Measurement of the resistivity of porous materials with an alternating air-flow method

Air-flow resistivity is a main parameter governing the acoustic behavior of porous materials for sound absorption. The international standard ISO 9053 specifies two different methods to measure the air-flow resistivity, namely a steady-state air-flow method and an alternating air-flow method. The latter is realized by the measurement of the sound pressure at 2 Hz in a small rigid volume closed partially by the test sample. This cavity is excited with a known volume-velocity sound source implemented often with a motor-driven piston oscillating with prescribed area and displacement magnitude. Measurements at 2 Hz require special instrumentation and care. The authors suggest an alternating air-flow method based on the ratio of sound pressures measured at frequencies higher than 2 Hz inside two cavities coupled through a conventional loudspeaker. The basic method showed that the imaginary part of the sound pressure ratio is useful for the evaluation of the air-flow resistance. Criteria are discussed about the choice of a frequency range suitable to perform simplified calculations with respect to the basic method. These criteria depend on the sample thickness, its nonacoustic parameters, and the measurement apparatus as well. The proposed measurement method was tested successfully with various types of acoustic materials.     

Spin structures with frustrated moments in RNiAl intermetallic compounds

Magnetic order in the ternary equiatomic intermetallic compounds PrNiAl, DyNiAl and HoNiAl is investigated by means of neutron powder diffraction and dc- and ac-susceptibility measurements. DyNiAl shows two magnetic phases: It is ferromagnetic between T₁ =15 K and T_c = 31 K and a tilted antiferromagnet below T₁. HoNiAl is a ferromagnet in a narrow temperature region just below T_c = 14.5 K, but furthermore exhibits two tilted antiferromagnetic phases (T₁ =5.5 K, T₂ = 12.5 K). The corresponding magnetic structures differ in the ordering of some of the magnetic moments which are coupled by frustrated bonds. The existence of those frustrated moments, which is induced by the topology is a common phenomenon in the hexagonal RNiAl series. They are manifested with reduced mean values of their moments in the elastic neutron spectra. Another interesting observation is the scaling of the second temperature T₁ at which the frustrated spins change their order: For all heavy rare earths investigated so far (R = Tb, Dy, Ho) we find T₁: T_N,C ≈ 1: 2. PrNiAl displays an incommensurate sinusoidal (TSW) structure with XY spin behaviour below T_N = 6.9 K, and only one ordered phase is found. The observed ordering temperatures roughly follow the de Gennes scaling factor.     

Path integral synthesis of lyapunov functionals for partial differential equations

A method is given for constructing Lyapunov Functionals for dynamical systems governed by partial differential equations. The functionals are obtained as path integrals in a suitably chosen state space of a generalized gradient operator, and the method may be viewed as an extension to infinite dimensional systems of the variable gradient technique. Some of the fundamental concepts underlying the formalism are reviewed, and examples of applications to some linear, non-linear and hybrid systems are given.     

Material electromagnetic fields and material forces

Electromagnetic fields address configurational forces in a natural way through an energy–stress tensor, which reduces to the Maxwell tensor in the simplest case. This tensor is related to physical forces and to the Cauchy traction in a continuum. Material forces, as opposed to physical forces, are of a different nature as they act upon a site of a continuum where the possible material inhomogeneity is located. A material energy–stress tensor, which is reminiscent of the Maxwell stress, is associated with these forces. Through appropriate balance laws, a material momentum is also associated with material forces. The material momentum is of particular interest in electromagnetic materials as it is intimately related to the pseudomomentum of light [Peierls in Highlights of Condensed Matter Physics, pp. 237–255 (1985) and in Surprises in Theoretical Physics, pp. 91–99 (1979); Thellung in Ann. Phys. 127, 289–301 (1980)]. The balance law for the material momentum can be derived either from the classical physical laws or independently of them. This derivation, which is based on the material electromagnetic potentials and the related gauge transformations, is discussed and commented on for an electromagnetic body.     

