Effects of Ferromagnetic Magnetic Ordering and Phase Transition on the Resistivity of Spin Current

It has been shown experimentally a long time ago that the magnetic ordering causes an anomalous behavior of the electron resistivity in ferromagnetic crystals. Phenomenological explanations based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. We show by extensive Monte Carlo simulation that this behavior is also observed for the resistivity of the spin current calculated as a function of temperature ($T$) from low-$T$ ordered phase to high-$T$ paramagnetic phase in a ferromagnet. We show in particular that across the critical region, the spin resistivity undergoes a huge peak. The origin of this peak is shown to stem from the formation of magnetic domains near the phase transition. The behavior of the resistivity obtained here is compared to experiments and theories. A good agreement is observed.Comment: 7 pages, 3 figures, accepted, to appear in J. Appl. Phy

The role of social interactions on the development and honesty of a signal of status

Badges of status are supposed to have insignificant production costs, so use costs are thought to be most important in ensuring signal reliability. Use costs arise from the use of the status signal in social interactions. Social experiences that arise from the use of inappropriate signals in social interactions may drive mechanisms that result in reduced fitness for inappropriate signalers. The role of social control, probing and familiarity in producing use costs was explored. There was no evidence that social control by dominants produced a cost for cheaters and no evidence that social control by subordinates produced a cost for inappropriate signaling by Trojans. Probing produced a cost for cheating when resource value was high but not when resource value was low. Familiarity had some effect on the cost and benefit of cheating but in patterns that were not predicted. Familiarity both eliminated a benefit of cheating and reduced a cost of cheating; therefore it is uncertain how familiarity affects honest signaling. The status signal of the receiver had no effect on the cost or benefits of cheating, and there was no evidence of punishment. Social experiences have the potential to affect signal development to produce a correspondence between signal and status. The effects of social experience on signal production were examined and there was little evidence that social experience influenced bib development. Neither aggression expressed nor aggression received was not predictive of bib size. Additionally, tests on the different measures of winning experience produced conflicting conclusions regarding the relationship between winning experience and bib development

Spin Resistivity in Frustrated Antiferromagnets

In this paper we study the spin transport in frustrated antiferromagnetic FCC films by Monte Carlo simulation. In the case of Ising spin model, we show that the spin resistivity versus temperature exhibits a discontinuity at the phase transition temperature: an upward jump or a downward fall, depending on how many parallel and antiparallel localized spins interacting with a given itinerant spin. The surface effects as well as the difference of two degenerate states on the resistivity are analyzed. Comparison with non frustrated antiferromagnets is shown to highlight the frustration effect. We also show and discuss the results of the Heisenberg spin model on the same lattice

Frustration Effects in Antiferromagnetic FCC Heisenberg Films

We study the effects of frustration in an antiferromagnetic film of FCC lattice with Heisenberg spin model including an Ising-like anisotropy. Monte Carlo (MC) simulations have been used to study thermodynamic properties of the film. We show that the presence of the surface reduces the ground state (GS) degeneracy found in the bulk. The GS is shown to depend on the surface in-plane interaction $J_s$ with a critical value at which ordering of type I coexists with ordering of type II. Near this value a reentrant phase is found. Various physical quantities such as layer magnetizations and layer susceptibilities are shown and discussed. The nature of the phase transition is also studied by histogram technique. We have also used the Green's function (GF) method for the quantum counterpart model. The results at low-$T$ show interesting effects of quantum fluctuations. Results obtained by the GF method at high $T$ are compared to those of MC simulations. A good agreement is observed.Comment: 11 pages, 19 figures, submitted to J. Phys.: Condensed Matte

GdI_2: A New Ferromagnetic Excitonic Solid?

The two-dimensional, colossal magnetoresistive system GdI_2 develops an unusual metallic state below its ferromagnetic transition and becomes insulating at low temperatures. It is argued that this geometrically frustrated, correlated poor metal is a possible candidate for a ferromagnetic excitonic liquid. The renormalized Fermi surface supports a further breaking of symmetry to a charge ordered, excitonic solid ground state at lower temperatures via order by disorder mechanism. Several experimental predictions are made to investigate this unique orbitally correlated ground state.Comment: 4 pages, 4 figures, changed Fig. 1 with extended energy scale, added text and references, author list shortene

Extended Defects in the Potts-Percolation Model of a Solid: Renormalization Group and Monte Carlo Analysis

We extend the model of a 2$d$ solid to include a line of defects. Neighboring atoms on the defect line are connected by ?springs? of different strength and different cohesive energy with respect to the rest of the system. Using the Migdal-Kadanoff renormalization group we show that the elastic energy is an irrelevant field at the bulk critical point. For zero elastic energy this model reduces to the Potts model. By using Monte Carlo simulations of the 3- and 4-state Potts model on a square lattice with a line of defects, we confirm the renormalization-group prediction that for a defect interaction larger than the bulk interaction the order parameter of the defect line changes discontinuously while the defect energy varies continuously as a function of temperature at the bulk critical temperature.Comment: 13 figures, 17 page

Spin transport in magnetic multilayers

We study by extensive Monte Carlo simulations the transport of itinerant spins travelling inside a multilayer composed of three ferromagnetic films antiferromagnetically coupled to each other in a sandwich structure. The two exterior films interact with the middle one through non magnetic spacers. The spin model is the Ising one and the in-plane transport is considered. Various interactions are taken into account. We show that the current of the itinerant spins going through this system depends strongly on the magnetic ordering of the multilayer: at temperatures $T$ below (above) the transition temperature $T_c$, a strong (weak) current is observed. This results in a strong jump of the resistance across $T_c$. Moreover, we observe an anomalous variation, namely a peak, of the spin current in the critical region just above $T_c$. We show that this peak is due to the formation of domains in the temperature region between the low-$T$ ordered phase and the true paramagnetic disordered phase. The existence of such domains is known in the theory of critical phenomena. The behavior of the resistance obtained here is compared to a recent experiment. An excellent agreement with our physical interpretation is observed. We also show and discuss effects of various physical parameters entering our model such as interaction range, strength of electric and magnetic fields and magnetic film and non magnetic spacer thicknesses.Comment: 8 pages, 17 figures, submitted to J. Phys.: Cond Matte

On confined fractional charges: a simple model

We address the question whether features known from quantum chromodynamics (QCD) can possibly also show up in solid-state physics. It is shown that spinless fermions of charge $e$ on a checkerboard lattice with nearest-neighbor repulsion provide for a simple model of confined fractional charges. After defining a proper vacuum the system supports excitations with charges $\pm e/2$ attached to the ends of strings. There is a constant confining force acting between the fractional charges. It results from a reduction of vacuum fluctuations and a polarization of the vacuum in the vicinity of the connecting strings.Comment: 5 pages, 3 figure