摘要:

Mechanical milling and subsequent annealing have been applied in order to obtain FeRh compound, as an alternative to bulk sample preparation methods (plasma jet- and arc-melting, induction, etc). Structural and magnetic characterization of the as-milled sample show that heat treatment at the proper temperature is necessary in order to obtain the equilibrium ordered &agr;′-bcc phase, in which a first order ferro–antiferromagnetic transition accompanied by a large magnetovolume effect takes place. In order to study the magnetoelastic behavior of the mechanically alloyed FeRh compound, magnetostriction measurements were performed using high pulsed magnetic fields up to 14.2 T within the temperature range 4–450 K. The large magnetostriction observed in the room temperature range has been related to the field instability of the rhodium magnetic moment within the bcc ordered phase. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.364290

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The present analysis providesTb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0)magnetization and magnetostriction states during applied magnetic field and stress processes by minimizing the total magnetic free energy in the presence of a parameter selected inverse exponential magnetic free energy distribution. A comparison between experimental and model results shows that the model provides a qualitatively accurate description of the magnetostriction and magnetization inTb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0)during applied magnetic field processes for a range of constant uniaxial applied stresses. Therefore, the model appears to be a useful tool for the analysis ofTb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0)applications with constant stress or strain dependent stress states. Furthermore, the good qualitative agreement between model and experiment supports the contention that the magnetostriction and magnetization response ofTb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0)is influenced by a defect induced defocusing of local magnetization direction about the average magnetization direction. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.364235

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The antiferromagnetic (AF) coupling and magnetoresistance in magnetron-sputtered polycrystalline NiFeCo/Cu superlattices grown on Fe, Cr, or Zr buffer layers have been studied. The results show that the buffer layers significantly affect AF coupling and magnetoresistance through modulating the growth of specifically oriented crystalline multilayered structures. Two types of buffer layer effects on AF coupling and magnetoresistance are considered: modifying interfacial roughness and producing layer-thickness fluctuations. Interfacial roughness is formulated in the first Born approximation using data obtained from specular and off-specular (diffuse) x-ray scattering measurements. For the roughness at interfaces, correlated interfacial roughness is found to predominate and estimated to be as large as 5–7 Å with a moderately large lateral correlation length scale &xgr; that ranges from 120 to 200 Å. Upon approximately the same AF coupling, for Fe or Cr buffer layered superlattices characteristic of well-defined, comparatively flat layer structures throughout the entire specimen, magnetoresistance turns out to increase as the correlated interfacial roughness increases. For the superlattices grown on Zr buffer layers in which the presence of columnar structures is confirmed, a constant, weak AF coupling and moderate magnetoresistance concur and are both almost independent of buffer layer thickness. These features are explained in terms of the averaging of AF coupling and magnetoresistance through parallel layer-thickness fluctuation structures approximating the columnar structures. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364236

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A ferromagnetic crystal with cubic and induced orthorhombic and uniaxial anisotropies is considered. The hard uniaxial anisotropy axis is parallel to the [110] direction, the hard orthorhombic axis parallel to the [1¯10] direction. Localized spin waves propagating along a single domain wall in such materials are analyzed by numerical solution of the Landau–Lifshitz and magnetostatic equations. The spin wave spectra are calculated for propagation directions either perpendicular or parallel to the direction of the magnetization in the two adjacent domains. There exist critical combinations of magnetic anisotropy constants for which the uniform Bloch wall becomes unstable before the Neel wall is preferred. In such parameter regions Goldstone modes show a gap in the wave number and a loop of the dispersion branch for the case of perpendicular propagation. This behavior is very similar to that of hydrodynamic waves excited in stratified shear flows. The existence of negative energy waves and the appearance of a Kelvin–Helmholtz instability are to be expected for spin waves propagating along an initially plane domain wall in the absence of dissipation. All spectra for perpendicular propagation are asymmetric with respect to wave vector inversion. They also contain the unidirectional Gilinskii-type mode which has a finite frequency at zero wave number. This activation frequency coincides with the frequency of the bottom of the bulk spin wave manifold for propagation parallel to domain magnetizations. Measurements of resonant excitations of single domain walls in bismuth-substituted garnet films with in-plane magnetization are presented. The resonances are observed in the frequency range from 50 to 1500 MHz. They are qualitatively discussed on the basis of the developed theory. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364237

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The dielectric permittivities parallel and perpendicular to thecaxis (optic axis) of ice Ih were measured using an open resonator at 39 GHz in the temperature range 194–262 K. The dielectric anisotropy in ice at microwave frequencies is important for understanding remote sensing data in polar regions, obtained by ice radar and satellite-born microwave radar and radiometer. The measured samples were natural single-crystal ice collected from Mendenhall Glacier, Alaska. A very precise measurement was achieved by detecting two resonant peaks, one from the ordinary component and the other from the extraordinary component, simultaneously, from one sample. The real part of dielectric anisotropy,&Dgr;&egr;′=&egr;∥c′−&egr;⊥c′,at 39 GHz was 0.0339±0.0007 (1.07&percent;±0.02&percent;) at 252 K and slightly depended on temperature. Reference measurements at 1 MHz using parallel plate electrodes were also carried out. The measured dielectric anisotropy at microwave frequencies agrees very well with the value at 1 MHz. The absolute values of&egr;∥c′and&egr;⊥c′at 39 GHz were, respectively, smaller than those at 1 MHz and the difference was about 0.044 at 252 K. The results suggest that a small dispersion exists between GHz and MHz frequencies, but there is no frequency dependence in the value of anisotropy. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364238

