Monte Carlo Simulation of Ising Models with Dipole Interaction

Recently, a new memory effect was found in the metamagnetic domain structure of the diluted Ising antiferromagnet $Fe_x Mg_{1-x} Cl_2$ by domain imaging with Faraday contrast. Essential for this effect is the dipole interaction. We simulate the low temperature behavior of diluted Ising-antiferromagnets by a Monte Carlo simulation considering long range interaction. The metamagnetic domain structure occurring due to the dipole interaction is investigated by graphical representation. In the model considered the antiferromagnetic state is stable for an external magnetic field smaller than a lower boundary $B_{c1}$ while for fields larger than an upper boundary $B_{c2}$ the system is in the saturated paramagnetic phase, where the spins are ferromagnetically polarized. For magnetic fields in between these two boundaries a mixed phase occurs consisting of ferromagnetic domains in an antiferromagnetic background. The position of these ferromagnetic domains is stored in the system: after a cycle in which the field is first removed and afterwards applied again the domains reappear at their original positions. The reason for this effect can be found in the frozen antiferromagnetic domain state which occurs after removing the field.Comment: Latex, 10 pages; 3 postsript-figures, compressed tar-file, uuencoded, report 10109

Critical behavior of the Random-Field Ising model at and beyond the Upper Critical Dimension

The disorder-driven phase transition of the RFIM is observed using exact ground-state computer simulations for hyper cubic lattices in d=5,6,7 dimensions. Finite-size scaling analyses are used to calculate the critical point and the critical exponents of the specific heat, magnetization, susceptibility and of the correlation length. For dimensions d=6,7 which are larger or equal to the assumed upper critical dimension, d_u=6, mean-field behaviour is found, i.e. alpha=0, beta=1/2, gamma=1, nu=1/2. For the analysis of the numerical data, it appears to be necessary to include recently proposed corrections to scaling at and beyond the upper critical dimension.Comment: 8 pages and 13 figures; A consise summary of this work can be found in the papercore database at http://www.papercore.org/Ahrens201

The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud

We report the discovery of a relatively faint (V=15.5) early-type WN star in the SMC. The line strength and width of He II lambda 4686 emission is similar to that of the other SMC WNs, and the presense of N V lambda 4603,19 emission (coupled with the lack of N III) suggests this star is of spectral type WN3-4.5, and thus is similar in type to the other SMC WRs. Also like the other SMC WN stars, an early-type absorption spectrum is weakly present. The absolute magnitude is comparable to that of other (single) Galactic early-type WNs. The star is located in the Hodge 53 OB association, which is also the home of two other SMC WNs. This star, which we designate SMC-WR12, was actually detected at a high significance level in an earlier interference-filter survey, but the wrong star was observed as part of a spectroscopic followup, and this case of mistaken identity resulted in its Wolf-Rayet nature not being recognized until now.Comment: Accepted by PASP (November 2003 issue

A new type of temperature driven reorientation transition in magnetic thin films

We present a new type of temperature driven spin reorientation transition (SRT) in thin films. It can occur when the lattice and the shape anisotropy favor different easy directions of the magnetization. Due to different temperature dependencies of the two contributions the effective anisotropy may change its sign and thus the direction of the magnetization as a function of temperature may change. Contrary to the well-known reorientation transition caused by competing surface and bulk anisotropy contributions the reorientation that we discuss is also found in film systems with a uniform lattice anisotropy. The results of our theoretical model study may have experimental relevance for film systems with positive lattice anisotropy, as e.g. thin iron films grown on copper.Comment: 7 pages, 4 figures, accepted for publication in EPJ

Mean-field scaling function of the universality class of absorbing phase transitions with a conserved field

We consider two mean-field like models which belong to the universality class of absorbing phase transitions with a conserved field. In both cases we derive analytically the order parameter as function of the control parameter and of an external field conjugated to the order parameter. This allows us to calculate the universal scaling function of the mean-field behavior. The obtained universal function is in perfect agreement with recently obtained numerical data of the corresponding five and six dimensional models, showing that four is the upper critical dimension of this particular universality class.Comment: 8 pages, 2 figures, accepted for publication in J. Phys.

