摘要:

We investigated the effect of thermal relaxation on reversible transverse susceptibility (RTS), &khgr;, of single domain Stoner–Wohlfarth particles. We studied such an effect as a function of the orientation &bgr; of the easy axis with respect to the drive field (thezaxis) within a non‐interacting bi‐state model. At &bgr;=00one prominent cusp occurs for both &khgr;t1and &khgr;t2along the direction perpendicular to the drive field atH&bartil;HC<HN(nucleation field) at high valuesKV/kBT(the ratio of the anisotropy energy versus thermal energy). The height of the cusp varies with &bgr; and reduces with decreasing values ofKV/kBT. Correspondingly, the switching field distribution (SFD) becomes broadened with decreasing values ofKV/kBT. The RTS may exhibit one, two, or three prominent cusps at high values ofKV/kBTdepending upon the textural distribution of the easy axes. For cos2&thgr; distribution of the easy axis about thexaxis,yaxis, andzaxis there exists two, three, and one cusps, respectively. For random distribution of the easy axis in 2‐D in theXZandYZplane there are one and three cusps, respectively, while there are three for random distribution in 3‐D. Finally, we studied coercivity, RTS and SFD as a function of sweep rate. All these quantities are found to increase with sweeping frequency of the drive field. The peak field value of RTS is consistently the smallest of the three, while that of SFD is equal to or a shade larger thanHCdepending upon whether the hysteresis loop is rectangular or severely sheared. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358618

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Measurements of structure and magnetoresistance in Ag‐Co0.4Ni0.6granular alloy films with 10–23.5 vol % Co0.4Ni0.6grown epitaxially on MgO{100} are reported. These results elucidate the effects of varying both the concentration and the size of the Co0.4Ni0.6precipitates in the Ag matrix. It is shown that the magnetic‐field‐dependent contribution to the resistivity, &Dgr;&rgr;mag, is consistent with the predictions of a two‐current model in which spin‐dependent scattering is dominated by the interfaces between the precipitates and the matrix. Specifically we observe: (a) A regime where &Dgr;&rgr;magis linear inc/D, withcthe volume fraction of precipitates with diameterD. (b) Extrapolation of these linear dependences to &Dgr;&rgr;mag=0 gives values of 1/Dthat are proportional to 1/c. (c) WithD≳100 A˚ there is a reduced dependence on particle size that is consistent with transition to a quadratic dependence. In contrast, at the lower concentrations the behavior of the maximum resistivity, &rgr;max, is not well described by the two‐current model that suffices for &Dgr;&rgr;mag. This suggests that refinement of the model may be needed for granular films that have relatively large and widely spaced ferromagnetic precipitates. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359563

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Liquid‐quenched metallic amorphous alloys (FeCo)75(SiB) have been studied by57Fe Mo¨ssbauer spectroscopy at room temperature. Direct insight in the short‐range order (SRO) and the bonding properties involving Fe atoms in these alloys is presented, taking apart clearly the metal and metalloid influence over such properties. A large increase of the SRO is found in the Co‐rich alloys which is also affected by variations in the relative concentration of metalloids Si and B. Such SRO changes influence the bonding of Si and B with Fe. The substitution of B for Si decreases the Fe 3dpolarization in the Co‐rich alloys with large SRO. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358619

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

1H nuclear magnetic resonance (NMR) is widely used for the study of water kinetics into various porous systems like sandstones, zeolites, clays, and various cementitious materials. In the case of hydrating cement pastes the evolution of the cement structure ultimately leads to a broad pore size distribution, and consequently to a multiexponential magnetization recovery represented in many cases by a Kohlraush–Williams–Watts ‘‘stretched exponential’’ curve. However, it has been proven an extremely difficult task to fully clarify and exploit the information contained in the multiexponential magnetization recovery of cementitious materials. This difficulty is due to the fact that spin‐lattice relaxation timesT1exhibit broad multimodal distributions, which sometimes extend over more than two orders of magnitude. In this work, a numerical routine using a combination of Monte Carlo and Simplex iterations (MCS) is examined which appears to estimate faithfully broad and complex spin‐lattice relaxation time distributions. First, the reliability of the routine has been tested on simulated data and then the algorithm has been applied to the analysis of1H NMR time domain measurements performed on different kinds of cement paste. The development of the pore structure is directly reflected in the time evolution of theT1distribution function. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358620

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This article reports the fabrication of thick films of lead zirconate titanate (PZT) on platinum‐buffered silicon substrates by screen printing. Crack‐free films, up to 12 &mgr;m on a single pass, show a dielectric permittivity of 200, tangent losses of 0.05, remanent polarization of 2.5 &mgr;C/cm2, and coercive field of 40 kV/cm. The field‐induced longitudinal piezoelectric coefficientd33at 40 kV/cm dc bias and 4 kV/cm alternating field corresponded to 50 pC/N. The magnitude of the piezoelectric voltage coefficientg33, computed from the strain coefficient and dielectric permittivity, under the same conditions, was found to be 36×10−3V m/N, higher than that of a poled PZT bulk ceramic in comparison. These results are promising for a broad variety of sensor applications. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358621

