Non-uniform Exchange Research Articles

Electromagnetic and absorption properties of urchinlike Ni composites at microwave frequencies

In this paper, nearly monodispersed urchinlike Ni powders were synthesized by a simple hydrogen-thermal reduction method. Electromagnetic and absorption characteristics were then investigated at 0.5–18 GHz. The permeability spectra present four resonance peaks over the whole frequency range. The resonance absorption property was discussed by fitting the permeability spectrum using the well-known Landau-Lifshitz-Gilbert equation and Maxwell-Garnett mixing rule. Correspondingly, the magnetic loss of the first band observed is attributed to the natural resonance, while the other three bands are considered to originate from non-uniform exchange resonance in the permeability spectra. The maximum reflection loss can reach −43 dB at about 10 GHz with 2 mm in absorber thickness.

Nonlinear excitations in strongly coupled Fermi-Dirac plasmas

In this paper, we use the conventional quantum hydrodynamics (QHD) model in combination with the Sagdeev pseudopotential method to explore the effects of Thomas-Fermi nonuniform electron distribution, Coulomb interactions, electron exchange, and ion correlation on the large-amplitude nonlinear soliton dynamics in Fermi-Dirac plasmas. It is found that in the presence of strong interactions, significant differences in nonlinear wave dynamics of Fermi-Dirac plasmas in the two distinct regimes of nonrelativistic and relativistic degeneracies exist. Furthermore, it is remarked that first-order corrections due to such interactions (which are proportional to the fine-structure constant) are more significant on soliton characteristics (particularly the amplitude) in the nonrelativistic plasma degeneracy regime rather than the relativistic one. In the relativistic degeneracy regime, however, these effects become less important and the electron quantum-tunneling and Pauli-exclusion dominate the nonlinear wave dynamics. Hence, application of non-interacting Fermi-Dirac QHD model to study the nonlinear wave dynamics in quantum plasmas, such as in compact stars is most appropriate for the relativistic degeneracy regime rather than nonrelativistic one.

Electromagnetic waves reflected from the plate of a magnetic with a ferromagnetic spiral

The spectrum of coupled spin and electromagnetic waves is obtained for a magnetic with a ferromagnetic spiral structure determined by nonuniform exchange and relativistic interactions. It is shown that resonant interaction between spin and electromagnetic waves is possible. The electromagnetic wave reflectance from the plate of a magnetic with a ferromagnetic spiral is calculated for different spiral angles.

Multi-resonances behavior of Ni nanobelt/paraffin composites at microwave frequencies

Ni nanobelts are synthesized through a facile hydrothermal method to form magnetic composites for high frequency applications. Four resonance peaks of the relative complex permeability illustrate the multi-resonance behavior in Ni nanobelt/paraffin composites in a frequency range of 2–18GHz. The resonance absorption characteristics are analyzed by fitting the permeability spectrum with the well-known Landau–Lifshitz–Gilbert (LLG) equation and Maxwell–Garnett mixing rule. Unlike previous studies, LLG equation has been deduced on account of the random distribution of Ni nanobelts in paraffin matrix first, and then applied for data fitting and resonance identification. Correspondingly, the first band is attributed to natural resonance, while the other three bands are considered to originate from non-uniform exchange resonance.

Negative permeability due to exchange spin-wave resonances in thin magnetic films with surface pinning

We report a theory of the effective permeability of multilayered metamaterials containing thin ferromagnetic layers with magnetization pinned on either one or both surfaces. Because of the pinning and small film thickness, the lowest frequency magnetic resonances are due to nonuniform exchange spin waves with frequencies far above those expected for uniform ferromagnetic resonance in known magnetic materials. Yet, the coupling of the nonuniform spin-wave modes to the electromagnetic field is shown to be strong enough to lead, for magnetic parameters characteristic for conventional transition metal alloys, to negative values of the effective permeability at frequencies of several hundred gigahertzs. The permittivity of metals is already negative in this frequency range. Hence, this system represents a negative refractive index metamaterial at subterahertz frequencies. The ways by which to maximize the frequency and the strength of the negative magnetic response are analyzed.

