A novel antiferromagnetic organic superconductor kappa-(BETS)(2)FeBr(4) [where BETS = bis(ethylenedithio)tetraselenafulvalene].

The electrical and magnetic properties of kappa-(BETS)(2)FeBr(4) salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (T(N) = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at T(N) clearly shows the existence of the interaction between pi metal electrons and localized magnetic moments of Fe(3+) ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, kappa-(BETS)(2)FeBr(4) is the first antiferromagnetic organic metal exhibiting a superconducting transition below Néel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe(3+) 3d spins in contrast to the cases of the other conventional organic superconductors. Furthermore, the specific heat measurement exhibited a lambda-type large peak of zero-field specific heat corresponding to the three-dimensional antiferromagnetic ordering of high-spin Fe(3+) ions. The lack of distinct anomaly in the C(p) vs T curve at T(c) suggests the coexistence of the superconductivity and the antiferromagnetic order below T(c).     

Colossal magnetoresistance in Mn2+ oxypnictides NdMnAsO(1-x)F(x)

Colossal magnetoresistance is a rare phenomenon in which the electronic resistivity of a material can be decreased by orders of magnitude upon application of a magnetic field. Such an effect could be the basis of the next generation of memory devices. Here we report CMR in the antiferromagnetic oxypnictide NdMnAsO(1-x)F(x) as a result of competition between an antiferromagnetic insulating phase and a paramagnetic semiconductor upon application of a magnetic field. Mn(2+) oxypnictides are relatively unexplored, and tailored synthesis of novel compounds could result in an array of materials for further investigation and optimization.     

Effect of the substituents on the spin coupling between iminosemiquinone pi-radicals mediated by diamagnetic metal ions: l.s. Co(III) vs Ga(III)

Exchange coupling between the radical iminosemiquinone ligands changes from ferromagnetic to antiferromagnetic in l.s. Co(III) complexes, whereas the same coupling in Ga(III) complexes is always antiferromagnetic.     

Syntheses and Magnetic Properties Study of Isostructural BiM(2)BP(2)O(10) (M = Co, Ni) Containing a Quasi-1D Linear Chain Structure

We present here the structures and magnetism of two quasi-1D linear chain compounds of BiM(2)BP(2)O(10) (M = Co, Ni), which were synthesized by traditional solid-state reactions for the first time. Two title compounds crystallize in the monoclinic system with space group P2(1)/c and feature novel 3D structures with a linear chain structure of {MO(6)}(n) further connected by [BP(2)O(10)](7-) anionic groups. The results of magnetic property measurements evidence the antiferromagnetic properties of both compounds in low magnetic field and a field-dependent metamagnetic transition from the antiferromagnetic to ferromagnetic ground state of the BiCo(2)BP(2)O(10) complex.     

X-ray structure of [ReCl4(mu-ox)Cu(pyim)2]: a new heterobimetallic Re(IV)Cu(II) ferrimagnetic chain

A new heterobimetallic Re(IV)Cu(II) compound has been prepared and its crystal structure determined by single-crystal X-ray diffraction; magnetic susceptibility measurements show that this compound behaves as a ferrimagnetic chain with significant antiferromagnetic interactions between Re(IV) and Cu(II) metal ions.     

Spin dimer analysis of the spin exchange interactions in paramelaconite Cu(4)O(3) and its analogue Ag(2)Cu(2)O(3) and the spin ordering of the Cu(2)O(3) spin lattice leading to their magnetic phase transitions

The magnetic structures of the Cu(2)O(3) spin lattices present in Cu(4)O(3) and Ag(2)Cu(2)O(3) were analyzed by studying their spin exchange interactions on the basis of spin dimer analysis. Calculations of spin exchange parameters were calibrated by studying LiCuVO(4) whose intrachain and interchain antiferromagnetic spin exchange parameters are known experimentally. The magnetic phase transition of Cu(4)O(3) at 42.3 K doubles the unit cell along each crystallographic direction. The spin arrangements of the Cu(2)O(3) lattice consistent with this experimental observation are different from conventional antiferromagnetic ordering. Our analysis indicates that spin fluctuation should occur in Cu(4)O(3), low-dimensional magnetism should be more important than magnetic frustration in Cu(4)O(3), and Ag(2)Cu(2)O(3) and Cu(4)O(3) should have similar structural and magnetic properties.     

(BEDT-TTF)3Cu2(C2O4)3(CH3OH)2: an organic-inorganic hybrid antiferromagnetic semiconductor

An organic-inorganic hybrid combining a semiconducting BEDT-TTF layer and a Jahn-Teller distorted oxalato-bridged honeycomb antiferromagnetic layer [Cu(2)(C(2)O(4))(3)(2-)](n) was obtained and characterized.     

A Metamagnetic Two-Dimensional Molecular Material with Nickel(II) and Azide

Bridging azido ligands transmit both ferro- and antiferromagnetic interactions. The sheetlike structured complex [Ni(μ-N(3))(2)(N,N-Et(2)-N'-Me-en)](n) (see picture) exhibits a range of magnetic behaviors at low temperature. Short-range ferromagnetic and three-dimensional antiferromagnetic ordering compete, giving metamagnetic behaviors. N,N-Et(2)-N'-Me-en=N,N-diethyl-N'-methylethylenediamine.     

