摘要:

By the use of x‐ray and neutron topography, spin‐density‐wave (SDW) domains in pure chromium (IOCHROME), both in the virgin state and after field cooling, have been found to be of the order of a few mm in length and of several mm3in volume. The change in configuration of the domains before and after field cooling is reasonable. This was found in a nearly perfect well‐characterized sample. In a strain‐annealed sample, the domain size was below the resolution.

ISSN:0021-8979    

DOI:10.1063/1.324575

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Using a transit‐time technique, we have measured domain‐wall velocities versus applied fieldHfor Bloch, Neˆel, and the head‐to‐head walls in bulk single‐crystal bars of yttrium orthoferrite from 150 to 600 °K. Fields as high as 100 Oe were applied and wall velocities exceeding 106cm/sec were recorded. After correcting for magnetostatic effects, all three walls display a well‐defined linearv‐Hregion, with a common mobility &mgr; of 1.2×104cm/sec Oe at 300 °K. As a function of temperatureTthe wall mobility varies as 1/T2. From these results it is argued that the mobility arises from thermal spin‐wave scattering. Velocities for the three walls become field independent abruptly atvs=4.2×105cm/sec and maintain this velocity untilHexceeds ∼60 Oe, whereupon the velocity again increases. We demonstrate that this ’’saturation’’ atvsis due to the interaction between the wall and the bulk acoustic phonons.

ISSN:0021-8979    

DOI:10.1063/1.324576

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The mobility of domain walls in YFeO3is determined by calculating the damping of the uniform precession or k=0 spin‐wave mode. The presence of antisymmetric exchange is explicitly taken into account in obtaining the spin‐wave modes. The dominant spin‐wave damping mechanism is found to be a four‐magnon scattering process. This process leads to a calculated wall mobility that has the same order of magnitude and temperature dependence as that observed.

ISSN:0021-8979    

DOI:10.1063/1.324577

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The large &Dgr;Eeffect exhibited by the alloy Fe2(Tb0.2Dy0.22Ho0.58) has been measured dynamically at low frequencies under conditions which closely approach quasistatic equilibrium. It is found that, in common with several other materials, Young’s modulusEpasses through a distinct minimum as a function of magnetizing field. The modulus minimum is most apparent during initial magnetization, but occurs with reduced prominence and with typical hysteresis as the material is cyclically magnetized. The origin of the minimum is reviewed in terms of the behavior of the macroscopic &Dgr;Eeffect, and the frequency above which the minimum is expected to disappear from the measured response is discussed in terms of the theory of stress‐induced macroeddy currents. The magnetoelastic internal friction exhibits unexplained features which indicate a need for further study of these alloys.

ISSN:0021-8979    

DOI:10.1063/1.324578

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The magnetic‐wave interactions in a YIG film having periodically corrugated surfaces are investigated for the case of magnetization perpendicular to the propagation direction and parallel to the length of the corrugations. Simple and asymptotically exact analytical expressions for the characteristics of the resulting wave filter are deduced by a systematic singular‐perturbation procedure. Numerical results of the characteristics of the wave filter are obtained and compared with the available experimentally observed values.

ISSN:0021-8979    

DOI:10.1063/1.324526

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Chromium chrysoberyl undergoes a phase transformation from a paramagnetic state to a complex antiferromagnetic state at 28 K. The spiral spin structure of the antiferromagnetic state violates all the crystallographic symmetry elements, making Cr2BeO4potentially ferroelectric. Chynoweth experiments conducted at low temperatures reveal a weak pyroelectric effect which disappears above 28 K. Cr2BeO4ceramics can be poled electrically between 24 and 28 K, giving rise to remnant polarizations four to six orders of magnitude smaller than normal ferroelectrics. The pyroelectric coefficient and the remnant polarization reverse in sign with the poling field, but no anomalies in the electric permittivity or electric conductivity occur at the Ne`el point.

ISSN:0021-8979    

DOI:10.1063/1.324527

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Ferroelectric hysteresis of a copolymer of vinylidene fluoride with tetrafluoroethylene and of uniaxially stretched polyvinylidene fluoride has been observed at 60 Hz at room temperature. The measured remanent polarization of the copolymer was in good agreement with a calculated value using a frozen dipole model and measured piezoelectric coefficients. The dielectric constant for the copolymer has been measured at 1000 Hz as a function of temperature. Also, a set of copolymer films of varying thickness were poled above the melt temperature and their piezoelectric stress coefficientsg31were measured as a function of the poling field. These measurements indicate that the copolymer, when poled above the melt temperature, exhibits bulk polarization. These results suggest that this copolymer is ferroelectric and that poling aligns the dipoles in the &bgr; crystal phase of the material.

ISSN:0021-8979    

DOI:10.1063/1.324528

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

At certain wavelengths, depending on the thickness of a thin transparent dielectric layer on an absorbing substrate, a standing‐wave condition can be produced that efficiently couples theSwave to the substrate, simulating a cavity resonance in the dielectric and maximizing the complex reflectance ratio. Under these conditions high sensitivity is obtained to the nature of the interface between the dielectric and the substrate. We illustrate the method by applying it to plasma‐oxidized GaAs samples using scanning ellipsometry from 1.5 to 5.6 eV. As‐grown samples exhibit a composite transition layer at the interface consisting of a mixture of oxide, unoxidized GaAs, and elemental As. A crystalline As layer is formed at the interface by annealing in N2at 550 °C.

ISSN:0021-8979    

DOI:10.1063/1.324529

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Minority‐carrier lifetimes, internal quantum efficiencies, and values of the radiative recombination coefficientBare determined from photoluminescence time‐decay and external quantum efficiency data taken for LPE GaAs samples (germanium doped or undoped) in the doping range 1.9×1015?p0?1×1019cm−3at 300 °K. Measurements are made on isotype double heterostructure samples where the optically exicted GaAs layer is bounded by wider‐band‐gap Ga0.5Al0.5As layers which provide for confinement of minority carriers and also minimize the importance of surface or interfacial recombination on the measured lifetimes. Comparison with time‐decay data for samples without the ternary cladding layers shows the dominant effect of surface and substrate recombination on the decay time if confinement layers are not provided. Luminescence decay times are observed from 1.2 nsec for heavily doped samples up to 1.3 &mgr;sec for lightly doped samples. Values of the bulk minority‐carrier lifetimes, radiative lifetimes, and internal quantum efficiencies are determined. The experimental values forBagree well with existing theory for heavily doped material. For lightly doped material, the experimentalBcoefficient is larger than expected from the single‐electron model.

ISSN:0021-8979    

DOI:10.1063/1.324530

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We have investigated Ce3+ : Y3Al5O12(YAG) as a model system for a 5d‐4fsolid‐state tunable laser. This system has been chosen since YAG has been extensively studied as a laser host and good quality crystals are readily available. Despite providing apparently adequate conditions to achieve stimulated emission, we were unable to detect laser action in Ce3+ : YAG. We find, however, strong excited‐state absorption in this material at the wavelengths of its fluorescence and we report on this excited‐state absorption (ESA) here. This self‐absorption may explain the failure of all attempts to obtain stimulated emission in this material and has serious implications for any attempt to develop a 5d4fn−1‐4fnlaser.

ISSN:0021-8979    

DOI:10.1063/1.324531

出版商:AIP    

年代:1978

数据来源: AIP