Itinerant Ferromagnetism in ultracold Fermi gases

Itinerant ferromagnetism in cold Fermi gases with repulsive interactions is studied applying the Jastrow-Slater approximation generalized to finite polarization and temperature. For two components at zero temperature a second order transition is found at $ak_F\simeq0.90$ compatible with QMC. Thermodynamic functions and observables such as the compressibility and spin susceptibility and the resulting fluctuations in number and spin are calculated. For trapped gases the resulting cloud radii and kinetic energies are calculated and compared to recent experiments. Spin polarized systems are recommended for effective separation of large ferromagnetic domains. Collective modes are predicted and tri-critical points are calculated for multi-component systems.Comment: 8 pages, 6 figure

Quantum Hall Ferromagnetism in Graphene

Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like low-energy excitations. When Zeeman and spin-orbit interactions are neglected its Landau levels are four-fold degenerate, explaining the $4 e^2/h$ separation between quantized Hall conductivity values seen in recent experiments. In this paper we derive a criterion for the occurrence of interaction-driven quantum Hall effects near intermediate integer values of $e^2/h$ due to charge gaps in broken symmetry states.Comment: 4 pages, 1 figur

Effect of surface anisotropy on the hysteretic properties of a magnetic particle

We study the influence of surface anisotropy on the zero-temperature hysteretic properties of a small single-domain magnetic particle, and give an estimation of the anisotropy constant for which deviations from the Stoner-Wohlfarth model are observed. We consider a spherical particle with simple cubic crystalline structure, a uniaxial anisotropy for core spins and radial anisotropy on the surface, and compute the hysteresis loop by solving the local Landau-Lifshitz equations for classical spin vectors. We find that when the surface anisotropy constant is at least of the order of the exchange coupling, large deviations are observed with respect to the Stoner-Wohlfarth model in the hysteresis loop and thereby the limit-of-metastability curve, due to the non-uniform cluster-wise reversal of the magnetisation.Comment: 4 pages, 4 figures: this is a short version of cond-mat/0109411 presented at the 46th MMM Conference, Seattle November 12-16, 2001, to appear in J. Appl. Phy

Magnetic free energy at elevated temperatures and hysteresis of magnetic particles

We derive a free energy for weakly anisotropic ferromagnets which is valid in the whole temperature range and interpolates between the micromagnetic energy at zero temperature and the Landau free energy near the Curie point T_c. This free energy takes into account the change of the magnetization length due to thermal effects, in particular, in the inhomogeneous states. As an illustration, we study the thermal effect on the Stoner-Wohlfarth curve and hysteresis loop of a ferromagnetic nanoparticle assuming that it is in a single-domain state. Within this model, the saddle point of the particle's free energy, as well as the metastability boundary, are due to the change in the magnetization length sufficiently close to T_c, as opposed to the usual homogeneous rotation process at lower temperatures.Comment: 16 pages, 4 figure

Surveyor launch opportunities for mid-1965 through 1968

Lunar launch data - ascent characteristics of Atlas-Centaur AC-15 configuration and mission ground rules applied to models for Surveyor flight

The Treadmill of Destruction in Comparative Perspective: A Panel Study of Military Spending and Carbon Emissions, 1961-2014

This article analyzes a unique panel data set to assess the effect of militarism on per capita carbon dioxide emissions. We extend previous research examining the effects of military expenditures on carbon emissions by including in our analyses over 30 years of additional data. In addition, we compare our preliminary results to those obtained from other estimation procedures. Specifically, we report and visually illustrate the results of 54 cross-sectional models (one for each year) and 36 unique panel regression models on both balanced and unbalanced panels. We assess how this relationship has changed over time by testing for interactions between military spending and time and by systematically re-analyzing our data across 180 panel regressions with varying time frames. A strong and enduring association between military spending and per capita carbon emissions is indicated in cross-sectional comparisons. Our panel analyses reveal a much weaker and varying relationship that has become stronger in recent decades. Moreover, we find that the effect of military spending on per capita carbon emissions is moderated by countries’ level of economic development, with military spending of more wealthy countries having relatively larger net effects on carbon emissions. We partially confirm previous findings on the temporal stability of the environmental impacts of militarism. Our analyses show, however, that this temporal stability has emerged relatively recently, and that the relationship between military expenditures and carbon emissions is weaker prior to the 1990s

Hysteretic properties of a magnetic particle with strong surface anisotropy

We study the influence of surface anisotropy on the zero-temperature hysteretic properties of a small single-domain magnetic particle, and give an estimation of the anisotropy constant for which deviations from the Stoner-Wohlfarth model are observed due to non-uniform reversal of the particle's magnetisation. For this purpose, we consider a spherical particle with simple cubic crystalline structure, a uniaxial anisotropy for core spins and radial anisotropy on the surface. The hysteresis loop is obtained by solving the local (coupled) Landau-Lifschitz equations for classical spin vectors. We find that when the surface anisotropy constant is at least of the order of the exchange coupling, large deviations are observed with respect to the Stoner-Wohlfarth model in the hysteresis loop and thereby the limit-of-metastability curve, since in this case the magnetisation reverses its direction in a non-uniform manner via a progressive switching of spin clusters. In this case the critical field, as a function of the particle's size, behaves as observed in experiments.Comment: 12 pages, 15 eps figure

Inhomogeneous spin diffusion in traps with cold atoms

The spin diffusion and damped oscillations are studied in the collision of two spin polarized clouds of cold atoms with resonant interactions. The strong density dependence of the diffusion coefficient leads to inhomogeneous spin diffusion that changes from central to surface spin flow as the temperature increases. The inhomogeneity and the smaller finite trap size significantly reduce the spin diffusion rate at low temperatures. The resulting spin diffusion rates, spin drag and initial damped oscillations are compatible with measurements at low to high temperatures for resonant attractive interactions but are incompatible with a metastable ferromagnetic phase.Comment: 3 pages, 1 figure; Phys. Rev. Letters, 201

Magnetism of 3d transition metal atoms on W(001): submonolayer films

We have investigated random submonolayer films of 3d transition metals on W(001). The tight-binding linear muffin-tin orbital method combined with the coherent potential approximation was employed to calculate the electronic structure of the films. We have estimated local magnetic moments and the stability of different magnetic structures, namely the ferromagnetic order, the disordered local moments and the non-magnetic state, by comparing the total energies of the corresponding systems. It has been found that the magnetic moments of V and Cr decrease and eventually disappear with decreasing coverage. On the other hand, Fe retains approximately the same magnetic moment throughout the whole concentration range from a single impurity to the monolayer coverage. Mn is an intermediate case between Cr and Fe since it is non-magnetic at very low coverages and ferromagnetic otherwise.Comment: 5 pages, 3 figures in 6 files; presented at ICN&T 2006, Basel, Switzerlan