On the relevance of an antiferromagnetic dimer model for the spin-gapped magnetic solids Cu(terpy)Mo2O7 and Cu(OH)(p-pyc)H2O

The magnetic solids Cu(terpy)Mo2O7 (terpy = terpyridine) and Cu(OH)(p-pyc)H2O (p-pyc = p-pyridinecarboxylate) have a spin gap and possess chains of Cu2+ ions in which two different Cu...Cu distances alternate. On the basis of their reported crystal structures, the spin-exchange interactions of these compounds were examined by performing spin dimer analysis to determine whether an antiferromagnetic dimer or an alternating antiferromagnetic chain model is appropriate for their magnetic properties. Our analysis shows that an antiferromagnetic dimer model is correct for both compounds because of the anisotropic overlap between the magnetic orbitals of their Cu2+ sites.     

Antiferromagnetic Coupling in a Gadolinium(III) Semiquinonato Complex

The strongest antiferromagnetic coupling to Gd(III) so far reported was found for the complex [Gd(Hbpz(3))(2)(dtbsq)] small middle dot2 CHCl(3) (1; Hbpz(3)=hydrotris(pyrazolyl)borate; dtbsq=3,5-di-tert-butylsemiquinonato; see structure). At 245 K the magnetic susceptibility of 1 is lower than expected for two uncorrelated spins of 7/2 and 1/2, and the lowering of chiT with increasing temperature suggests that this is due to antiferromagnetic interaction between Gd(III) and the radical.     

Synthetic, spectral and solution studies on imidazolate-bridged copper(II)-copper(II) and copper(II)-zinc(II) complexes

Synthesis, spectral and solution studies on 2-ethyl imidazolate-bridged (2-EtIm) homo-binuclear copper(II)-copper(II) and hetero-binuclear copper(II)-zinc(II) homologue are described. Magnetic moment values of homo-binuclear complexes indicate that the imidazolate group can mediate antiferromagnetic interactions. Optical spectra of hetero-binuclear complex at varying pH values suggest that the imidazolate-bridged complex is stable over the pH-range 7.15–10.0.     

Ferromagnetic exchange in the ground and (4)A(1) excited states of (Et(4)N)(3)Fe(2)F(9). A magnetic and optical spectroscopic study

The synthesis, crystal growth, magnetic susceptibility, and polarized optical absorption spectra in the visible and near UV of (Et(4)N)(3)Fe(2)F(9) are reported. From single-crystal magnetic susceptibility data and high-resolution absorption spectra in the region of the (6)A(1) --> (4)A(1) spin-flip transition, exchange splittings in the ground and excited states are derived. Ferromagnetic ground and excited state exchange parameters J(GS) = -1.55 cm(-1) and J(ES) = -0.53 cm(-1) are determined, respectively, and the relevant orbital contributions to the net exchange are derived from the spectra. The results are compared with those reported earlier for the structurally and electronically analogous [Mn(2)X(9)](5-) pairs in CsMgX(3):Mn(2+) (X = Cl(-), Br(-)), in which the splittings are antiferromagnetic. This major difference is found to be due to the increased metal charge of Fe(3+) compared to Mn(2+), leading to orbital contraction and thus to a strong decrease of the orbital overlaps and hence the antiferromagnetic interactions.     

One-dimensional mu-chloromanganese(II)-tetrathiafulvalene (TTF) coordination compound

A new tetrathiafulvalene derivative 1 bearing a single pyridine group and its coordination complex 2, with stoichiometry [Mn(mu-Cl)Cl(1)2(CH3OH)]n, have been synthesized and fully characterized. The complex 2 shows an extended chain structure, which is potentially favorable for electrical conductivity. Notably, this is the first monohalogen-bridged Mn(II) polymer exhibiting a moderate antiferromagnetic coupling between the Mn(II) centers.     

A one-pot synthesis of a paramagnetic high-nuclearity nickel(II) cluster: an octadecanuclear Ni(II)16Na(I)2 metal aggregate

An unusual Ni(II)(16)Na(I)(2) cluster which features formate as a bridging clamp between two octanuclear nickel cages is reported; preliminary magnetic studies exhibit paramagnetic low-lying states resulting from dominating antiferromagnetic interactions between the nickel(II) centers.     

Magnetic structure of Ni(DCOO)2(D2O)2

Ni(HCOO)(2)(H(2)O)(2) is a structurally simple coordination polymer showing interesting magnetic phase transitions at low temperature (<16K). Previously published studies of these phase transitions have yielded inconsistent results, questioning the correctness of the published magnetic structure. Here heat capacity and magnetic susceptibility of a fully, a partly and a non-deuterated sample were measured, and they all exhibit magnetic phase transitions around 3 and 15 K. Neutron powder diffraction data was collected on the fully deuterated sample at various temperatures between 1.5 and 25 K. A magnetic model was refined against the neutron diffraction data using a spin system composed of two canted antiferromagnetic sublattices. The magnetic moments of the two sublattices show different magnitude, 1.7 μ(B) and 1.3 μ(B), and the temperature dependence of the magnetic sublattices is quite different. One of the sublattices shows the expected temperature behavior of an antiferromagnetic compound whereas the other sublattice follows a Brillouin like function with a slowly increasing magnetization below the Ne?el temperature.     

