E. Čižmár

Terahertz-range free-electron laser electron spin resonance spectroscopy: Techniques and applications in high magnetic fields

The successful use of picosecond-pulse free-electron-laser (FEL) radiation for the continuous-wave terahertz-range electron spin resonance (ESR) spectroscopy has been demonstrated. The combination of two linac-based FELs (covering the wavelength range of 4-250 microm) with pulsed magnetic fields up to 70 T allows for multifrequency ESR spectroscopy in a frequency range of 1.2-75 THz with a spectral resolution better than 1%. The performance of the spectrometer is illustrated with ESR spectra obtained in the 2,2-diphenyl-1-picrylhydrazyl and the low-dimensional organic material (C6H9N2)CuCl3.

Magneto-structural correlations in one-dimensional Ni(en)2Pd(CN)4: magnetic properties and redetermination of the crystal structure at two temperatures

Abstract The magneto-structural correlations in Ni(en)2Pd(CN)4 (NEPC) have been studied. The crystal structure is formed by infinite electroneutral 2,2-TT type chains [Ni(en)2NCPd(CN)2CN] (en=1,2-diaminoethane) in which the paramagnetic nickel atoms are bridged by diamagnetic tetracyanopalladate anions. Contrary to the previous room temperature X-ray analysis of NEPC, the results of the current structure determinations at both 293 and 150 K showed the nickel atom to be coordinated in the form of slightly elongated octahedron with two chelate bonded and disordered en molecules in the equatorial plane (mean NiN distance of 2.096 A at both temperatures) and N-bonded bridging cyano groups in the axial positions (NiN distances of 2.117 and 2.108 A at 293 and 150 K, respectively). Consequently, the zero field splitting parameter D is finite and positive. The chains are held together by weak hydrogen bonds of the NH⋯N(C) type. Magnetisation, susceptibility and specific heat measurements indicate that NEPC represents an S=1 Heisenberg magnetic chain characterised by strong planar anisotropy, D/kB≈5 K, weak in-plane anisotropy, E/D=0.1, and by a small exchange coupling constant, J/kB≈0.25 K. These properties are similar to those exhibited by an analogous compound Ni(en)2Ni(CN)4.

Anistropic photoinduced magnetism of a RbjCok[Fe(CN)6]l·nH2O thin film

A magneto-optically active thin film of RbjCok[Fe(CN)6]l·nH2O has been prepared using a sequential assembly method. Upon irradiation with light and at 5K, the net magnetization of the film increased when the surface of the film was oriented parallel to the external magnetic field of 0.1T. However, when the surface of the film was perpendicular to the field, the net magnetization decreased upon irradiation. The presence of dipolar fields and the low-dimensional nature of the system are used to describe the orientation dependence of the photoinduced magnetization. The ability to increase or decrease the photoinduced magnetization by changing the orientation of the system with respect to the field is a phenomenon that may be useful in future device applications.

A family of tri- and dimetallic pyridine dicarboxamide cryptates: unusual O,N,O-coordination and facile access to secondary coordination sphere hydrogen bonding interactions.

A series of tri- and dimetallic metal complexes of pyridine dicarboxamide cryptates are reported in which changes to the base and metal source result in diverse structure types. Addition of strong bases, such as KH or KN(SiMe3)2, followed by divalent metal halides allows direct access to trinuclear complexes in which each metal center is coordinated by a dianionic N,N,N-chelate of each arm. These complexes bind a guest K(+) cation within the central cavity in a trigonal planar coordination environment. Minor changes to the solvent and equivalents of base used in the syntheses of the triiron(II) and tricobalt(II) complexes affords two trinuclear clusters with atypical O,N,O-coordination by each pyridine dicarboxamide arm; the amide carbonyl O atoms are oriented toward the interior of the cavity to coordinate to each metal center. Finally, varying the base enables the selective synthesis of dinuclear nickel(II) and copper(II) complexes in which one pyridine dicarboxamide arm remains protonated. These amide protons are at one end of a hydrogen bonding network that extends throughout the internal cavity and terminates at a metal bound hydroxide, carbonate, or bicarbonate donor. In the dinickel complex, the bicarbonate cannot be liberated as CO2 either thermally or upon sparging with N2, which differs from previously reported monometallic complexes. The carbonate or bicarbonate ligands likely arise from sequestration of atmospheric CO2 based on the observed reaction of the di(hydroxonickel) analog.

