Abstract: Data of 23Na NMR spectra and relaxation measurements are interpreted as suggesting that, upon increasing temperature, the Na layers in Na0.8CoO2 adopt a 2D liquid state at T=291 K. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K, the 23Na NMR response is similar to that previously observed in superionic conductors with planar Na layers.
Abstract: Results of 27Al-NMR measurements on the heavy-electron compound CeAl3, recorded in low magnetic fields (of the order of 1 kG), very low temperatures View the MathML source, and at hydrostatic pressures up to 5.6 kbar are presented. We observe drastic changes of the NMR spectra and of the spin-lattice relaxation rates View the MathML source with increasing pressure. For View the MathML source, the 27Al spectrum is rather broad and complicated, but the situation changes significantly at higher pressures. The spectra narrow rapidly and can be reproduced by simulation calculations using reasonable parameters. The spin-lattice relaxation rate drops by almost two orders of magnitude upon increasing the pressure up to 5.6 kbar, indicating a substantial decrease of the density of electronic states at the Fermi energy. The data suggest a simple paramagnetic ground state for View the MathML source, without any evidence of magnetic order above 65 mK.
Abstract: We report the results of a 63,65Cu NMR/NQR study probing the intermetallic compound PrCu2. The previously claimed onset of magnetic order at 65 K, indicated in a muon spin resonance study, is not confirmed. Based on our data, we discuss different possible reasons for this apparent discrepancy, including a non-negligible influence of the implanted muons on their environment. Competing dipolar and quadrupolar interactions lead to unusual features of the magnetic-ion/conduction-electron system, different from those of common intermetallics exhibiting structural or magnetic instabilities.
Abstract: We report a study of the low-temperature high-pressure phase diagram of the intermetallic compound PrCu2, by means of molecular-field calculations and 63,65Cu nuclear-quadrupole-resonance (NQR) measurements under pressure. The pressure-induced magnetically-ordered phase can be accounted for by considering the influence of the crystal electric field on the 4f electron orbitals of the Pr3+ ions and by introducing a pressure-dependent exchange interaction between the corresponding local magnetic moments. Our experimental data suggest that the order in the induced antiferromagnetic phase is incommensurate. The role of magnetic fluctuations both at high and low pressures is also discussed.
Abstract: Pressure-induced variations of 27Al NMR spectra of CeAl3 indicate significant changes in the ground-state characteristics of this prototypical heavy-electron compound. Previously reported magnetic and electronic inhomogeneities at ambient pressure and very low temperatures are removed with external pressures exceeding 1.2 kbars. The spectra and results of corresponding measurements of the NMR spin-lattice relaxation rates indicate a pressure-induced emergence of a simple paramagnetic state involving electrons with moderately enhanced masses and no magnetic order above 65 mK.
Abstract: We present 125Te NMR measurements on CeTe powder at temperatures between 1 and 150 K and in magnetic fields between 5 and 8 T. CeTe is a rocksalt-type intermetallic compound. It orders antiferromagnetically at View the MathML source with a much reduced ordered moment [H.R. Ott, J.K. Kjems, F. Hulliger, Phys. Rev. Lett. 42 20 (1979) 1378]. From our low-temperature NMR spectra we infer the presence of at least three inequivalent Te sites at low temperatures. Considering the crystal structure this result is completely unexpected. The linewidths and the Knight shifts of the individual lines are significantly different and increase substantially with decreasing temperature. They follow the temperature dependence of the magnetic susceptibility above 20 K. Above TN, hyperfine fields of 1.6, 0.8 and 0.0 T at the three Te sites per Bohr magneton of Ce moment are deduced from Knight shift vs. magnetic susceptibility data. These values are typical for transferred hyperfine fields via conduction electrons.
Abstract: We present the results of 115In-NQR measurements of CePd2In under hydrostatic pressure up to 14.6 kbar. At zero external pressure this compound exhibits antiferromagnetic order below View the MathML source, involving the alignment of very small Ce moments. This unusual ground state is believed to be the result of a competition between Kondo screening and RKKY interaction involving the conduction electrons and the localised 4f-electrons of the Ce3+ ions. From our NQR spectra, we observe a pressure-induced increase of TN between ambient pressure and 14.6 kbar, such that View the MathML source.
Abstract: Previous investigations of the compound Na0.5CoO2 gave evidence for a metal–insulator transition at View the MathML source upon cooling, and another transition at View the MathML source whose nature has not been clarified yet. We report the results of NMR measurements on a powder sample of Na0.5CoO2. They include the mapping of 23Na and 59Co spectra and the evaluation of the 23Na NMR spin-lattice relaxation rate View the MathML source in the temperature range between 10 and 305 K. The NMR data reflect the transition at TX very well but give little evidence of the metal–insulator transition at TMI. A prominent peak in View the MathML source at TX is accompanied by an abrupt broadening of both the 23Na and 59Co spectra upon decreasing temperature. The shape of the spectra below TX implies the formation of a staggered internal field below TX.