Magnetic friction in Ising spin systems

A new contribution to friction is predicted to occur in systems with magnetic correlations: Tangential relative motion of two Ising spin systems pumps energy into the magnetic degrees of freedom. This leads to a friction force proportional to the area of contact. The velocity and temperature dependence of this force are investigated. Magnetic friction is strongest near the critical temperature, below which the spin systems order spontaneously. Antiferromagnetic coupling leads to stronger friction than ferromagnetic coupling with the same exchange constant. The basic dissipation mechanism is explained. If the coupling of the spin system to the heat bath is weak, a surprising effect is observed in the ordered phase: The relative motion acts like a heat pump cooling the spins in the vicinity of the friction surface.Comment: 4 pages, 4 figure

Theory for the reduction of products of spin operators

In this study we show that the sum of the powers of arbitrary products of quantum spin operators such as $(S^+)^l(S^-)^m(S^z)^n$ can be reduced by one unit, if this sum is equal to 2S+1, S being the spin quantum number. We emphasize that by a repeated application of this procedure \em all \em arbitrary spin operator products with a sum of powers larger than 2S can be replaced by a combination of spin operators with a maximum sum of powers not larger than 2S. This transformation is exact. All spin operators must belong to the same lattice site. By use of this procedure the consideration of single-ion anisotropies and the investigation of the magnetic reorientation within a Green's function theory are facilitated. Furthermore, it may be useful for the study of time dependent magnetic properties within the ultrashort (fsec) time domain.Comment: 11 pages, 1 table, uses rotatin

Massive Binary WR112 and Properties of Wolf-Rayet Dust

Some hot, massive, population-I Wolf-Rayet (WR) stars of the carbon subclass are known to be prolific dust-producers. How dust can form in such a hostile environment remains a mystery. Here we report the discovery of a relatively cool, extended, multi-arc dust envelope around the star WR112, most likely formed by wind-wind collision in a long-period binary system. We derive the binary orbital parameters, the dust temperature and the dust mass distributions in the envelope. We find that amorphous carbon is a main constituent of the dust, in agreement with earlier estimates and theoretical predictions. However, the characteristic size of the dust grains is estimated to be ~1 micron, significantly larger than theoretical limits. The dust production rate is 6.1*10^[-7] M_Sun / yr and the total detectable dust mass is found to be about 2.8*10^[-5] M_Sun (for d=4.15 kpc). We also show that, despite the hostile environment, at least ~20% of the initially-formed dust may reach the interstellar medium.Comment: 8pp, 3 figures (2 of them in colour). The Astrophysical Journal Letters (2002) in pres

Massive Stars in the Quintuplet Cluster

We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The total mass in observed stars is \sim 10^3 \Msun, and the implied mass is \sim 10^4 \Msun, assuming a lower mass cutoff of 1 \Msun and a Salpeter initial mass function. The implied mass density in stars is at least a few thousand \Msun pc^{-3}. The newly-identified stars increase the estimated ionizing flux from this cluster by about an order of magnitude with respect to earlier estimates, to 10^{50.9} photons/s, or roughly what is required to ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity from the massive cluster stars is $\approx 10^{7.5}$ \Lsun, enough to account for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a picture which integrates most of the major features in this part of the sky, excepting the non-thermal filaments. We compare the cluster to other young massive clusters and globular clusters, finding that it is unique in stellar content and age, except, perhaps, for the young cluster in the central parsec of the Galaxy. In addition, we find that the cluster is comparable to small ``super star clusters.'

Optically thick clumps: not the solution to the Wolf-Rayet wind momentum problem?

The hot star wind momentum problem η = Mν∞/(L/c)» much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather than in unison since clumping reduces multiple scattering. From basic geometric considerations, it is shown that this reduction in momentum delivery by clumping more than offsets the reduction in momentum required, for a specified ε. Thus the ratio of momentum deliverable to momentum required is maximal for a smooth wind and the momentum problem remains for the thick clump case. In the case of thin clumps, all of the benefit of clumping in reducing η lies in reducing M for a given ε so that extremely small filling factors f ≈ 10-4 are needed. It is also shown that clumping affects the inference of M from radio ε not only by changing the emission measure per unit mass but also by changing the radio optical depth unity radius Rrad, and hence the observed wind volume, at radio wavelengths. In fact, for free-free opacity proportional to αn2, contrary to intuition, Rrad increases with increasing clumpiness