Superparamagnetic amorphous iron oxide nanowires self-assembled into ordered layered structures

The thermal decomposition of μ3-oxo trinuclear iron(III) acetate in presence of dodecylamine and oleic acid, in trichloroacetic acid at 320 °C was optimized to obtain iron oxide nanoparticles with pure nanowire morphology. The amount of oleic acid having a surfactant role, beside the ligand exchange for acetate, was used as a control parameter. The size and morphology of the resulted particles were evaluated on the basis of transmission electron microscopy images. Thermal stability and the content of the organic component of the naoparticles were determined by thermogravimetrical analysis. The ordering degree of the organic and inorganic phases constituting the nanoparticles was investigated by wide angle X-ray diffraction, small angle X-ray scattering and polarized optical microscopy. Superparamagnetic state was verified by Mossbauer spectroscopy and the magnetic behavior was studied by SQUID magnetometry. The preservation of the morphology of the particles after complete thermal decomposition was verified by transmission electron microscopy.

Ferromagnetic dimer interactions in Cu2Cl4(CH3CN)2

The magnetic properties of Cu2Cl4(CH3CN)2 are reported. Initially reported by Rundle and Willett [J. Chem. Phys. 40, 838, (1964)], the title compound consists of dimeric Cu(II) ions doubly bridged by two chloride ions. Each Cu(II) ion is further bonded to an additional nonbridging chloride ion and a nonbridging acetonitrile molecule. The dimers stack along the crystallographic a axis to form a ladder structure. Static magnetization measurements indicate the two Cu(II) ions within a dimer interact ferromagnetically with J/kB=40±2 K, while a weak ferromagnetic interaction, J′/kB=0.9±0.1 K, also exists between dimers. Ferromagnetic exchange is unusual for this structural motif and is attributed to the reduction of electronic density on the bridging ions due to the electron-accepting nature of the nitrile group in acetonitrile.

Specific heat study of magnetic excitations in a one-dimensional S=1 Heisenberg magnet with strong planar anisotropy

The results of experimental studies of the specific heat of the magnetic chain compounds Ni(C2H8N2)2Ni(CN)4, Ni(C11H10N2O)2Ni(CN)4, and Ni(C10H8N2)2Ni(CN)4⋅H2O are reported. All compounds are identified as S=1 planar Heisenberg magnetic chains with large planar anisotropy and different values of the in-plane anisotropy constant. The low-temperature specific heat data are interpreted assuming the existence of noninteracting excitons and antiexcitons as elementary excitations from the singlet-ground state. The extended strong-coupling model is used for analysis of the data at higher temperatures. The applicability of the models used with respect to the value of the in-plane anisotropy is discussed.

Nonlinear excitations in CsNiF 3 in magnetic fields perpendicular to the easy plane

Centre de Recherche en Physique du Solide, De ´partement de Physique, Universite de Sherbrooke, Que´bec, Canada J1K 2R1~Received 9 September 2003; revised manuscript received 19 December 2003; published 6 May 2004!Experimental and numerical studies of the magnetic-field dependence of the specific heat and magnetizationof single crystals of CsNiF

[Ni(bpy)(ox)]: a candidate in the class of Haldane gap systems (bpy = 2,2ʹ-bipyridine, ox = oxalate)

[Ni(bpy)(ox)] (bpy = 2,2ʹ-bipyridine; ox = oxalate) was solvothermally prepared in the microcrystalline form and identified by chemical analyses and IR spectroscopy. X-ray powder diffractometry indicates its isostructural character with analogous complexes [M(bpy)(ox)], and thus, its chain-like crystal structure formed of {Ni(bpy)} building units linked by bridging oxalate anions coordinated in a bis(chelate) fashion. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization reveal that the studied compound belongs to the class of systems with a Haldane gap. The estimated magnitude of the gap Eg ≈ 13.4 cm−1 is comparable with those found in archetypal Haldane gap systems. Graphical abstract

Correlating Bridging Ligand with Properties of Ligand-Templated [MnII3X3]3+ Clusters (X = Br–, Cl–, H–, MeO–)

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type Mn3X3L (X = Cl-, H-, and MeO-) where L3- is a tris(β-diketiminate) cyclophane. The tri(chloride) complex (2) is structurally similar to the reported tri(bromide) complex (1) with the Mn3X3 core having a ladder-like arrangement of alternating M-X rungs, whereas the tri(μ-hydride) (3) and tri(μ-methoxide) (4) complexes contain planar hexagonal cores. The hydride and methoxide complexes are synthesized in good yield (48% and 56%) starting with the bromide complex employing a metathesis-like strategy. Compounds 2-4 were characterized by combustion analysis, X-ray crystallography, X-band EPR spectroscopy, SQUID magnetometry, and infrared and UV-visible spectroscopy. Magnetic susceptibility measurements indicate that the Mn3 clusters in 2-4 are antiferromagnetically coupled, and the spin ground state of the compounds (S = 3/2 (1, 2) or S = 1/2 (3, 4)) is correlated to the identity of the bridging ligand and structural arrangement of the Mn3X3 core (X = Br, Cl, H, OCH3). Electrochemical experiments on isobutyronitrile solutions of 3 and 4 display broad irreversible oxidations centered at 0.30 V.