373 research outputs found
Nonlinear and spin-glass susceptibilities of three site-diluted systems
The nonlinear magnetic and spin-glass susceptibilities
in zero applied field are obtained, from tempered Monte Carlo simulations, for
three different spin glasses (SGs) of Ising spins with quenched site disorder.
We find that the relation ( is the temperature),
which holds for Edwards-Anderson SGs, is approximately fulfilled in
canonical-like SGs. For nearest neighbor antiferromagnetic interactions, on a
0.4 fraction of all sites in fcc lattices, as well as for spatially disordered
Ising dipolar (DID) systems, and appear to diverge in
the same manner at the critical temperature . However, is
smaller than by over two orders of magnitude in the diluted fcc
system. In DID systems, is very sensitive to the systems
aspect ratio. Whereas near , varies by approximately a
factor of 2 as system shape varies from cubic to long-thin-needle shapes,
sweeps over some four decades.Comment: 7 pages, 7 figure
Continuous spin reorientation in antiferromagnetic films
We study anisotropic antiferromagnetic one-layer films with dipolar and
nearest-neighbor exchange interactions. We obtain a unified phase diagram as a
function of effective uniaxial D_e and quadrupolar C anisotropy constants. We
study in some detail how spins reorient continuously below a temperature T_s as
T and D_e vary.Comment: 3 LaTeX pages, 3 eps figures. Submitted to JMMM on 25 May 2006.
Accepted on 21 July 200
Magnetization Process of Single Molecule Magnets at Low Temperatures
We show that correlations established before quenching to very low
temperatures, later drive the magnetization process of systems of single
molecule magnets, after a magnetic field is applied at t=0. We also show that
in SC lattices, m \propto sqrt(t), as observed in Fe_8, but only for
1+2*log_10(h_d/h_w) time decades, where h_d is a nearest neighbor dipolar
magnetic field and a spin reversal can occur only if the field on it is within
(-h_w,h_w). However, the sqrt(t) behavior is not universal. For BCC and FCC
lattices, m \propto t^p, but p \simeq 0.7. The value to which m finally levels
off is also given.Comment: 5 LaTeX pages, 2 figures. Submitted to Phys. Rev. Lett. on 26 January
2003, accepted on 24 July 200
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
Phases of anisotropic dipolar antiferromagnets
We study systems of classical magnetic dipoles on simple cubic lattices with
dipolar and antiferromagnetic exchange interactions. By analysis and Monte
Carlo (MC) simulations, we find how the antiferromagnetic phases vary with
uniaxial and fourfold anisotropy constants, C and D, as well as with exchange
strength J. We pay special attention to the spin reorientation (SR) phase, and
exhibit in detail the nature of its broken symmetries. By mean field theory and
by MC, we also obtain the ratio of the higher ordering temperature to the SR
transition temperature, and show that it depends mainly on D/C, and rather
weakly on J. We find a reverse SR transition.Comment: 10 LaTeX pages, 14 eps figures. Submitted to PRB on 03 October 2005.
Accepted on 13 December 200
Theoretical simulation of the anisotropic phases of antiferromagnetic thin films
We simulate antiferromagnetic thin films. Dipole-dipole and antiferromagnetic
exchange interactions as well as uniaxial and quadrupolar anisotropies are
taken into account. Various phases unfold as the corresponding parameters, J, D
and C, as well as the temperature T and the number n of film layers vary. We
find (1) how the strength Delta_m of the anisotropy arising from dipole-dipole
interactions varies with the number of layers m away from the film's surface,
with J and with n; (2) a unified phase diagram for all n-layer films and bulk
systems; (3) a layer dependent spin reorientation (SR) phase in which spins
rotate continuously as T, D, C and n vary; (4) that the ratio of the SR to the
ordering temperature depends (approximately) on n only through (D+Delta/n)/C,
and hardly on J; (5) a phase transformation between two different magnetic
orderings, in which spin orientations may or may not change, for some values of
J, by varying n.Comment: 10 LaTeX pages, 13 eps figures. Submitted to PRB on 30 June 2006.
Accepted on 10 October 200
On Weierstra{\ss} semigroups at one and two points and their corresponding Poincar\'e series
The aim of this paper is to introduce and investigate the Poincar\'e series
associated with the Weierstra{\ss} semigroup of one and two rational points at
a (not necessarily irreducible) non-singular projective algebraic curve defined
over a finite field, as well as to describe their functional equations in the
case of an affine complete intersection.Comment: Beginning of Section 3 and Subsection 3.1 were modifie
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