Changes in the magnetic properties of nanocomposite co-based films at the percolation transition

The magnetic and magnetotransport properties of nanocomposite (Co)_x(SiO₂)_{1 ? x} films have been investigated. Small-angle neutron scattering study showed that fractal magnetic structures begin to form near the percolation threshold. In the range of the structural percolation threshold, the size of the magnetically correlated regions exceeds several times the structural size of Co grains.     

Directed percolation phase transition at the onset of STI in an inhomogeneous coupled map lattice

A model for inhomogeneously coupled logistic maps is considered to find some critical exponents in the transition from inhomogeneous steady state to spatiotemporal chaos through spatiotemporal intermittency. The laminar state in the model is described by inhomogeneous steady state with spatial period two. We obtain a complete set of static exponents which match with the corresponding directed percolation (DP) values in (1+1) dimension. We also find four nonuniversal spreading exponents in which three exponents are in agreement with DP values. The model in which absorbing state is inhomogeneous steady state, contributes a new example in evidence of Pomeau's [18] conjecture that the onset of STI in a deterministic system belongs to DP universality class.     

Chaotic domain patterns in periodic inhomogeneous magnetic films

A study of domain structures in thin inhomogeneous ferromagnetic films is presented. It is shown that smooth and small inhomogeneities in the exchange and anisotropy parameters yield very complex domain structures. We show that the domain walls are fixed near certain inhomogeneities but do not repeat their space distribution. We found that there are metastable chaotic domain patterns in periodic inhomogeneous films. These results are relevant for magnetoresistive devices.     

Magnetic resonance studies of three-layer FeNi/Bi/FeNi films

The interlayer coupling in three-layer FeNi/Bi/FeNi films is studied by electron magnetic resonance. The magnetic anisotropy at the permalloy–bismuth interface is shown to play a significant role in the formation of the magnetic state of the film structure. The interlayer coupling oscillation period is found to be about 8 nm. The interlayer coupling and the interface anisotropy and their temperature dependences are determined.     

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.     

Impedance spectra of thin permalloy films with a nanoisland structure

The behavior of the impedance spectra of island permalloy films prepared through vacuum evaporation onto optically polished glass-ceramic substrates has been investigated in the frequency range from 0.0001 to 100 MHz. A resistor-capacitor model of the films has been developed and the model parameters, for which there is a good agreement with experimental data on the frequency dependences of the real and imaginary components of the impedance, have been determined. The specific features in the behavior of the electrical and physical characteristics with variations in the thickness of the sample and the gap between the measuring electrodes have been investigated. It has been found that the relative permittivity of the films under investigation reaches values ɛ ∼ 10⁷–10⁸. The structural relaxation times have been calculated.     

Magnetooptical images of inhomogeneous magnetic fields in metallic films with in-plane anisotropy

The magnetooptical (MO) images of the inhomogeneous field created by permanent magnets in magnetic metallic films with in-plane anisotropy are experimentally studied. The MO images recorded using the longitudinal Kerr effect are the superposition of two pictures, namely, a polar-sensitivity MO image and a longitudinal-sensitivity MO image. An analysis of these images after separation shows that the polar-sensitivity MO image reflects the distribution of the inhomogeneous field component that is normal to the surface of an indicator film in an analog manner. The longitudinal-sensitivity MO image reflects the angular distribution of the in-plane component of a stray field in an analog manner. The coincidence of the experimental and corresponding simulated MO images makes it possible to interpret the experimental images. In particular, it is shown that the specific features detected in the topological characteristics of the inhomogeneous field correspond to experimental singular points. Hidden magnetic images (magnetic bar codes) are shown to be visualized with metallic CoFe films. As an example, the stray field of a magnetic system made of cylindrical magnets is mapped.     

Nonmonotonic flux flow in inhomogeneous superconductors above the percolation threshold

Flux flow resistivity above critical temperature is analyzed based on the increased evidences that high temperature superconductors are intrinsically inhomogeneous, and local nonpercolating superconducting domains persist high above the critical temperature. It was found that above a certain field, the resistivity originated in the flow of flux motion starts to decrease with increasing field, due to the predominance of the suppression of the superconducting droplets over the increase of vortex core density.     

