Elmar Dormann

Multiple spin echoes for protons in water

Abstract Multiple spin echoes that are generated by the dipolar demagnetizing field have been studied for protons in pure water at Larmor frequencies of 90 and 200 MHz. Their amplitudes were analyzed as a function of the tipping angle, the pulse separation, the strength of the magnetic-field gradient, and the diffusion coefficient (via temperature). The experimental results are compared with recent theoretical predictions.

«Real» single crystals of the organic conductor (fluoranthene)2PF6: an N.M.R. study

Abstract We report on extensive n.m.r. investigations of the quasi-one-dimensional organic conductor (fluoranthene)2PF6. N.m.r. signatures of sample aging and decomposition were established. We analysed the frequency dependence of proton spin-lattice relaxation of individual, well-characterized single crystals. These data could be described quantitatively based on parameters that were independently derived earlier. It is concluded that the intrinsic anisotropy of the conduction electron scattering timesτ⊥/τ| for (fluoranthene)2PF6 must be larger than 3 × 104, i.e., more than one order of magnitude larger than for TTF-TCNQ and its derivatives.

Magnetic properties of spin-labelled polyacetylene

Abstract Polyacetylene has been chemically modified by the incorporation of spin-labelled moieties. The magnetic properties were characterized by static magnetic susceptibility measurements and ESR spectroscopy. In spite of the very low spin concentrations of about 10 −3 per CH unit employed, magnetic spin-spin interactions were observed. Magnetically correlated units consisting at least of pairs of spins with ferromagnetic exchange coupling ( J / k B ⩾ 8 K) were found in galvinoxyl-modified polyacetylene.

Pyroelectric properties for single crystals of the disubstituted diacetylene DNP

Dielectric and pyroelectric properties of single crystals of 1,6-bis(2,4 dinitrophenoxy)-2,4-hexadiyne (DNP) were measured for different crystal orientations and as a function of solid state polymerization. In monomer crystals at Tc = (46.4 ± 0.8)K an anomaly of the dielectric constant was observed with e(Tc)/e(293 K) = 18 ± 3. Below Tc spontaneous electric polarization appears with values up to 1.3 · 10-7Ccm-2 at 10 K. Orientation dependence of these anomalies indicates that point symmetry must be reduced from 2/m of the monoclinic nonpolar high temperature phase to 2, if the low temperature phase is a uniform one. Details of the phase transition and dependence of the electric properties on sample quality were analysed. With increasing amount of polymerization of DNP, the transition temperature Tc is lowered and the size of the dielectric anomaly and of the low temperature electric polarization decreases.

Magnetic properties of new charge-transfer complexes based on porphyrins

Tetra-p-tolyl and tetrathienyl substitution of manganese(III) porphyrins in charge-transfer compounds with organic acceptors like tetracyanoethylene is used to modify molecular packing of ferrimagnetically correlated chains in the solid. Ferri- and antiferromagnetically ordered compounds and also borderline situations are realized. The strong influence of the solvent used for crystal growth is exemplified.

Polypyrrole with free radical-derived counterions

Polypyrrole films have been prepared that contain counteranions derived from stable free radicals, i.e. 1,3,5-triphenylverdazyl (TPV) sulfonic acid or Fremys salt. These polymer films as well as different crystalline molecular TPV derivatives have been characterized by ESR, static magnetic susceptibility and electrical conductivity measurements. Sizable (antiferromagnetic) exchange coupling is observed with asymptotic Curie temperatures amounting to − 12 K < ϑp < −6 K for only the large spin concentrations achieved for TPV and its derivatives.

Magnetic properties of organic free radicals in polyacetylene and polypyrrole

Abstract Polyacetylene and polypyrrole have been chemically modified by incorporation of different organic free radicals. The magnetic properties were characterized by static magnetic susceptibility measurements and ESR spectroscopy. We analyzed the potentialities of indirect exchange interaction between spin labels via the conduction electrons for iodine doped polyacetylene.

Chapter 94 NMR in intermetallic compounds

Publisher Summary Nuclear magnetic resonance (NMR) is a radiofrequency technique using frequencies v rf up to several hundreds of MHz. An NMR spectroscopist has to tackle the problem of the skin depth for electromagnetic radiation in metals. The most frequently adopted procedure of solving the sensitivity problem is, therefore, to use powder samples for the NMR of intermetallic compounds with grain sizes smaller than the skin depth. An annealing treatment after powdering is generally recommended to heal spurious strains to reduce line broadening at least if nuclei susceptible to quadrupolar broadening are studied. Most investigations use pulsed NMR spectrometers. Appropriate instruments for the lower radiofrequency range (3–200 MHz) are commercially available; they have to be equipped with a reliable system for controlling tuning, sensitivity, rf field amplitude, and pulse shape to allow the analysis of broad line profiles or relative intensities. An automatic broadband frequency variable spectrometer working in the frequency range from 10 to 1000 MHz has been introduced recently for the study of hyperfine fields in ferromagnetic materials. The NMR spectrum can be measured in two different modes: by varying the frequency or by varying the magnetic field strength. The latter is generally much easier to perform.

Dielectric properties of the unsymmetrically substituted diacetylenes NP/4-MPU and TS/FBS

Abstract The permittivities of monomer single crystals of the unsymmetrically substituted diacetylenes NP/4-MPU and TS/FBS were measured as a function of temperature (10–300 K) in the respective crystal orientations and values partly larger than 10 were obtained. The polar modification of NP/4-MPU single crystals cannot be polymerized in the solid state whereas the TS/FBS single crystals can be completely converted to polymer single crystals. The temperature dependences of the spontaneous polarization and of the permittivities were measured for different polymer contents.

The magnetic properties of an organic conducting crystal

Abstract The magnetic properties of single crystals of the organic conductor (Fluoranthene) 2 °+ PF 6 − have been analysed by static magnetic susceptibility measurements, ESR, NMR and Overhauser shift technique. Anisotropic molecular diamagnetism, isotropic temperature-dependent conduction electron spin paramagnetism and sample-dependent contribution of paramagnetic defects can be distinguished in the static susceptibility. Defects are of decisive importance for the electron spin dynamics in this highly-1D conductor. They lead to pronounced orientation-, frequency- and temperature-dependences of the electron-spin relaxation times and to a sample dependence of the low-frequency proton spin-lattice relaxation time.