A. Grayevsky

ESR in new high temperature superconductors

Abstract We report on ESR experiments on the 90K superconductor YBa 2 Cu 3 O 9−y . We observed the spectrum at X band as function of temperature down to 10K. At room temperature a weak line was observed which, on cooling, disappears around 60K. The striking result is the appearance of a broad Dysonian line below about 55K whose intensity grows exponentially with decreasing temperature. In samples doped with 1% of Gd the room temperature Gd line is split by the crystal field and narrowed by the Koringa mechanism. It again disappears at about 60K. Below 55K the spectrum is similar to that observed in the undoped sample. Some tentative interpretation regarding the magnetic behaviour in the superconducting state is offered.

Macroscopic and microscopic kinetics of hydrogen in magnesium-rich compounds

An overview of the kinetic properties in magnesium-rich systems is proposed in this report and our results on “macroscopic” and “microscopic” diffusion in MgH2 and in a mixed system of 12MgH2 + LaH2.8 are presented. We have obtained the diffusion coefficient D of the hydrogen in the MgH2 phase using nuclear magnetic resonance and note that this coefficient is nearly independent of temperature in the range 300 K < T < 600 K. In contrast, the macroscopic hydrogen absorption shows different behaviour at different temperatures and the absorption is much slower in pure magnesium than in the mixed hydride. A consistent approach which accounts for all the above observations is presented.

Magnetic excitations in single crystal PrNi5

Abstract Inelastic neutron scattering has been employed to determine the crystal-field and exchange interaction in PrNi 5 . The observed energy spectra exhibit a sufficient number of well resolved crystal-field transitions, so that we have been able to unambiguously determine the four crystal-field parameters required for hexagonal symmetry. From the dispersion behavior of the lowest lying ground-state transition we determined both the nearest- and next-nearest neighbor exchange parameters to be ferromagnetic. The results allow to calculate the anisotropic exchange enhanced static susceptibility which agrees closely with corresponding experimental data. These observations are at variance with recent μSR-results which suggest that around the muon probe a large part of the susceptibility is associated with the Ni-ions.

Evidence for a background neutron enhanced fusion in deuterium absorbed palladium

Abstract Experimental evidence for a correlation between the background neutron flux and the fusion reaction rate in deuterium charged palladium is demonstrated. A reaction rate of 10−24 per sec per deuterium atom is obtained under uncontrolled background conditions. The same specimen does not exhibit the fusion reaction under low background conditions. The reaction rate is accelerated when the specimen is irradiated by an external neutron source. Similar effect is obtained from a compressed deuterium gas. A recoiled deuteron mechanism may explain the result in the gas phase. The application of this model to the metal-deuterium system yields a discrepancy of three orders of magnitude between theory and experimental observation of the reaction rate.

Mapping the dynamic properties of electron spins in the organic conductor (FA)2PF6

Abstract Pulsed ESR is used for microscopic three-dimensional imaging of the conduction electron spin properties in the organic crystal (FA) 2 PF 6 (FA: fluoranthene). The measurements enable a study of the local properties of the electron spins on the scale of ∼30xa0μm. The ‘local’ aspect of the information gained is found to be important, as the images generally exhibit pronounced inhomogeneity. The properties investigated so far are spin relaxation times and mobility. The relaxation times, T 1 and T 2 , were found each to vary by a factor of ∼2 within the samples, and their variations are highly correlated with one another. The spin diffusion might be varying too, in negative correlation with the relaxation times. Viewing the relaxation maps, surface effects are also discernible in some of the samples.

Restricted electron motion in 1D organic conductors: PGSE-ESR in (PE)2PF6 and (FA)2PF6

Abstract The pulsed-gradient spin echo electron spin resonance technique has been used to study the restricted motion of the conduction electrons in the one-dimensional organic conductors (PE) 2 PF 6 and (FA) 2 PF 6 . Data were collected as a function of temperature, between room temperature and the Peierls transition temperature. Improved signal-to-noise ratio and reduced eddy currents were achieved. Echo attenuation for (PE) 2 PF 6 was found to fit a model assuming diffusional motion between scattered, totally reflecting, barriers. The free diffusion constant and average barrier distance were successfully best fitted simultaneously for all temperatures, and the temperature dependence of the diffusion constant was found to scale with the previously measured AC conductivity. In contrast, PGSE results for the (FA) 2 PF 6 crystals do not fit the above model, possibly due to the existence of basically different restricting factors.

Thermal desorption spectra of hydrogen from LaNi5

Abstract Thermal desorption of hydrogen from LaNi 5 H x shows spectra which consist of a single peak for x ≲ 6 and develop into a two-peak structure for x ≈ 6.7. The desorption of hydrogen starts at a low temperature of about 160 K and terminates at about 420 K. Analysis of the spectra leads to a conclusion that the rate-determining step of the desorption is a diffusion process for temperatures lower than about 175 K and either a diffusion or a phase transformation for higher temperatures. This is dissimilar to the PdH x system, where the desorption that occurs in the temperature range 150–380 K is a surface-controlled process. The activation energy obtained for hydrogen desorption from the β phase of the LaNi 5 H x system was 32 ± 4 kJ (mol H 2 ) −1 .

Relaxation and exchange in a nuclear cooling agent: NMR and EPR in single crystal PrNi5☆☆☆

Abstract NMR of Pr and ESR of Gd in single crystals of PrNi5 are observed to exhibit a dramatic increase of the resonance linewidth with temperature in the range 1.2K ⩽ T ⩽ 10K. The observations are interpreted, within the framework of a crystal field spin Hamiltonian model calculation, in terms of low frequency fluctuations of the Pr ions induced by Pr-Pr exchange coupling. A fitting procedure of the theory to the experimental data enabled us to extract the various Pr-Pr and Pr-Gd exchange parameters. The former yield independent information concerning the lowest temperature that can be achieved in nuclear cooling experiments.

Nuclear magnetic resonance and magnetization studies in Ti4Fe2O0.4Hx

The proton relaxation times T1, T2 and T2∗ were measured in the Ti4Fe2O0.4Hx system (0.64 < x < 4.80) for temperatures in the range 100–430 K. On the basis of these results, and in accord with the crystal structure of the hydride and magnetic susceptibility measurements, a self-consistent approach, which describes the hydrogen diffusion process and the magnetic behaviour of the system, is presented.

High temperature hydride tank using MmMg12 compounds (Mm ≡ misch metal)☆

Abstract The design and performance of a 1 kg MmMg 12 hydride tank (Mm ≡ misch metal) is described. It operates at temperatures of 250–400 °C using electrical heaters. Provisions were made to operate it using hot gas. The desorption rate is a function of the heat supply and at 200 W it can supply 12 1 min −1 . Cycling measurements performed on a smaller tank with a similar design containing 0.17 kg MmMg 12 showed that the absorption rate and the total capacity were not affected by cycling up to 70 cycles. However, the desorption rate decreased by 50%. We suggest that the discrepancy between the deterioration of the desorption rate compared with almost no effect on the absorption rate and capacity as a function of cycling is due to the decrease in the heat conductivity of the hydride. Such a decrease is expected as the grain size of the MmMg 12 decreases with cycling.