4

Magnetization and heat capacity were measured down to 0.4 K in a 2:1 charge-transfer (CT) salt of a new donor molecule, ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide (1) with a magnetic FeBr(4)(-) ion (1(2).FeBr(4)). The Fe(III) d spins of FeBr(4)(-) ions were subject to apparently ferromagnetic interaction with each other through the interaction with the pi spins developed by localization of the conducting pi electrons on the donor columns, eventually giving rise to ferrimagnetic ordering (FI) near 1 K, which provides the first example in a molecular pi-d system.     

Mixed valence in YBaFe(2)O(5)

YBaFe(2)O(5) has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe(2)O(5) adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO(5) square pyramids separated by Y(3+) ions. At T(N) congruent with 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe(2)O(5) orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions. The magnetic structure is characterized by AFM Fe-O-Fe superexchange coupling within the double layers and a ferromagnetic Fe-Fe direct-exchange coupling between neighboring double layers. Upon cooling below approximately 335 K, a premonitory charge ordering (2Fe(2.5+) --> Fe(2.5+delta) + Fe(2.5)(-delta)) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At approximately 308 K, a complete charge ordering (2Fe(2.5+) --> Fe(2+) + Fe(3+)) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe(2)O(5). The magnetic structure rearranges to a G-type AFM structure, where both the Fe-O-Fe superexchange and the Fe-Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe(2+) and Fe(3+) running along b and an unexpectedly large cooperative Jahn-Teller distortion about the high-spin Fe(2+) ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe(2+) d(xz) orbitals. Comparisons with YBaMn(2)O(5) and YBaCo(2)O(5) are made to highlight the impact of changing the d-electron count.     

Cyano-bridged bimetallic assemblies from hexacyanometalate, [M(CN)6](3-) (M = Mn(III) and Fe(III)), and [M(N4-macrocycle)]2+ (M=Fe(III), Ni(II) and Zn(II)) building blocks. Syntheses, multidimensional structures, and magnetic properties

Reactions between [M(N(4)-macrocycle)](2+) (M = Zn(II) and Ni(II); macrocycle ligands are either CTH = d,l-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane or cyclam = 1,4, 8, 11-tetrazaazaciclotetradecane) and [M(CN)(6)](3-) (M = Fe(III) and Mn(III)) give rise to cyano-bridged assemblies with 1D linear chain and 2D honeycomblike structures. The magnetic measurements on the 1D linear chain complex [Fe(cyclam)][Fe(CN)(6)].6H(2)O 1 points out its metamagnetic behavior, where the ferromagnetic interaction operates within the chain and the antiferromagnetic one between chains. The Neel temperature, T(N), is 5.5 K and the critical field at 2 K is 1 T. The unexpected ferromagnetic intrachain interaction can be rationalized on the basis of the axially elongated octahedral geometry of the low spin Fe(III) ion of the [Fe(cyclam)](3+) unit. The isostructural substitution of [Fe(CN)(6)](3-) by [Mn(CN)(6)](3-) in the previously reported complex [Ni(cyclam)](3)[Fe(CN)(6)](2).12H(2)O 2 leads to [Ni(cyclam)](3)[Mn(CN)(6)](2).16 H(2)O 3, which exhibits a corrugated 2D honeycomblike structure and a metamagnetic behavior with T(N) = 16 K and a critical field of 1 T. In the ferromagnetic phase (H > 1 T) this compound shows a very important coercitive field of 2900 G at 2 K. Compound [Ni(CTH)](3)[Fe(CN)(6)](2).13H(2)O 4, C(60)H(116)Fe(2)N(24)Ni(3)O(13), monoclinic, A 2/n, a = 20.462(7), b = 16.292(4), c = 27.262(7) A, beta = 101.29(4) degrees, Z = 4, also has a corrugated 2D honeycomblike structure and a ferromagnetic intralayer interaction, but, in contrast to 2 and 3, does not exhibit any magnetic ordering. This fact is likely due to the increase of the interlayer separation in this compound. ([Zn(cyclam)Fe(CN)(6)Zn(cyclam)] [Zn(cyclam)Fe(CN)(6)].22H(2)O.EtOH) 5, C(44)H(122)Fe(2)N(24)O(23)Zn(3), monoclinic, A 2/n, a = 14.5474(11), b = 37.056(2), c = 14.7173(13) A, beta = 93.94(1) degrees, Z = 4, presents an unique structure made of anionic linear chains containing alternating [Zn(cyclam)](2+) and [Fe(CN)(6)](3)(-) units and cationic trinuclear units [Zn(cyclam)Fe(CN)(6)Zn(cyclam)](+). Their magnetic properties agree well with those expected for two [Fe(CN)(6)](3-) units with spin-orbit coupling effect of the low spin iron(III) ions.     