Phase-controlled proximity effect in ferromagnetic Josephson junctions: Calculation of the density of states and the electronic specific heat

We study the thermodynamic properties of a dirty ferromagnetic S$\mid$F$\mid$S Josephson junction with s-wave superconducting leads in the low-temperature regime. We employ a full numerical solution with a set of realistic parameters and boundary conditions, considering both a uniform and non-uniform exchange field in the form of a Bloch domain wall ferromagnetic layer. The influence of spin-active interfaces is incorporated via a microscopic approach. We mainly focus on how the electronic specific heat and density of states (DOS) of such a system is affected by the \textit{proximity effect}, which may be tuned via the superconducting phase difference. Our main result is that it is possible to \textit{strongly modify the electronic specific heat} of the system by changing the phase difference between the two superconducting leads from 0 up to nearly $\pi$ at low temperatures. An enhancement of the specific heat will occur for small values $h\simeq\Delta$ of the exchange field, while for large values of $h$ the specific heat is suppressed by increasing the phase difference between the superconducting leads. These results are all explained in terms of the proximity-altered DOS in the ferromagnetic region, and we discuss possible methods for experimental detection of the predicted effect.

Finite size effects with variable range exchange coupling in thin-film Pd/Fe/Pd trilayers

The magnetic properties of thin-film Pd/Fe/Pd trilayers in which an embedded ∼ 1.5 Å - thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8 to 56 Å . As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decreases, whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The critical exponent β of the order parameter (magnetization) is found to approach the mean field value of 0.5 as the thickness of the top Pd layer increases. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understand nanomagnetism in two-dimensions.

Effects of nonuniform exchange and magnetostatic interactions on the determination of intrinsic switching field distributions

A systematic numerical study is performed on the ΔH(M,ΔM) method and its ability to determine the intrinsic switching field distributions of perpendicular recording media in the presence of locally varying intergranular exchange and magnetostatic interactions. It is found by means of evaluating various reliability measures that the overall robustness of the ΔH(M,ΔM) method is only slightly reduced due to the presence of randomness in both the exchange and magnetostatic interactions, at least within the parameter range that is relevant for typical recording media.

Phenomenological model of the magnetic states of ferromagnetic film with competing surface and bulk anisotropies

The magnetic properties of a film with competing surface and bulk anisotropies and nonuniform ferromagnetic exchange interaction between atomic layers in the surface region have been investigated. The analytic expressions for the criteria of stability of the perpendicular and in-plane states of a film of arbitrary thickness have been derived in the framework of the layer by layer approach. We find that in films characterized by an arbitrary layer-dependent ferromagnetic exchange interaction between atomic layers and layer-dependent anisotropy constants, a canted noncollinear magnetic state becomes favorable when neither the perpendicular nor in-plane state is stable. We construct the diagram of the magnetic states of such a film and analyze the behavior of the magnetic state depending on film thickness. The developed theory can be applied to describe a wide class of quasi-two-dimensional magnetic systems demonstrating phase transitions between collinear and noncollinear magnetic states. In particular, we use this theory to describe the two-step spin-reorientation transition observed in bare $\mathrm{Co}∕\mathrm{Au}$ films.

Synthesis, Structure and Magnetic Properties of a Novel Hexanuclear Copper Methylsiloxane Complex

AbstractA new hexanuclear copper(II) sandwich complex based on two 12‐membered macrocyclic methylsiloxanolate ligands, [Cu6{(MeSiO2)6}2]·6DMF, was synthesized and characterized by single‐crystal X‐ray diffraction analysis and magnetic measurements. The cluster compound crystallizes in the monoclinic system, space group P21/n (No. 14), with a = 13.3728(5) Å, b = 15.4281(7) Å, c = 17.4335(7) Å, β = 98.932(3)° and Z = 2. The unit cell contains two identical molecular clusters, each consisting of six interacting Cu2+ (S = ${1 \over 2}$) ions. Within the cluster the six oxygen‐bridged Cu2+ ions are arranged in an almost regular hexagon. An analysis of the high‐temperature part of the magnetic susceptibility reveals that the complex has a strong average ferromagnetic Cu–Cu exchange interaction of |Jav/kB| = (50.4 ± 1) K with a high‐spin S = 3 ground state. A satisfactory fit for the magnetic susceptibility and the magnetization in the whole accessible temperature range is obtained from a Heisenberg model with nonuniform exchange couplings within a ring, corresponding to a system of two weakly coupled trimers with a ferromagnetic intratrimer exchange coupling of |J/kB| = 72.5 K and a ferromagnetic intertrimer exchange coupling of |J′/kB| = 7 K. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Microwave permeability change of FeCo nanocrystalline during high energy ball milling

The microwave magnetic properties of the ball milled FeCo particles were investigated as functions of ball milling time (t) using microwave electromagnetic parameters analysis techniques. The results show that the imaginary part of intrinsic dynamic permeability (μi) of the ball-milled particles is much bigger than that of raw powders. μi strongly depends on t and exhibits several slightly damped ferromagnetic resonances. These phenomena are in qualitative agreement with the formation of the corresponding microstructure or the Aharoni s model of non-uniform exchange resonance modes. The present microwave permeability behavior indicates that nanocrystalline materials with the same grain size may exhibit different properties that depend upon the microstructure, which provides a possibility for manufacturing high performance microwave absorber.

