Nature of the spin-glass phase in dense packings of Ising dipoles with random anisotropy axes

By Monte Carlo simulations, we study the character of the spinglass (SG) phase in dense disordered packings of magnetic nanoparticles (NPs). We focus on NPs which have large uniaxial anisotropies and can be well represented as Ising dipoles. Dipoles are placed on SC lattices and point along randomly oriented axes. From the behaviour of a SG correlation length we determine the transition temperature Tc between the paramagnetic and a SG phase. For temperatures well below Tc we find distributions of the SG overlap parameter q that are strongly sample-dependent and exhibit several spikes. We find that the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with the system sizes studied. We compare these results with the ones found previously for 3D site-diluted systems of parallel Ising dipoles and with the behaviour of the Sherrington-Kirkpatrick model.We thank financial support from MINECO FIS2013-43201-P Gran

Low-temperature spin-glass behaviour in a diluted dipolar Ising system

Using Monte Carlo simulations, we study the character of the spin-glass (SG) state of a site-diluted dipolar Ising model. We consider systems of dipoles randomly placed on a fraction x of all L^3 sites of a simple cubic lattice that point up or down along a given crystalline axis. For x < 0.65 these systems are known to exhibit an equilibrium spin-glass phase below a temperature T_sg proportional to x. At high dilution and very low temperatures, well deep in the SG phase, we find spiky distributions of the overlap parameter q that are strongly sample-dependent. We focus on spikes associated with large excitations. From cumulative distributions of q and a pair correlation function averaged over several thousands of samples we find that, for the system sizes studied, the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with L. This is compared with the behaviour found for the Sherrington-Kirkpatrick model.Comment: 9 LaTeX pages, 9 pdf figures, 2 table

Time relaxation of interacting single--molecule magnets

We study the relaxation of interacting single--molecule magnets (SMMs) in both spatially ordered and disordered systems. The tunneling window is assumed to be, as in Fe8, much narrower than the dipolar field spread. We show that relaxation in disordered systems differs qualitatively from relaxation in fully occupied cubic and Fe_8 lattices. We also study how line shapes that develop in ''hole--digging'' experiments evolve with time t in these fully occupied lattices. We show (1) that the dipolar field h scales as t^p in these hole line shapes and show (2) how p varies with lattice structure. Line shapes are not, in general, Lorentzian. More specifically, in the lower portion of the hole, they behave as (h/t^p)^{(1/p)-1} if h is outside the tunnel window. This is in agreement with experiment and with our own Monte Carlo results.Comment: 21 LaTeX pages, 6 eps figures. Submitted to PRB on 15 June 2005. Accepted on 13 August 200

¿Dios hoy? Creyentes y no creyentes ante un nuevo paradigma. [Reseña]

Reseña de Enrique MARTfNEZ LoZANO, ^Dios hoy? Creyentesy no creyentes ante un nuevo paradigma, Narcea, Madrid 2005, 171 pp., 13 X 21, ISBN 84-277-1485-8

Catecumenado e iniciación cristiana. Un desafío para la Iglesia de hoy. [Reseña]

Reseña de Dionisio BOROBIO, Catecumenado e iniciación cristiana. Un desafio para la Iglesia de hoy, Barcelona: Centre de Pastoral Litúrgica (Colección «Biblioteca Litúrgica», 30), 2007, 229 pp., 16 x 22, ISBN 978-84-9805-210-7

Ser laico en la Iglesia y en el mundo. Claves teológicoespirituales a la luz del Vaticano II y Christifideles laici. [Reseña]

Reseña de Raúl BERZOSA MARTÍNEZ, Ser laico en la Iglesia y en el mundo. Claves teológicoespirituales a la luz del Vaticano II y Christifideles laici, Desclée de Brouwer, Bilbao 2000, 241 pp., 16 x 24, ISBN 84-330-1458-7

Conversión filosófica y conversión cristiana

En los orígenes del cristianismo se observa una cierta prevención de los cristianos hacia la práctica filosofía del momento, pues Cristo, el verdadero Filósofo y el auténtico Pastor, ofrecía las respuestas adecuadas a las cuestiones fundamentales de la existencia humana. Este artículo analiza la novedad radical de la cosmovision cristiana a partir del análisis de la noción de conversión. La comparación entre conversión filosófica y conversión cristiana ofrece una explicación sobre esa inicial desconfianza del cristianismo hacia la filosofía. Indirectamente, este estudio pone de manifiesto el alcance de la categoría conversión como piedra de toque para el análisis de las distintas corrientes de pensamiento o de las visiones del mundo

Dal senso cercato al senso donato. Pensare la ragione nell'orizzonte della fede. [Reseña]

Reseña de Antonio SABETTA, Dal senso cercato al senso donato. Pensare la ragione nell'orizzonte della fede, Lateran University Press (Collana «Prospettive», 1), Città del Vaticano 2007, 191 pp., 24 x 17, ISBN 978-88-465-0596-5