N. Kaplan

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.

NMR in single crystal metallic ferromagnets: GdAl2

Abstract Necessary conditions to observe NMR in wall-free metallic single crystal ferromagnets are formulated. 27 Al NMR study in a magnetized single crystal GdAl 2 confirms the validity of the preconditions. Improved resolution and control over orientation, both inherent to single crystal study, enable for the first time an angular dependent NMR measurement of hyperfine interaction in metallic ferromagnetic system. The significance of the measured hyperfine tensor A (A χχ /λ h = A yy /λ h = - 14.825 kG ; A zz /gl h = - 15.313 kG ) is briefly discussed.

Anisotropic hyperfine interactions in cubic ferromagnets: EuO

Abstract Quadrupolar splittings of Eu153 are observed in an n.m.r. study of a fully magnetised single crystal sphere of the cubic ferromagnet EuO. The electrifield gradient, q, and the magnetic hyperfine frequency, vL = A 〈S〉/γh are anisotropic, e.g. for M rotation in a 〈100〉 plane: q = qs.i. + q′ + q″ cos22θ, vL = v0 + vL″ cos22θ, with vqs.i. = −305 KHz, vq″ = −291 KHz, vq″ = 97 KHz, v0 = 142.3 MHz and vL″ = 126 KHz. A model calculation based on magnetoelastic crystal distortion predicts the actually observed values of vq′, vq″.

Observations related to hydrogen in powder and single crystal samples of YBa2Cu3O7−δ

Abstract New observations related to hydrogenation of YBa 2 Cu 3 O 7−δ (YBCO) are reported: (a) The effects of sample preparation on the H concentration in “uncharged” YBCO samples is investigated, and it is shown through nuclear magnetic resonance measurements that samples of YBCO prepared by “standard” solid-state reaction procedures may contain ab initio up to 0.2 atoms formula −1 of hydrogen. (b) It is demonstrated that one may introduce up to 0.3 atoms formula −1 into single crystal samples of YBCO without destroying the macroscopic crystal. The significance of the above observations is discussed briefly.

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.

The spin susceptibility of Ni in the ferromagnetic ground state: High field 61Ni Knight shift

Abstract NMR of naturally abundant 61 Ni was used to obtain the high field Knight shift, K = 90 × 10 -4 , in a single crystal sphere of nickel at 1.8 K. In combination with available bulk susceptibility data, the electronic contributions χ p = χ s p + χ d p = 44 × 10 -6 emu mole -1 and χ vv = 76 × 10 -6 emu mole -1 could be separated. The resulting experimental χ p value enabled for the first time a meaningful test of some predictions of basic band ferromagnetism models in Ni, and confirmed the validity of several recent band structure calculations.