Spin-glass behaviour in the coordination polymer [Co(C3H3N2)2]n

The magnetic behaviour of the coordination polymer [Co(C(3)H(3)N(2))(2)](n) has been investigated by magnetization and specific heat measurements. Low-field magnetic susceptibility shows the presence of two maxima at approximately 8 and 4 K (T(f)), which reflect short-range low-dimensional antiferromagnetic behaviour and the existence of a spin-glass-like state, respectively. The latter state was observed by magnetic irreversibility in both the zero-field cooled and field-cooled data, and was also confirmed by specific heat measurements. The magnetic specific heat (C(mag)) shows a lack of any long-range ordered peaks. Instead, a broad maximum near T(f) was observed in the C(mag)(T)/T-curve. Below T(f), the C(mag)(T) data follow the relation: C(mag)(T)/T = gamma + AT. We suggest that the competition of the antiferromagnetic (AF) intra-chain and the ferromagnetic (F) inter-chain interactions in a low-dimensional arrangement of magnetic Co(2+) ions can produce the spin-glass behaviour in the sample. The susceptibility data was analyzed in terms of a spin S = 3/2 Heisenberg linear-chain model with a small exchange energy and is consistent with the presence of both F and AF interactions. The splitting of the crystal field energy levels of the Co(2+) ions causes a Schottky-type specific heat anomaly of around 60 K.     

X-ray single-crystal structure and magnetic properties of Fe[CH(3)PO(3))] x H(2)O: a layered weak ferromagnet

The crystal and molecular structure of the layered weak-ferromagnet Fe[CH(3)PO(3)] x H(2)O has been solved by X-ray single-crystal diffraction techniques. Crystal data for Fe[CH(3)PO(3)] x H(2)O are the following: orthorhombic space group Pna2(1); a =17.538(2), b = 4.814(1), c = 5.719(1) A. The structure is lamellar, and it consists of alternating organic and inorganic layers along the a direction of the unit cell. The inorganic layers are made of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. Each phosphonate group coordinates four metal ions, through chelation and bridging, making in this way a cross-linked Fe-O network. The resultant layers are then separated by bilayers of the methyl groups, with van der Waals contacts between them. The compound is air stable, and it dehydrates under inert atmosphere at temperatures above 120 degrees C. The oxidation state of the metal ion is +2, and the electronic configuration is d(6)( )()high spin (S = 2), as determined from dc magnetic susceptibility measurements from 150 K to ambient temperature. Below 100 K, the magnetic moment of Fe[CH(3)PO(3)] x H(2)O rises rapidly to a maximum at T(max) approximately equal to 24 K, and then it decreases again. The onset of peak at T = 25 K is associated with the 3D antiferromagnetic long-range ordering, T(N). The observed critical temperature, T(N), is like all the other previously reported Fe(II) phosphonates, and it appears to be nearly independent of the interlayer spacing in this family of hybrid organic-inorganic layered compounds. Below T(N), the compound behaves as a "weak ferromagnet", and represents the third kind of magnetic materials with a spontaneous magnetization below a finite critical temperature, ferromagnets and ferrimagnets being the other two types.     

A spin crossover ferrous complex with ordered magnetic ferric anions

The first tetrahaloferrate spin crossover compound, [Fe(Metz)(6)](FeBr(4))(2) (Metz = 1-methyltetrazole), is reported. The FeBr(4)(-) ions form ferromagnetically coupled 1D stacks and exhibit an antiferromagnetic order at 2.2 K, which coexists with the gradual spin crossover centred at 165 K.     

Co5(mu3-OH)2(btec)2(bpp)]n: a three-dimensional homometallic molecular metamagnet built from the mixed hydroxide/carboxylate-bridged ferrimagnetic-like chains

A three-dimensional homometallic complex [Co(5)(mu(3)-OH)(2)(btec)(2)(bpp)](n) is built from the mixed hydroxide/carboxylate bridged cobalt(ii) chains linked by the 1,2,4,5-benzenetetracarboxylate (btec(4-)) anion and 1,3-bis(4-pyridyl)-propane molecule (bpp). Within each chain, two mu(3)-OH-bridged metal triangles connect to each other by sharing a common vertex to give rise to a bow-tie type Co(5)(mu(3)-OH)(2) subunit, which is joined to adjacent subunits by four mu(1,1)-carboxylate bridges to form a step-like metal-oxygen backbone. The magnetic studies revealed that the coexistence of ferromagnetic and antiferrimagnetic interactions resulted in a ferrimagnetic-like behavior of the homometallic chains. Below a critical temperature (T(N) = 12.5 K), bulk antiferromagnetic ordering was observed at low field due to the weak interchain antiferromagnetic interactions. A metamagnetic transition occurred at a magnetic field of ca. 5 kOe at 2 K.     

