15 research outputs found
Specific heat of two-dimensional diluted magnets
Using Monte Carlo techniques, the two-dimensional site-diluted Ising model is
studied. In particular, properties of the specific heat, its critical behaviour
and the emergence of a non-singular maximum above the transition temperature at
moderate concentration of defects, are discussed.Comment: 10 pages, 5 eps-figures, elsart-style, submitted to Physica
Observation of Irregular Labyrinthine Magnetic Domains with Dendritic Edges in a Co-Rich CoCu Alloyed Film
Phase Diagram of the BCC S=1/2 Heisenberg Antiferromagnet with First and Second Neighbor Exchange
We use linked-cluster series expansions, both at T=0 and high temperature, to
analyse the phase structure of the spin-\half Heisenberg antiferromagnet with
competing first and second-neighbor interactions on the 3-dimensional
body-centred-cubic lattice. At zero temperature we find a first-order quantum
phase transition at between AF (Ne\'el)
and AF ordered phases. The high temperature series yield quite accurate
estimates of the bounding critical line for the AF phase, and an apparent
critical line for the AF phase, with a bicritical point at , . The possibility that this latter transition is
first-order cannot be excluded.Comment: 10 pages, 4 figure
Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers
Ni/NiO multilayers were grown by magnetron sputtering at room temperature,
with the aid of the natural oxidation procedure. That is, at the end of the
deposition of each single Ni layer, air is let to flow into the vacuum chamber
through a leak valve. Then, a very thin NiO layer (~1.2nm) is formed.
Simulated x-ray reflectivity patterns reveal that layering is excellent for
individual Ni-layer thickness larger than 2.5nm, which is attributed to the
intercalation of amorphous NiO between the polycrystalline Ni layers. The
magnetization of the films, measured at temperatures 5–300K, has almost bulk-
like value, whereas the films exhibit a trend to perpendicular magnetic
anisotropy (PMA) with an unusual significant positive interface anisotropy
contribution, which presents a weak temperature dependence. The power-law
behavior of the multilayers indicates a non-negligible contribution of higher
order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the
temperature dependence of the magnetization in the spin-wave regime show that
the magnetization in the multilayers decreases faster as a function of
temperature than the one of bulk Ni. Finally, when the individual Ni-layer
thickness decreases below 2nm, the multilayer stacking vanishes, resulting in
a dramatic decrease of the interface magnetic anisotropy and consequently in a
decrease of the perpendicular magnetic anisotropy
First-order transition features of the triangular Ising model with nearest- and next-nearest-neighbor antiferromagnetic interactions
We implement a new and accurate numerical entropic scheme to investigate the
first-order transition features of the triangular Ising model with
nearest-neighbor () and next-nearest-neighbor ()
antiferromagnetic interactions in ratio . Important aspects
of the existing theories of first-order transitions are briefly reviewed,
tested on this model, and compared with previous work on the Potts model. Using
lattices with linear sizes and 480 we
estimate the thermal characteristics of the present weak first-order
transition. Our results improve the original estimates of Rastelli et al. and
verify all the generally accepted predictions of the finite-size scaling theory
of first-order transitions, including transition point shifts, thermal, and
magnetic anomalies. However, two of our findings are not compatible with
current phenomenological expectations. The behavior of transition points,
derived from the number-of-phases parameter, is not in accordance with the
theoretically conjectured exponentially small shift behavior and the well-known
double Gaussian approximation does not correctly describe higher correction
terms of the energy cumulants. It is argued that this discrepancy has its
origin in the commonly neglected contributions from domain wall corrections.Comment: 34 pages, 11 figure
DIMENSIONAL-CROSSOVER STUDIES OF RANDOMLY DILUTED FERROMAGNETIC THIN FILMS
Using Monte Carlo simulations, we studied the Ising model with random, nonmagnetic impurities on an N x N x L simple cubic lattice. Systems with ferromagnetic nearest-neighbor coupling and periodic boundary conditions in the x - y plane were studied for N = 40 and 1 ≤ L < 40 for several values of p, the concentration of magnetic ions. The transition temperature Tc decreased monotically to zero as the concentration p decreased towards the percolation threshold for the width L. The shift of Tc was consistent with L-λ, where λ is 1.56 in the critical region