E. Dormann

High field magnetic susceptibility of individual single crystals of the organic conductor (fluoranthene)2PF6

Using single crystals of a sufficient size (4–7 mg) and a high quality (about 5 × 10-4 paramagnetic defects per formula unit) we measured the anisotropic static magnetic properties of the quasi-1d organic conductor (FA)2PF6 in external fields of up to 70 kOe. Thus the strong anisotropy of the molecular diamagnetism was determined for the first time. The contribution of the conduction electrons to the static susceptibility of (FA)2PF6 was separated. We analysed the influence of fluctuations (pseudogaps) in the metallic high temperature phase and we derived the temperature dependence of the energy gap in the low-temperature (T < 185 K) semiconducting phase of (FA)2PF6.

One-dimensionality of the organic conductor (FA)+·2PF-6

Photon spin lattice relaxation is analysed in the radical cation salt (FA)2PF6 for 14 MHz ≦ vp ≦ 200 MHz at T = 293 K. From the observed variation of T1 with frequency the time constant τ∗ describing the interruption of the one-dimensional diffusive motion is determined. In our samples, τ∗ -values in the range (24)·10-12 s were derived, depending on sample quality. Analysis of the available data gives a consistent picture of (FA)2PF6 in its high temperature phase as a quasi-1 D organic conductor whose extreme one-dimensionality is spoiled by sample imperfection.

Application of the organic conductor (fluoranthene)⋯+2PF-6 as an ESR magnetic field probe

Abstract Experimental conditions are specified that render possible the application of (FA)⋯ + 2 X - crystals (X = PF 6 , AsF 6 , SbF 6 ,… ) as an ESR magnetic field probe for magnetic flux densities ranging from the earths field to tens of kG with a resolution of at least 0.1 mG.

Single crystal magnetic susceptibility of the quasi-one-dimensional organic conductor (fluoranthene) 2PF6

Abstract Magnetic susceptibility was measured on a bundle of single crystals of difluoranthene phosphor hexafluoride [(FA) 2 ·+ PF 6 − ] with external field parallel or perpendicular to the molecular stacking direction. Except for lowest temperatures, where the paramagnetic contribution of sample defects is of importance, magnetic susceptibility is dominated by the strongly anisotropic molecular diamagnetism. Spin paramagnetism is separated in the metallic and semiconducting phase and compared with magnetic resonance results and predictions of available models.

Quadrupolar interaction of gadolinium nuclei at the cubic sites of ferromagnetic GdAl2

Abstract We show that unresolved quadrupolar splitting is the origin of the large width commonly observed for zero-field NMR lines of Gd on cubic sites in ferromagnetic intermetallic compounds such as GdAl 2 . In contrast to earlier published results, this quadrupolar splitting could be well resolved in GdAl 2 , even in powder samples provided they were appropriately prepared: e 2 qQ/h is 1.48 (1.34) MHz for 157 Gd ( 155 Gd) at 4.2 K. The main potential mechanisms for the quadrupolar splitting are discussed. The observed splitting is broken down into different contributions with the aid of their distinctive dependences on temperature and/or magnetization. We conclude that the largest of them, namely the isotropic relativistic quadrupolar interaction, cannot be disregarded in any metallic host. The results reported should also be taken into account for other compounds, rare-earth isotopes and measurement techniques if accurate results for the hyperfine interactions are to be obtained.

Indirect exchange in intermetallic europium compounds

Abstract Spin echo NMR was measured in EuCu 5 , EuPt 2 , EuPd 2 and related pseudobinary compounds. The hyperfine fields measured were analyzed together with data available in the literature. By comparing the transferred hyperfine field at the Eu site in various intermetallics with the corresponding magnetic ordering temperature we have attempted to determine to what extent non-s conduction electrons are responsible for the indirect exchange interaction in these compounds.

Analysis of the low-temperature phase of the organic conductor (fluoranthene)2PF6 with magnetic resonance

Abstract In order to clear up the electronic structure of the low-temperature phase of the organic conductor (fluoranthene)2PF6, we combined the knowledge available from the electrical conductivity and the static magnetic susceptibility with the information accessible by different magnetic resonance techniques: ESR of defects and conduction electrons (intensity, g-tensor, linewidth, T2, diffusion constant and T1) and proton-NMR (linewidth), all measured for different qualities of (FA)2PF6-single crystals in dependence of the sample orientation, the temperature and the Larmor frequency. Thus we report on comprehensive results for a model system of the organic conductor.

Magnetic properties of MgCo2 studied by means of magnetic dilution and 59Co spin echo NMR

Abstract The compounds MgCo2−2xZn2x form a series of solid solution over the whole concentration range. The crystal structures correspond to hexagonal (C14) when x ⪕ 0.2 and x ⪖ 0.63. In the intermediate range the structure is cubic (C15). Replacement of Co in MgCo2 by Fe is only possible up to 10% Fe. In the first series increasing Zn concentration leads to a gradual reduction of the magnetization and the Curie temperature. In MgCo1.8Fe0.2 the magnetization is similar to that in MgCo2 but the Curie temperature is higher. The substitution of Zn and also of Fe for Co leads to a disappearance of the anomalous reduction of the low temperature magnetization observed in MgCo2. 59Co Spin Echo NMR has been used to determine the magnitude and sign of the hyperfine fields at the two Co sites in MgCo2 and to study the temperature dependence of these Co hyperfine fields. No discontinuity is found in the hyperfine field near the temperature where the bulk magnetization rises steeply. This excludes a possible explanation of the anomaly in terms of changes of the intrinsic Co moments. Furthermore, the intensity of the spin echo signals (NMR enhancement factor η ≈ 70) remains roughly the same within wide limits of the temperature where the anomaly is observed. This refutes an explanation in terms of a ferromagnetic-to-antiferromagnetic transition, proposed earlier. The appearance of a broadened spin echo spectrum (NMR enhancement factor η ≈ 7000) at low temperatures, in addition to the relatively sharp resonances observed also at higher temperatures, has been interpreted as being due to easy nucleation of domain walls at defect sites with a lower magnetic coupling.

NMR analysis of an antiferromagnetic intermetallic rare earth compound

Abstract Zero field NMR of 139La probes in antiferromagnetically ordered GdAg was measured using phase-independent linear spin echo detection. Lineshape analysis gives information about the magnetic structure. 139La NMR of a La0.995Gd0.005Ag single crystal in an external field yields hyperfine field contributions from distinct Gd neighbours, especially of the third nearest one. The comparison of both sets of data allows one to derive a negative sign of the transferred hyperfine field in the antiferromagnetic state. The investigation shows that indirect magnetic interaction via conduction electrons with RE-d character can lead to antiferromagnetic coupling.

Electric field gradient in cubic intermetallic europium compounds with unstable europium valence

Abstract Spin echo NMR measurements in EuPd 2 and EuPt 2 are reported. The europium hyperfine fields were determined and found to be equal to B hf = + (2.08 ± 0.04) T in EuPd 2 and B hf = -(3.33 ± 0.02) T in EuPt 2 . The Eu 153 resonance lines show well resolved quadrupolar structures with a line separation of 0.7 MHz in EuPd 2 and 1.4 MHz in EuPt 2 . Analysis of the NMR results obtained in the isoelectronic pseudobinary series Eu x Ca 1− x Pd 2 , Eu x Ba 1− x Pd 2 , Eu x Ca 1− x Pt 2 and Eu 0.95 Ba 0.05 Pt 2 suggests that the observed quadrupolar interaction is partially related to a not completely divalent character of Eu in these compounds. This is in keeping with thermodynamic arguments given earlier by Miedema.