A unique single carboxylate-bridged spin-frustrated chiral Mn(II) metallatriangle

The reaction between Hmbpymca ligand (prepared in situ from the hydrolysis of 5-methyl-4-cyano-bispyrimidine with NaOH and further neutralization with 2 M HCl) and Mn(ClO(4))(2)·4H(2)O in 1:1 molar ratio afforded the triangulo-trimanganese(II) complex [Mn(3)(bpymca)(3)(H(2)O)(6)]Cl(3)·6H(2)O 1. The chloride anions in this complex come from the HCl used in the neutralization process. The molecular structure of 1 consists of cationic molecular triangles [Mn(3)(μ-mbpymca)(3)(H(2)O)(6)](3+) with C(3) symmetry, chloride anions and crystallization water molecules, all of them involved in an extensive network of hydrogen bonds, leading to a chiral network. Within the [Mn(3)(μ-mbpymca)(3)(H(2)O)(6)](3+) cations, seven-coordinated Mn(II) ions are bridged by both oxygen atoms of the carboxylate groups and exhibit a MnO(5)N(2) compressed pentagonal bipyramidal coordination environment. The temperature dependence of the magnetic susceptibility shows the presence of weak antiferromagnetic interactions between Mn(II) ions mediated by the carboxylate group of the mbpymca ligand and the existence of a 3D antiferromagnetic ordering below 4 K, which has its origin in the AF inter-trimer exchange interactions mediated by the strong hydrogen bonds present in the crystal of 1. The experimental magnetic susceptibility data above 7 K could be satisfactorily fitted to the theoretical analytical expression derived from the spin Hamiltonian H = -J(S(1)S(2) + S(1)S(3) + S(2)S(3)) with J = -0.782(3) cm(-1) and g = 2.092(3). The model predicts a degenerate ground state with an S = 1/2, which is typical of triangular trimetallic spin frustrated systems containing metal with non-integer spins. DFT calculations were performed on the molecular structure as found in the solid state to support the experimental J value and the Mn-O(carb)-Mn as the primarily exchange pathway.     

d-Electron-induced negative magnetoresistance of a pi-d interaction system based on a brominated-TTF donor

A new pi-d interaction system (EDT-TTFBr2)2FeBr4 (EDT-TTFBr2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr2)2GaBr4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D pi-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr4- and GaBr4- salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr4- salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (TN) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the pi-d interaction in the magnetism. The surprisingly strong pi-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting pi electrons through the strong pi-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr4- salt at TN. Below TN, the FeBr4- salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B=15 T), whereas only a small positive magnetoresistance is observed in the pi-electron-only GaBr4- salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the pi electrons by the periodic magnetic potential of the anion d spins in the FeBr4- salt, which is modified by applying the external magnetic field.     

Electrical conducting and magnetic properties of (ethylenedithiotetrathiafulvalenothioquinone-1,3-diselenolemethide)2.FeBr4 (GaBr4) crystals with two different interlayer arrangements of donor molecules

Two donor molecules newly synthesized, dimethylthio- and ethylenedithio-tetrathiafulvalenothioquinone-1,3-diselenolemethides (1 and 2), were used to prepare their charge-transfer (CT) salts with a magnetic FeBr(4)(-) counteranion. For 1, a low electrical conducting 1:1 salt (1.FeBr(4)) was obtained, in which molecules of 1 are tightly dimerized in a one-dimensional (1D) stacking column. On the other hand, 2 gave a 2:1 salt (2(2).FeBr(4)) as two different kinds of plate crystals (I and II). Both I and II possess similar stacking structures of molecules of 2 in each 1D column with a half-cut pipelike structure along the c axis. However, for I, the stacking columns are aligned in the same direction along the a and b axes, while for II they are in the same direction along the a axis, but in the reverse direction along the b axis, resulting in the difference in the relative arrangement of molecules of 2 and FeBr(4)(-) ions between the two crystals. The room-temperature electrical conductivities of the single crystals of I and II were 13.6 and 12.7 S cm(-)(1), respectively. The electrical conducting behavior in I was metallic above 170 K but changed to be semiconducting with a very small activation energy of 7.0 meV in the temperature range 4-170 K. In contrast, II showed the semiconducting behavior in the whole temperature range 77-285 K. The corresponding nonmagnetic GaBr(4)(-) salts with almost the same crystal structure as I and II showed definitively different electrical conducting properties in the metal to semiconductor transition temperature in I as well as in the magnitude of activation energy in the semiconducting region of I and II. The interaction between the d spins of FeBr(4)(-) ions was weak and antiferromagnetic in both I and II, but the magnitude of the spin interaction was unexpectedly larger compared with that in the FeBr(4)(-) salt of the corresponding sulfur derivative of 2 with closer contact between the neighboring FeBr(4)(-) ions. These electrical conducting and magnetic results suggest a significant interaction between the conducting pi electrons and the d spins of FeBr(4)(-) ions located near the columns or layers.     