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Single-crystal thin films ofPb(ZrxTi1−x)O3(PZT) covering the full compositional range (0⩽x⩽1) were deposited by metal-organic chemical vapor deposition. Epitaxial SrRuO3(001) thin films grown on SrTiO3(001) substrates by rf-magnetron sputter deposition served as template electrode layers to promote the epitaxial growth of PZT. X-ray diffraction, energy-dispersive x-ray spectroscopy, atomic force microscopy, transmission electron microscopy, and optical waveguiding were used to characterize the crystalline structure, composition, surface morphology, microstructure, refractive index, and film thickness of the deposited films. The PZT films were single crystalline for all compositions exhibiting cube-on-cube growth epitaxy with the substrate and showed very high degrees of crystallinity and orientation. The films exhibited typical root mean square surface roughness of ∼1.0–2.5 nm. For tetragonal films, the surface morphology was dominated by grain tilting resulting from ferroelectric domain formation. We report the systematic compositional variation of the optical, dielectric, polarization, and electronic transport properties of these single-crystalline PZT thin films. We show that the solid-solution phase diagram of the PZT system for thin films differs from the bulk due to epitaxy-induced strains and interfacial defect formation. High values of remanant polarization (30–55 &mgr;C/cm2) were observed for ferroelectric compositions in the range of 0.8⩽x⩽0.2. Unlike previous studies, the dielectric constant exhibited a clear dependence on composition with values ranging from 225 to 650. The coercive fields decreased with increasing Zr concentration to a minimum of 20 kV/cm forx=0.8. The undoped films exhibited both high resistivity and dielectric-breakdown strength (1013–1014&OHgr; cm at 100 kV/cm and 300–700 kV/cm, respectively). ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364239

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

SrTiO3thin films have been grown on Si(100) substrates by pulsed laser deposition using an epitaxial TiN film as a buffer layer, and their structural and dielectric properties were investigated as functions of deposition parameters. X-ray diffraction analysis showed that theSrTiO3films were grown epitaxially in the wide range of substrate temperatures(400–650 °C)and ambient oxygen pressure(10−5 Torr–150mTorr) with an orientation relationship ofSrTiO3(100) // TiN(100) // Si(100) andSrTiO3⟨010⟩// TiN⟨010⟩// Si⟨010⟩. The crystallinity of the epitaxial films was improved with the increase of the substrate temperature and decrease of the ambient oxygen pressure, while the film surface morphology was degraded with increasing either of the two parameters. The relative dielectric constant of the films was revealed to depend both on the crystallinity and on the surface roughness of the films, the highest value of which was&Vegr;r=270at 1 MHz, comparable to that of bulkSrTiO3. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364240

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The electrical breakdown of low density polyethylene films 100 &mgr;m thick was investigated experimentally and theoretically under high hydrostatic pressure (1 to 500 bar) at temperatures of 20, 70, and 90 °C. The theoretical analysis was based on combining the two basic mechanisms of thermal and electronic breakdowns by introducing a new parameter, pressure. This model mainly takes into account the transient current using the hopping transport phenomenon and local electric field distortion due to the presence of space charges in the material. The results of simulation show good agreement with experiments compared to our previous works on the modeling of electrical breakdown in which the effect of space charges was neglected. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364241

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The frequency response of the dielectric constant(&egr;r), dielectric loss tangent (tan &dgr;) and impedance (Z) of an organic electro-optic crystal,(−)−2−&agr;- methylbenzylamino-5-nitropyridine (MBA-NP) monitored along the polar axis, is characterized by the presence of strong peaks in the kHz region. The observed resonance behavior, which is strongly dependent on the geometric size and shape of the sample, is attributed to its piezoelectric nature. The resonance peaks were found to shift steadily towards lower frequencies with increasing temperature and towards higher frequencies with increasing pressure. The temperature dependence of the stiffness coefficient(C)of the crystal has been defined using the resonance data. The temperature dependence of the damping of these piezoelectric vibrations has been monitored by the resonance–anti-resonance method. The influence of the application of pressure and the variation of temperature on&egr;r,monitored along the polar axis of the crystal, has been studied. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364292

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The randomness in the structure of two-component dense composite materials influences the scalar effective dielectric constant, in the quasistatic limit. A numerical analysis of this property is developed in this paper. The computer-simulation models used are based on both the finite element method and the boundary integral equation method for two- and three-dimensional structures, respectively. Owing to possible anisotropy the orientation of spatially fixed inhomogeneities of permittivity&Vegr;1,embedded in a matrix of permittivity&Vegr;2,affects the effective permittivity of the composite material sample. The primary goal of this paper is to analyze this orientation dependence. Second, the effect of the components geometry on the dielectric properties of the medium is studied. Third the effect of inhomogeneities randomly distributed within a matrix is investigated. Changing these three parameters provides a diverse array of behaviors useful to understand the dielectric properties of random composite materials. Finally, the data obtained from this numerical simulation are compared to the results of previous analytical work. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364276

出版商:AIP    

年代:1997

数据来源: AIP