4,136 research outputs found
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
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
¿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
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
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