Sharpness of the phase transition in percolation models

The equality of two critical points — the percolation thresholdp _H and the pointp _T where the cluster size distribution ceases to decay exponentially — is proven for all translation invariant independent percolation models on homogeneousd-dimensional lattices (d1). The analysis is based on a pair of new nonlinear partial differential inequalities for an order parameterM(,h), which forh=0 reduces to the percolation densityP — at the bond densityp=1–e ^– in the single parameter case. These are: (1)MhM/h+M ²+MM/, and (2) M/|J|MM/h. Inequality (1) is intriguing in that its derivation provides yet another hint of a ³ structure in percolation models. Moreover, through the elimination of one of its derivatives, (1) yields a pair of ordinary differential inequalities which provide information on the critical exponents and . One of these resembles an Ising model inequality of Fröhlich and Sokal and yields the mean field bound 2, and the other implies the result of Chayes and Chayes that . An inequality identical to (2) is known for Ising models, where it provides the basis for Newman's universal relation and for certain extrapolation principles, which are now made applicable also to independent percolation. These results apply to both finite and long range models, with or without orientation, and extend to periodic and weakly inhomogeneous systems.Research supported in part by the NSF Grant PHY-8605164Also in the Physics Department     

Tunable resonance frequency of FeNi films by oblique sputtering

FeNi thin films were fabricated by radio frequency magnetron sputtering on Si(1 1 1). Dynamic properties at remanence of the films were systematically investigated in a wide frequency range from 100 MHz to 5 GHz. The results show that both thickness of FeNi films and oblique angle have important effects on the magnetic properties of the films, the magnetic resonant frequency of the films can also be adjusted by the two factors. The in-plane uniaxial magnetic anisotropy field can be adjusted from 82 Oe to 220 Oe by increasing the oblique angle. As a consequence, the magnetic resonant frequency of the films increased from 2.7 GHz to 4.2 GHz.     

Is the percolation transition of hard spheres a thermodynamic phase transition?

In hard-sphere systems, there is a fluid-solid transition, but no gas-liquid transition. In the fluid region, however, one can find a purely geometric percolation transition, which is studied in detail. The van der Waals model of hard spheres is treated. In this model, a uniform negative background potential is added. This modification does not change the structure, but induces a gas-liquid transition. In fact, percolation and the gas-liquid transition can be related to each other.     

Crossover of polaron conductivity and the inhomogeneous state of lanthanum manganites in the magnetic phase transition region

The transport properties of lanthanum manganites over a wide range of temperatures below the magnetic phase transition point are discussed within the model of a two-phase composite whose phases differ in the magnetic order and charge carrier concentration. The volume ratio of the phases depends on the temperature and the magnetic field. The magnetoelastic polarons are charge carriers in both phases, and the metal-dielectric transition occurs as a percolation transition accompanied by the crossover of the polaron conductivity. The results obtained by numerical simulation of the resistivity, magnetoresistance, and thermopower are compared with the experimental data for La_0.7Mn_1.3O_3?δ thin films. The theoretical and experimental data are in good agreement.     

The nature of explosive percolation phase transition

In this Letter, we show that the explosive percolation is a novel continuous phase transition. The order-parameter-distribution histogram at the percolation threshold is studied in Erd?s-Rényi networks, scale-free networks, and square lattice. In finite system, two well-defined Gaussian-like peaks coexist, and the valley between the two peaks is suppressed with the system size increasing. This finite-size effect always appears in typical first-order phase transition. However, both of the two peaks shift to zero point in a power law manner, which indicates the explosive percolation is continuous in the thermodynamic limit. The nature of explosive percolation in all the three structures belongs to this novel continuous phase transition. Various scaling exponents concerning the order-parameter-distribution are obtained.     

A model for magnetic abnormalies of FeNi Invar alloys

A model based on the idea of localized magnetic moments is presented which allows to calculate the local magnetic moment expectation values of FeNi alloys. The only parameters of the model are the exchange integralsJ _FeFe,J _FeNi,J _NiNi. By assuming a “mixed” exchange interaction the concentration dependence of the exchange integralsJ _FeFe andJ _FeNi is calculated. The model allows the iron magnetic moments to orient parallel or antiparallel to the magnetization axis, depending on the local environment. It explains the magnetic abnormalies of FeNi Invar alloys as for example the concentration dependence of the mean magnetic moment and the Curie temperatures as well as the characteristic “flat” courves of the spontaneous magnetization.     

QCD phase transition in the inhomogeneous universe

We investigate a new mechanism for the cosmological QCD phase transition: inhomogeneous nucleation. The primordial temperature fluctuations, measured to be deltaT/T approximately 10(-5), are larger than the tiny temperature interval in which bubbles would form in the standard picture of homogeneous nucleation. Thus the bubbles nucleate at cold spots. We find the typical distance between bubble centers to be a few meters. This exceeds the estimates from homogeneous nucleation by 2 orders of magnitude. The resulting baryon inhomogeneities may affect primordial nucleosynthesis.