Lanthanide-ion-tuned magnetic properties in a series of three-dimensional cyano-bridged Ln(III)W(V) assemblies

The reaction of [W(CN)(8)](3-) with Ln(3+) and pyrazine in acetonitrile yielded a series of isostructural compounds formulated as Ln(H(2)O)(4)(pyrazine)(0.5)W(CN)(8) (Ln = La(1), Ce(2), Pr(3), Nd(4), Sm(5), Eu(6), Gd(7)). The Ln(iii) and W(v) centers in the structure are linked through cyanide groups to form two-dimensional (2D) layers, which are further pillared by pyrazine, generating 3D frameworks. The magnetic behavior for compounds 1-7 were driven by the lanthanide ions involved. The Ln(iii) and W(v) ions in compounds 2 and 5 are ferromagnetically coupled with magnetic ordering occurring at 2.8 K, comparable with magnetic ordering with the critical temperature of 1.9 K for compound 4. In addition, the antiferromagnetic interactions were observed in compounds 3 and 7, while no significant magnetic couplings were found in compounds 1 and 6.     

Ferromagnetic ordering of Fe(III) d spins of FeBr4- ions in (ethylenedithiotetrathiafulvalenothioquinone-ethylenedithio-1,3-dithiolemethide) x FeBr4

The 1:1 salt of a new donor molecule, ethylenedithiotetrathiafulvalenothioquinone-ethylenedithio-1,3-dithiolemethide (1), with FeBr4- ion, 1 x FeBr4, was prepared and found to exhibit a room-temperature electrical conductivity of 4 x 10(-2) S cm(-1) and semiconducting behavior with an activation energy of 170 meV. The paramagnetic susceptibility obeyed the Curie-Weiss law with a Curie constant of 4.42 emu K mol(-1) and a Weiss temperature of +3.4 K, and below 15 K, this short-range ferromagnetic interaction increasingly extended to two- and/or three-dimensional interactions, eventually giving rise to a ferromagnetic ordering, whose temperature (TC) was determined to be 1.8 +/- 0.2 K using a resonant circuit method. The magnetic field dependence of magnetization showed that the saturation of magnetization was accomplished at ca. 60 kOe and the saturated value was ca. 5 microB, which is very close to the value obtained only due to Fe(III) (S = 5/2) d spins of one FeBr4- ion.     

Stable metallic behavior and antiferromagnetic ordering of Fe(III) d spins in (EDO-TTFVO)2.FeCl4

We report the crystal structure and physical properties of the 2:1 FeCl4- salt of a new donor molecule, EDO-TTFVO. Crystal structure analysis of this salt revealed that the donor molecules formed a beta' '-type two-dimensional conducting layer, and there is a short S...Cl contact between the donor molecules and the FeCl4- ions, which is expected to mediate a strong pi-d interaction. This salt showed a stable metallic conducting behavior down to 0.3 K and an antiferromagnetic ordering at TN approximately 3.0 K, indicating that this salt becomes a new antiferromagnetic molecular metal at ambient pressure. The appearance of the magnetic ordering is considered to originate from the strong pi-d interactions between the donor molecules and the FeCl4- ions because the field dependence of magnetoresistances was remarkably affected below the antiferromagnetic transition temperature.     

Fe(bpb)(CN)2]- as a versatile building block for the design of novel low-dimensional heterobimetallic systems: synthesis, crystal structures, and magnetic properties of cyano-bridged Fe(III)-Ni(II) complexes [(bpb)(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate

A dicyano-containing [Fe(bpb)(CN)2]- building block has been employed for the synthesis of cyano-bridged heterometallic Ni(II)-Fe(III) complexes. The presence of steric bpb(2-) ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe2 trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]2+ and [Fe(bpb)(CN)2]- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J(NiFe) values (based on the spin exchange Hamiltonian H = -2J(NiFe) S(Ni) (S(Fe(1)) + S(Fe(2))): J(NiFe) = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm(-1), respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N[triple bond]C bond angle, the stronger the Ni- - -Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T(N) = 3.5 K. Below T(N) (1.82 K), a metamagnetic behavior was observed with the critical field of approximately 6 kOe. The present research shows that the [Fe(bpb)(CN)2]- building block is a good candidate for the construction of low-dimensional magnetic materials.