Flow Visualization Study of a Pulsating Respiratory Assist Catheter

Our group is currently developing an intravenous respiratory assist device that uses a centrally located pulsatile balloon within a hollow fiber bundle to enhance gas exchange rate via active mixing mechanism. We tested the hypothesis that the non-symmetric inflation and deflation of the balloon lead to both nonuniform balloon-generated secondary flow and nonuniform gas exchange rate in the fiber bundle. The respiratory catheter was placed in a 1-in. internal diameter rigid test section of an in vitro flow loop (3 L/min deionized water). Particle image velocimetry (PIV), which was used to map the velocity vector field in the lateral cross-section, showed that the balloon pulsation generated a nonuniform fluid flow surrounding the respiratory assist catheter. PIV was also used to characterize the fiber bundle movement, which was induced by the balloon pulsation. Gas permeability coefficient of the device was evaluated by using both the fluid velocity and the relative velocity between the fluid and the fiber bundle. The highest difference in the gas permeability coefficient predicted by using the relative velocity was about 17% to 23% (angular direction), which was more uniform than the 49% to 59% variation predicted by using the fluid velocity. The movement of the fiber bundle was responsible for reducing the variation in the fluid velocity passing through the bundle and for minimizing the nonuniformity of the gas permeability coefficient of the respiratory assist catheter.

Evaluation of Local Gas Exchange in a Pulsating Respiratory Support Catheter

An intravenous respiratory support catheter, the next generation of artificial lungs, is being developed in our laboratory to potentially support acute respiratory failure or patients with chronic obstructive pulmonary disease with acute exacerbations. A rapidly pulsating 25 ml balloon inside a bundle of hollow fiber membranes facilitates supplemental oxygenation and CO2 removal. In this study, we hypothesized that non-uniform gas exchange in different regions of this fiber bundle was present because of asymmetric balloon collapse and the interaction of longitudinal flow. Four quarter regions and two rings around the central balloon were selectively perfused to evaluate local gas exchange in a 3.18 cm test section using helium as the sweep gas. Quarter region CO2 exchange rates at 400 beats per minute were 156.8 +/- 0.8, 162.5 +/- 1.8, 157.2 +/- 0.2, and 196.6 +/- 0.8 ml/min/m2 (top, front, bottom, and back, respectively). The back section, adjacent to convex balloon collapse, had 17-20% higher exchange than the other sections caused by higher relative velocities past its stationary fibers. Inner and outer ring maximum pulsation gas exchange rates were 174.4 +/- 1.8 and 174.6 +/- 0.9 ml/min/m2, respectively, showing that fluid flow was equally distributed throughout the fiber bundle.

Peculiarities of ferrimagnetism of Gd/Co multilayers

The features of the ferrimagnetic ordering in multilayered Gd/Co films have been investigated with a torquemeter, vibrating sample and SQUID magnetometers, and ferromagnetic resonance. A phenomenological analysis of the possible mechanisms of magnetic interaction in these structures was made using the molecular field theory. The model of non-uniform interlayer exchange between Gd and Co layers has some advantages in comparison with the model of partial mixing of the layers.

Theory of Nonlinear Surface Spin Waves on a Pure Exchange Ferromagnet

In terms of the total energy of a ferromagnet and the Landau–Lifshitz equation, the nonlinear surface spin waves in a ferromagnet with pure nonuniform exchange anisotropies are studied. It is found that the nonlinear dispersion relation for these waves is modified by the nonuniform exchange anisotropy comstants, and the nonlinear excitation appears as surface magnetic solitons localized near the surface of a crystal. A two-dimensional nonlinear Schrödinger equation for the spin–wave envelope is obtained and the conditions for the existence of a single soliton solution are discussed.

