Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10

The dynamics of the magnetic properties of polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K. The exact nature of Ru spins magnetic ordering is still debated and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac susceptibility vs. T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with magnetically interacting clusters. Such interpretation is supported by themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations are well described by a stretched exponential relation, and present significant ageing effects.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

Aging in a topological spin glass

We have examined the nonconventional spin glass phase of the 2-dimensional kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc magnetic measurements. The frequency dependence of the ac susceptibility peak is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature below Tg, aging effects are found which obey the same scaling law as in spin glasses or polymers. However, in clear contrast with conventional spin glasses, aging is remarkably insensitive to temperature changes. This particular type of dynamics is discussed in relation with theoretical predictions for highly frustrated non-disordered systems.Comment: 4 pages, 4 figure

Real spin glasses relax slowly in the shade of hierarchical trees

The Parisi solution of the mean-field spin glass has been widely accepted and celebrated. Its marginal stability in 3d and its complexity however raised the question of its relevance to real spin glasses. This paper gives a short overview of the important experimental results which could be understood within the mean-field solution. The existence of a true phase transition and the particular behaviour of the susceptibility below the freezing temperature, predicted by the theory, are clearly confirmed by the experimental results. The behaviour of the complex order parameter and of the Fluctuation Dissipation ratio are in good agreement with results of spontaneous noise measurements. The very particular ultrametric symmetry, the key feature of the theory, provided us with a simple description of the rejuvenation and memory effects observed in experiment. Finally, going a step beyond mean-field, the paper shortly discusses new analyses in terms of correlated domains characterized by their length scales, as well as new experiments on superspin glasses which compare well with recent theoretical simulations.Comment: To appear in the proceedings of "Wandering with Curiosity in Complex Landscapes", a scientific conference in honour of Giorgio Parisi for his 60th birthday, Roma, September 8-10 2008 (submitted for the special issue of the Journal of Statistical Physics, 2009

Remanent magnetization of spin glasses

International audienc

NUCLEAR INDUCED FERROMAGNETISM IN Pr Cu5

Nous avons observé dans le composé de Van Vleck Pr Cu5, l'apparition à basse température d'un ordre ferromagnétique nucléaire ; la température de Curie Tc est de 24 mK. Au dessus de Tc, l'énorme décroissance du renforcement de la constante de Curie nucléaire d'un facteur 180 avec le champ appliqué ne peut être expliquée qu'en tenant compte d'interactions indirectes entre les noyaux de Pr.In the Van Vleck compound PrCu5, we have observed, at low temperature, a nuclear ferromagnetic ordering with a Curie temperature of 24 mK. Above Tc, to explain the huge decrease by a factor 180 of the enhanced nuclear Curie constant with the applied magnetic field, one has to take into account indirect interactions between Pr nuclei

Bilan des properties magnetiques des alliages dilues de -Fe au-dessous de TK

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FREQUENCY DEPENDENCE OF THE SUSCEPTIBILITY MAXIMUM IN A SPIN GLASS

La température de gel du verre de spins dilué de (La1-xGdx)Al2 avec 0,6 et 1 at.%Gd, definie par le maximum de la susceptibilité a.c. augmente avec la fréquence (6 Hz < ν < 2 000 Hz).The freezing temperature of the dilute rare earth spin glass (La1-xGdxAl2 with 0.6 and 1 at.%Gd, as measured by the maximum of the ac susceptibility increases with-frequency (6 Hz < ν < 2 000 Hz)