A preconditioning approach to the pagerank computation problem

Some spectral properties of the transition matrix of an oriented graph indicate the preconditioning of Euler-Richardson (ER) iterative scheme as a good way to compute efficiently the vertexrank vector associated with such graph. We choose the preconditioner from an algebra U of matrices, thereby obtaining an ER_U method, and we observe that ER_U can outperform ER in terms of rate of convergence. The proposed preconditioner can be updated at a very low cost whenever the graph changes, as is the case when it represents a generic set of information. The particular U utilized requires a surplus of operations per step and memory allocations, which make ER_U superior to ER for not too wide graphs. However, the observed high improvement in convergence rate obtained by preconditioning and the general theory developed, are a reason for investigating different choices of U, more efficient for huge graphs.     

Raman scattering in the spin glass system EuxSr1-xS due to spin correlation effects

Raman measurements in all magnetic phases of Eu_xSr_1-xS show a new type of q ≠ 0 magnetic scattering at low temperatures. The intensity is quantitatively described using a modified Ornstein-Zernike ansatz for the spin correlation function in the scattering cross section. Systematic variations of spin correlations as function of composition and temperature in high magnetic fields allow a consistent interpretation of the Raman data in correspondence with the magnetic phase diagram.     

From itinerant antiferromagnetism towards localized ferromagnetism in UNi(Al,Ga)

UNiAl is an itinerant antiferromagnet with a transition temperature of 18 K and a huge low-temperature Sommerfeld coefficient of 167 mJ/K² mole U atoms. A systematic study of the pseudoternary series UNiAl₁-xGa _x by means of magnetic and specific-heat measurements is presented. The data are interpreted as evidence for a transition from antiferromagnetism to ferromagnetism forx>0.4 with more localized features in the Ga-rich compounds. Forx=0.4 competition of ferromagnetic and antiferromagnetic interactions causes a complicated behaviour characterized by two types of magnetic transitions. Huge magnetocrystalline anisotropy is responsible for the rather anomalous behaviour of the ferromagnetic compositions.     

Lattice defects and its influence on hyperfine interactions on EuS evaporated thin films

In a study of the Mössbauer effect on¹⁵¹Eu in bulk EuS and evaporated thin films the influence of lattice defects on the hyperfine interactions has been investigated by variation of the substrate temperature during evaporation. The changes in the hyperfine and the lattice parameters with lattice defects are similar to the changes in those parameters observed in substances exhibiting an increasing portion of covalency.     

I–V characteristics and critical currents in superconducting/ferromagnetic bilayers

Current–voltage (I–V) characteristics and critical current density, J_c, for the onset of vortex motion were measured at different magnetic fields, H, and temperatures, T, in a superconducting (S)/ferromagnetic (F) bilayer and in a single Nb film. We choose Nb as a superconductor and a weak ferromagnetic alloy, Pd_1−xNi_x with x = 16, as F. We found that J_c was smaller for the S/F bilayer with respect to the single Nb film. The result was related to the reduced value of the superconducting order parameter in the bilayer.     

Dynamic response of converse magnetoelectric effect in a PMN-PT-based multiferroic heterostructure

A multiferroic heterostructure, consisting of a 25 μm thick Metglas® ribbon affixed to a lead magnesium niobate–lead titanate (PMN-PT) crystal, was systemically studied to investigate the time response of converse magnetoelectric coupling under the application of electric fields at low frequencies (0.05<f<10 Hz). This multiferroic heterostructure exhibits a considerably strong converse magnetoelectric effect, CME=?80%, where CME=[M(E)?M(0)]/M(0), and a converse ME coupling constant, A=22.5 Oe-cm/kV, at frequencies below 1 Hz and near saturation electric polarization. A switching time (t _s), representing the response time of the CME coupling, is measured to be 0.6 seconds for this heterostructure under the application of instantaneous electric fields. The switching time results in significant influences on the magnetoelectric effect especially at frequencies higher than 2 Hz. The dynamic response of CME coupling is predominantly determined by ferroelectric relaxation within the PMN-PT crystal, as opposed to the magnetic relaxation of the Metglas® ribbon. A model was used to describe the dynamic behavior of CME coupling in disordered systems such as PMN-PT.