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Double hysteresis loops have been observed in ferroelectric lead zirconate titanate ceramics with a Zr/Ti ratio of 65/35 (PZT 65/35). In addition, dielectric dispersion was found near 50 °C, corresponding to the high‐temperature to low‐temperature rhombohedral ferroelectric [FER(HT)‐FER(LT)] transformation. Structurally this transformation is an oxygen octahedra tilting along the ⟨111⟩. It is proposed that an inability of the oxygen octahedra to coherently rotate within a rigid lattice generates random internal stresses which constrain the polarization. Small amounts of impurities on either the A site (La) or B site (Nb) were found to result in normal square‐loop hysteresis behavior. It is believed that aliovalent impurities relax the lattice, resulting in increased coherent of the ⟨111⟩ oxygen rotations. Electron diffraction studies revealed a strong increase of the reflection intensity of 1/2⟨111⟩ superlattice spots after doping, supporting this possibility. In addition, thermal expansion measurements revealed a lattice contraction after La‐modification, possibly reflecting enhanced octahedral tilting. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358622

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Lead scandium tantalate was prepared by a polymeric sol‐gel method, and the crystallization into the perovskite phase was determined at significantly reduced temperatures (600–700 °C). The extent of B‐site cation ordering was also determined as a function of cooling conditions. Results are reported for the integration of capacitors on silicon with a capacitance density greater than 200 fF/&mgr;m2. A preferred (111) orientation was obtained, with (1/21/21/2)B‐site cation partial ordering. No evidence was obtained forA‐site Pb vacancy ordering. Results are reported for x‐ray diffraction and selected‐area electron diffraction investigations. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358623

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy carried out on bulk samples of Er:As2S3glasses demonstrate that Er3+is incorporated in optically active sites in the glass and gives rise to a broad ∼1500–1600 nm,4I13/2→4I15/2emission spectrum similar to those observed in Er‐doped oxide glasses. In addition to the expected 980 nm (4I15/2→4I11/2) Er3+PLE band, the 1500–1600 nm Er3+PL band in the glass exhibits a remarkably broad PLE spectrum which extends from the As2S3Urbach absorption edge at ∼550 nm to beyond 1000 nm. This broad PLE band corresponds closely to an exponential PLE band observed in the ‘‘band tail’’ spectral range for Er‐doped Ge33As12Se55glasses. These unusual PLE spectra indicate that in Er‐doped chalcogenide glasses there is an additional broad‐band, below gap, extrinsic absorption mechanism which efficiently excites the characteristic 1550 nm,4I13/2→4I15/2Er3+emission band. It is not possible to determine at present whether the Er dopants themselves are responsible for the broad band absorption which excites the Er3+PL bands, or if photoexcited carriers attributable to absorption by other impurities transfer their energy to the excited states of the Er3+ 4fshells. Microscopic characterization techniques reveal that the Er2S3‐doped glasses also contain residual Er2S3crystallites which give rise to sharp, narrow line 1550, 980, and 810 nm Er3+optical spectra characteristic of polycrystalline Er2S3. The temperature dependence of the Er2S3PL and PLE spectra enables the energy levels of the4I13/2excited state and4I15/2ground state manifolds to be determined. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358624

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present the results of theoretical studies numerically analyzing the optical absorption and gain properties in AlGaAs fractional‐layer‐superlattice (FLS) quantum wires. With our calculation method we can treat any asymmetrical FLS wire with arbitrary dimensionality from 2D to 1D and to calculate not only absorption spectra but also gain properties. Our theory accurately reproduces the optical absorption anisotropy, experimentally evaluated for an AlGaAs FLS quantum wire. It is numerically shown how optical features evolve as the quantum confinement changes from 2D to 1D by varying the FLS lateral modulation. The small modulation of Al content in the AlGaAs FLS layer is found to change the optical properties markedly and improve the gain characteristics largely. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358625

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Hydrogen incorporation into Si‐doped InP grown by gas‐source molecular beam epitaxy was studied. P‐H sites were identified by infrared spectroscopy. Proton‐implanted reference samples were used to quantify the infrared results. Approximately 0.1 at. % hydrogen was found to be incorporated into InP:Si. Hall measurements indicated that most of the Si atoms were electrically active as donors. Rapid thermal annealing at 600 °C removed most of the bonded hydrogen from the samples. However, this resulted in relatively little change in either the room‐temperature free‐carrier concentration or Hall mobility. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358626

出版商:AIP    

年代:1995

数据来源: AIP