Magnetic fluctuations in frustrated Laves hydrides R(Mn_{1-x}Al_{x})_{2}H_{y}

By neutron scattering, we have studied the spin correlations and spin fluctuations in frustrated Laves hydrides, where magnetic disorder sets in the topologically frustrated Mn lattice. Below the transition towards short range magnetic order, static spin clusters coexist with fluctuating and alsmost uncorrelated spins. The magnetic response shows a complexe lineshape, connected with the presence of the magnetic inhomogeneities. Its analysis shows the existence of two different processes, relaxation and local excitations, for the spin fluctuations below the transition. The paramagnetic fluctuations are discussed in comparison with classical spin glasses, cluster glasses, and non Fermi liquid itinerant magnets

Magnetism, Critical Fluctuations and Susceptibility Renormalization in Pd

Some of the most popular ways to treat quantum critical materials, that is, materials close to a magnetic instability, are based on the Landau functional. The central quantity of such approaches is the average magnitude of spin fluctuations, which is very difficult to measure experimentally or compute directly from the first principles. We calculate the parameters of the Landau functional for Pd and use these to connect the critical fluctuations beyond the local-density approximation and the band structure.Comment: Replaced with the revised version accepted for publication. References updated, errors corrected, other change

Electronic structure of the strongly hybridized ferromagnet CeFe2

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare earth-iron Laves phase compounds. Our experimental results indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.Comment: 10 pages, 8 figures, to appear in Phys. Rev. B in December 200

Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}

For describing the first-order isostructural valence phase transition in mixed valence compounds we develop a new approach based on the lattice Anderson model. We take into account the Coulomb interaction between localized f and conduction band electrons and two mechanisms of electron-lattice coupling. One is related to the volume dependence of the hybridization. The other is related to local deformations produced by f- shell size fluctuations accompanying valence fluctuations. The large f -state degeneracy allows us to use the 1/N expansion method. Within the model we develop a mean-field theory for the first-order valence phase transition in YbInCu_{4}. It is shown that the Coulomb interaction enhances the exchange interaction between f and conduction band electron spins and is the driving force of the phase transition. A comparison between the theoretical calculations and experimental measurements of the valence change, susceptibility, specific heat, entropy, elastic constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a good quantitative agreement is found. On the basis of the model we describe the evolution from the first-order valence phase transition to the continuous transition into the heavy-fermion ground state in the series of compounds YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.

RECENT EXPERIMENTAL STUDIES OF DYNAMICS OF SPIN GLASSES

On présente une revue des résultats expérimentaux sur la dynamique de spin des verres de spin en alliages binaires, obtenus par des techniques esr, nmr, précession de muons, effet Mössbauer et de diffusion de neutrons. Les résultats peuvent être décrits de façon consistante en termes de spectre de temps de relaxation évoluant continuellement avec la température et s'étendant de temps relativement courts (τ ~ 10-13s) à des temps très longs (τ→∞) à des températures au voisinage du point de congélation.A review of the experimental results of the spin dynamics of binary spin glass alloys obtained with the help of esr, nmr, muon precession. Mössbauer effect and neutron scattering techniques is given. The results may be consistently described in terms of a spectrum of relaxation times evolving continuously with temperature and extending from relatively short (τ ~ 10-13s) to very long times (τ→∞) at temperatures around and below the freezing temperature

Neutron scattering study of the quasi-elastic spectral width in CeMg, CeMg 3 and NdMg3 intermetallic compounds

We report neutron scattering measurements of quasi-elastic magnetic spectra in cubic compounds CeMg, CeMg3 and NdMg3 as well as in compounds diluted with Y and La. The quasi-elastic linewidth measured up to 800 K in CeMg3 shows a linear Korringa law dependence at high temperature with a negligible intercept but deviates below 100 K and tends to a residual value γ0 as T → 0 K. The magnitude of γ0 (≃ 0.5 meV) is similar to that in CeMg (≃ 0.6 meV) and NdMg3 (≃ 0.8 meV). In CeMg 3 and NdMg3 the residual linewidth decreases with dilution by La or Y, whereas in CeMg the linewidth is only slightly affected on dilution with La and increases significantly on addition of Y. This difference in behaviour suggests different origins of the low temperature relaxation mechanisms in the two systems CeMg and CeMg3.Des mesures de diffusion de neutrons ont été faites en vue d'étudier la partie quasi élastique du spectre magnétique dans les composés cubiques CeMg, CeMg, et NdMg3 ainsi que dans des composés dilués d'yttrium ou de lanthane. La largeur de raie quasi élastique a été mesurée jusqu'à 800 K dans CeMg3. A haute température elle suit une loi de Korringa qui intercepte l'origine à T = 0 K, mais dévie au-dessous de 100 K et tend vers une valeur résiduelle γ0 pour T → 0 K. La valeur de γ0 (≃ 0,5 meV) est du même ordre de grandeur que dans CeMg (≃ 0,6 meV) et NdMg3 (≃ 0,8 meV). Dans CeMg3 et NdMg3, la largeur résiduelle décroît lorsqu'on dilue avec le lanthane ou l'yttrium alors que dans CeMg elle est très peu affectée par la dilution avec le lanthane et même augmente significativement dans le composé avec l'yttrium. Cette différence de comportement suggère une différence dans l'origine des mécanismes de relaxation dans les deux systèmes CeMg et CeMg3 à basse température