A bike-wheel microcell for measurement of thin-film forces

A thin-film balance employing either a Sheludko capillary or a Mysels-inspired porous-medium film holder provides a direct measurement of disjoining-pressure isotherms in free, liquid thin films. However, each film holder suffers its own distinct disadvantages spanning non-uniform and slow liquid exchange, a limited range of measurable disjoining pressures, an inability for reuse, and a requirement for significant chemical amounts. In an attempt to alleviate these disadvantages, we have designed and constructed a miniaturized and microfabricated ‘bike-wheel’ cell as a replacement film holder. Essentially, an inner hole holding the thin film (the hub) is connected radially by 24 small channels (the spokes) to an outer, larger size annulus (the wheel). This design provides a hybrid of the Sheludko capillary and the Exerowa–Sheludko porous-plate film holders and eliminates the undesirable features of each. Moreover, due to its miniaturized dimensions and concomitant fast drainage rates, the bike-wheel film holder is particularly suited for investigation of polymer and/or protein-based systems where thin-film force laws depend on the degree of aging at the interface. The new bike-wheel microcell is validated quantitatively by reproducing a known disjoining-pressure isotherm for 0.1 M aqueous sodium dodecyl sulfate (SDS) foam films, including dynamic stratification, and reversible and oscillatory isotherm branches. Finally, application of the new bike-wheel film holder is made to thin-film forces in aqueous protein-stabilized foam films of bovine serum albumin (BSA) at the isoelectric point. Here we find a repulsive, steric stabilized disjoining-pressure isotherm for fresh protein films, but surface aggregation and non-equilibrium forces for aged films. The new bike-wheel microcell incorporated into the thin-film balance provides a useful tool for studying thin-film forces, especially for larger molecular weight stabilizing species.

Finite linear theory of viscoelastic generalized thermorheologically simple ferromagnets

A finite linear theory of viscoelastic generalized thermorheologically simple ferromagnets is constructed with allowance for nonuniform exchange, when the derivative with respect to intrinsic time is an arbitrary functional of the independent field variables and their derivatives. Free energy is treated as an arbitrary functional of the past history of these variables. Specific constitutive equations are presented for such a model. Variants of the history of the variables are also examined for the case when a small nonsteady perturbation is superimposed on the steady state.

Two-Dimensional Modeling of Heat Transfer Through Sandwich Plates With Inhomogeneous Boundary Conditions on the Faces

This paper presents a thermal model describing heat transfer in multispan sandwich rectangular plates. The model is time-dependent and two-dimensional. Complex thermal phenomena occurring in light cores (honeycomb) and thermal contact resistance are taken into account in the model. Particular attention is paid to the boundary conditions on the faces of the plate: radiative and nonuniform convective exchange are taken into account. The global temperature solution for stationary cases is developed on trigonometric and exponential functions bases. The properties of the Laplace transform are used to solve time-dependent cases. Validation experiments are carried out in a wind tunnel ring for two-span plates submitted to convective exchanges on one face; the same configuration is reproduced for an aluminum sandwich plate in an industrial set-up bench. Experimental results agree well with the numerical simulation.

Magnetic properties of the diluted ferrimagnet Li 0.5Fe 1.1Ga 1.4O 4 with spatially nonuniform exchange

The isotherms of the specific magnetization, σ T ( H), and temperature dependences of the initial susceptibility, χ 0( T), have been investigated in the temperature interval 77–300 K and in magnetic fields from 0 to 17 kOe as well as at T=4.2 K and H⩽40 kOe on the diluted ferrimagnet, Li 0.5Fe 1.1Ga 1.4O 4. In this specimen, there is a spatial nonuniformity of the short-range indirect exchange interaction Fe 3+–O 2–Fe 3+ due to the composition disorder of nonmagnetic ions Ga 3+ in the spinel lattice. In order to change the extent and/or the character of this disorder, we used different regimes of heat treating of the initially synthesized polycrystalline samples at 1550 K: the quenching in air from T=1550 K (the Q sample) and the slow cooling to the room temperature together with the furnace (the SC sample). As a result of quenching an increase in the Curie point, T c, from ∼160 K (SC) to 240 K (Q) is observed, while the isotherm behaviour at 4.2 K differs only slightly (by ∼5% in magnitude of σ). For the Q sample at T⩾77 K irregular curves σ T ( H) are observed and the magnetization relaxation follows an exponential ( T=77 K, H=0.71 kOe) or logarithmic law ( T=123 K, H=0.74 kOe; T=177 K, H=0.73 kOe; T=77 K, H=1.55 kOe). Magnetic properties, the effects induced by the sample quenching included, are discussed within the model of the spatially nonuniform magnetic structure which may be visualized as two exchange-interacting subsystems: the matrix with a high concentration of the frustrated exchange bonds and the clusters, where spins are coupled by a strong exchange.

Exchange mechanism for localization of phonons near the surface of a magnetically ordered crystal

It is shown that first-principles inclusion of nonuniform exchange interaction leads to the creation of a new type of generalized shear acoustic wave propagating near the mechanically free surface of a magnet. Criteria are formulated that can be used to specify conditions under which an “exchange” type of surface acoustic wave can exist at the boundary between a magnetic and a nonmagnetic medium once the spectrum of magnetoelastic oscillations of the unbounded magnetic crystal is known.