1:1 complexes of 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine with manganese(II) and copper(II) hexafluoroacetonylacetonate

Mn(hfac)2 and Cu(hfac)2 form 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine that exhibit strong metal-nitroxide exchange; spin polarization models do not explain the antiferromagnetic exchange behavior between spin sites in these complexes.     

Structure and magnetism of the first strictly dinuclear compound containing paramagnetic 3d and 5f metal ions. Major influence of the Cu(II) ion coordination on the exchange Cu(II)-U(IV) interaction

The dinuclear compound [CuL2(py)U(acac)2] has been synthesized by treating [Cu(H2L2)] with U(acac)4 (L2 = N,N'-bis(3-hydroxysalicylidene)-2-methyl-1,2-propanediamine) and shows the antiferromagnetic Cu-U interaction; the distinct magnetic behaviour of the trinuclear complexes [(CuL2)2U] (antiferromagnetic) and [[CuL1(py)]U[CuL1]] (ferromagnetic) revealed the major influence of the Cu(II) ion coordination on the exchange interaction (L1 = N,N'-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine).     

A novel tetranuclear copper(II) complex with alternating mu 1,1-azido and phenoxo bridges: synthesis,structure, and magnetic properties of [Cu4(mu-salen)2(mu 1,1-N3)2(N3)2

A novel tetranuclear copper(II) complex containing alternating mu(1,1)-azido and monophenoxo bridges has been synthesized and characterized by spectroscopic methods, X-ray single-crystal analysis, and variable-temperature magnetic measurements. The magnetic behavior, investigated in the temperature range 2-300 K, indicates that the interactions between copper ions are antiferromagnetic in nature for both azido and phenoxo bridges. The temperature dependence of the magnetic susceptibility was fitted with J(1) = -12.8 cm(-1), J(2) = -10 cm(-1), g = 2.171, 2.1% paramagnetic component, and negligible temperature-independent paramagnetism (5 x 10(-8)). At variance with the earlier reports of these types of complexes containing a mu(1,1)-azido group, the end-on double-azido-bridged copper(II) center in this complex shows an antiferromagnetic interaction.     

Synthesis, crystal structure, and magnetic properties of oxime-bridged polynuclear Ni(II) and Cu(II) complexes

Two new polynuclear complexes [Ni6(amox)6(mu6-O)(mu3-OH)2](Cl2).6H2O and [Cu3(amox)3(mu3-OH)(mu3-Cl)](ClO4).4H2O (amox- = anion of 4-amino-4-methyl-2-pentanone oxime) have been synthesized and characterized structurally and magnetically. The Ni(II) complex contains a novel Chinese-lantern-like Ni6 cage centered by an oxo ion. It contains the nearest octahedral Ni(II)...Ni(II) separation (<2.8 A) and exhibits strong antiferromagnetic properties. The Cu(II) complex has a cyclic trinuclear copper(II) core bridged by both mu3-OH(-) and mu3-Cl(-) ions. The magnetic susceptibilities of both antiferromagnetic complexes were fitted by using approximate models.     

Structural and magnetic investigations of the mixed-valence Fe(II,III) two-dimensional layer complex, [Fe2(II) Fe2(III)(HCOO)10(C6H7N)6]n.

The structure of the complex, [Fe2(II)Fe2(III)(HCOO)10(C6H7N6)n, (1) exhibits a neutral two-dimensional layer network of alternating iron(II) and iron(III) ions, bridged equatorially by formate groups. All iron atoms are octahedrally coordinated, with iron(III) coordinating axially to one gamma-picoline and one formate group, while the iron(II) centers interact axially with two gamma-picoline groups, above and below the layer plane. The complex crystallizes in the triclinic space group P1 at all studied temperatures [at 120 K, the cell dimensions are: a = 10.228(1), b = 12.071(1), c = 12.072(1) A, alpha = 89.801(2), beta = 71.149(2), gamma = 73.371(2) degrees]. An intralayer antiferromagnetic exchange interaction of J = -2.8 cm(-1) between iron(II) and iron(III) was observed in the magnetic studies. Decreasing the temperature to close to 20 K causes a magnetic-ordering phenomenon to occur and a low-temperature phase with a long-range antiferromagnetic spin orientation appears. The magnetic phase transition was confirmed by M?ssbauer spectroscopic studies at temperatures above and below the critical temperature. Structural information of 1 from synchrotron X-ray diffraction data collected at room temperature and 16 K suggests that the antiferromagnetic ordering is caused by an enhanced pi-pi interaction between chi-picoline groups from adjacent layers.