Abstract: Based on experimental 59Co-NMR data in the temperature range between 0.1 and 300 K, we address the problem of the character of the Co 3d-electron based magnetism in Na0.7CoO2. Temperature-dependent 59Co-NMR spectra reveal different Co environments below 300 K and their differentiation increases with decreasing temperature. We show that the 23Na- and 59Co-NMR data may consistently be interpreted by assuming that below room temperature the Co 3d electrons are itinerant. We also argue that the 59Co-NMR response is inconsistent with well-defined local magnetic moments on the Co sites. We identify a substantial orbital contribution chiorb to the d-electron susceptibility. At low temperatures chiorb seems to acquire some temperature dependence, suggesting an increasing influence of spin-orbit coupling. The temperature dependence of the spin-lattice relaxation rate T<sub>1</sub><sup>-1</sup>(T) confirms significant variations in the dynamics of this electronic subsystem between 250 and 300 K, as previously suggested. Below 100 K, Na0.7CoO2 may be viewed as a weak antiferromagnet with TN below 1 K, but this scenario still leaves a number of open questions.
Abstract: We report the results of 87Rb NMR measurements on RbOs2O6 , a member of the family of the superconducting pyrochlore-type oxides with a critical temperature Tc=6.4 K . In the normal state, the nuclear spin-lattice relaxation time T1 obeys the Korringa-type relation T1T=const and the Knight shift is independent of temperature, indicating the absence of strong magnetic correlations. In the superconducting state, T1−1(T) exhibits a tiny coherence enhancement just below Tc , and decreases exponentially with further decreasing temperatures. The value of the corresponding energy gap is close to that predicted by the conventional weak-coupling BCS theory. Our results indicate that RbOs2O6 is a conventional s -wave-type superconductor.
Abstract: We report on measurements of the magnetic susceptibility χ(T) and 23Na nuclear magnetic resonance (NMR) on polycrystalline samples of Na0.7CoO2. The χ(T) data suggest that for View the MathML source, the Co ions adopt an effective configuration of Co3.4+. The temperature evolution of 2D exchange 23Na-NMR spectra and of the 23Na spin-lattice relaxation time T1 reveal significant anomalies between 230 and 300 K. These features provide evidence for a crossover phenomenon or a phase transition and suggest that, upon decreasing temperature, a dramatic change occurs in the Co 3d–electron spin dynamics, starting just below 300 K and being completed at 230 K. This phenomenom may involve a partial charge ordering within the Co subsystem.
Abstract: We report the results of measurements of the low-temperature specific heat Cp(T) and the ac susceptibility χac(T) in low applied magnetic fields for a series of samples of Eu1-xCaxB6. The anomalies in Cp(T), together with the results for χac(T) and M(H), confirm the onset of phase transitions to long range magnetic order for x < 0.7 and provide evidence that for x ≥ 0.7, the Eu moments, which are captured in large magnetic clusters with magnetic moments of the order of 260 μB, adopt a spin-glass type ground state. The data set allows to establish the low-temperature [ T,x] phase diagram of this alloy series.
Abstract: Na0.5CoO2 exhibits a metal-insulator transition at TMI=53 K upon cooling. The nature of another transition at TX=88 K has not been fully clarified yet. We report the results of measurements of the electrical conductivity σ , the magnetic susceptibility χ and 23Na NMR on a powder sample of Na0.5CoO2 , including the mapping of NMR spectra, as well as probing the spin-lattice relaxation rate T 1−1(T) and the spin-spin relaxation rate T 2−1(T) , in the temperature range between 30 and 305 K . The NMR data reflect the transition at TX very well but provide less evidence for the metal-insulator transition at TMI . The temperature evolution of the shape of the spectra implies the formation of a staggered internal field below TX , not accompained by a rearrangement of the electric charge distribution. Our results thus indicate that in Na0.5CoO2 , an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, which finally induces an insulating ground state.
Abstract: We compare the magnetic susceptibilities χ(T) and electrical resistivities of well-characterized icosahedral and amorphous phases of Al75Cu15V10 between 2 and 350 K. The structure of both phases was confirmed by X-ray powder diffraction and TEM measurements. Above 100 K, the icosahedral phase exhibits a temperature-independent Pauli-type susceptibility, but below 100 K it decreases with decreasing temperature, hinting that there is a tiny gap in the electronic density of states at the Fermi level. The differences of χ between the amorphous and icosahedral phases suggest that the icosahedral structure does not favor the formation of localized V magnetic moments. No substantial changes are observed in the electrical resistivities of the two phases.
