摘要:

Using a wide-field Kerr microscope, we have studied the magnetic domain structures of epitaxial and polycrystallineLa0.67Sr0.33MnO3thin films as well as a film having thermally induced ⟨110⟩ microcracks. The epitaxial film on a (001)SrTiO3substrate has different magnetic domain behaviors for in-plane fields applied along the ⟨100⟩ and ⟨110⟩ directions. Magnetic domain orientation and contrast suggest a biaxial magnetic anisotropy with ⟨110⟩ easy axes. Defects such as microcracks and grain boundaries have a strong perturbing effect on the local magnetization and can lead to an enhanced and controllable spin-dependent scattering. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365700

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The coupling mechanism of the electromagnetic acoustic transducer (EMAT) for bulk waves has been studied by examining the magnetic-field dependence of the wave amplitudes. A spiral elongated coil placed on a thin plate of low carbon steel excites and receives the longitudinal and the shear waves propagating in the thickness direction in the presence of the bias magnetic field. The field dependences of the bulk-wave amplitudes are measured using the electromagnetic acoustic resonance both for the normal and tangential bias fields, which showed different features, depending on the bias-field direction and the wave mode. A two-dimensional model is presented for the explanation of the observed results. The present analysis emphasizes the inclined total field in the derivation of the magnetostriction constants, which is revealed to play an essential role for the wave generation. Both the measurement and the model analysis conclude that the magnetostrictive effect dominates the EMAT phenomena for the bulk waves in ferromagnetic metal, regardless of the bias field direction. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365701

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In-plane electronic transport in thin layered magnetic structures composed of two ferromagnetic films separated by a nonmagnetic spacer is analyzed theoretically in the Born approximation. Particular attention is paid to the role of interface roughness in the giant magnetoresistance (GMR) effect. The analysis applies to self-affine interfaces described by thek-correlation model. Our results show that GMR is sensitive to the roughness exponentH(0⩽H⩽1)in a manner that depends on spin asymmetries for bulk and interfacial scattering. The limit of low electron concentration is also considered. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365702

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Spin valve[FeNi/Cu/Co/Cu]Nmultilayers were prepared using the electron beam method. The influence of the thickness of magnetic and nonmagnetic layers, preparation process, and coercive forceHcon the magnetoresistance (MR) were studied. Using fairly optimum preparing technology, we prepared many samples with low center fieldH0in the range of 10–20 Oe, the MR merit is larger than 0.2&percent;/Oe, whereH0is the center magnetic field of the linear region in the MR-Hcurve. The variation of MR in this linear region is reversible during repeated magnetizing. The change of MR is smaller than 1&percent; for several samples after aging for one year in a dry air chamber at room temperature. Moreover, the MR value decreases greatly when Cr or Cu is added into the Co layer, or Co layers are replaced bySmCo5magnetic layers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365703

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Given a fitting model, such as the Kohlrausch–Williams–Watts (KWW)/stretched-exponential response, three plausible approaches to fitting small-signal frequency or time-response data are described and compared. Fitting can be carried out with either of two conductive-system formalisms or with a dielectric-system one. Methods are discussed and illustrated for deciding which of the three approaches is most pertinent for a given data set. Limiting low- and high-frequency log–log slopes for each of the four immittance levels are presented for several common models; cutoff effects are considered; and an anomaly in the approach to a single-relaxation-time Debye response for one of the conductive-system approaches is identified and explained. It is found that the temporal response function for the most appropriate conductive-system dispersion (CSD) approach, designated the CSD1, one long used in approximate form for frequency-response data analysis, does not lead to stretched-exponential transient behavior when a KWW response model is considered. Frequency-domain fitting methods and approaches are illustrated and discriminated using 321 and 380 KNa2O–3SiO2data sets. The CSD1 approach using a KWW model is found to be most appropriate for fitting these data exceedingly closely with a complex nonlinear least-squares procedure available in the free computer programLEVM.Detailed examination and simulation of the approximate, long-used CSD1 modulus fitting formalism shows the unfortunate results of its failure to include separately the effects of the always present high-frequency-limiting dielectric constant,&egr;D∞.The stretched-exponential exponent, &bgr;, associated with this fitting approach has always been misidentified in the past, and even after its reinterpretation, the result is likely to be sufficiently approximate that most physical conclusions derived from such fitting will need reevaluation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365704

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Using anin situx-ray diffraction (XRD) technique, the electric-field induced change in thed-spacings&Dgr;d/dof the individual microcrystals or “crystallographic” strain is measured on lead magnesium niobate—lead titanate—barium titanate electrostrictive ceramics of composition 0.970(0.900PMN−0.100PT)−0.030BT from zero field to saturation. This crystallographic strain is then compared to the bulk longitudinal strain as measured by&Dgr;l/lusing a photonic sensor. The crystallographic electric-field induced strain behavior for virgin samples could be fit to a constitutive model, where the induced strain is proportional to the square of the polarization with an electrostrictive coefficientQdetermined to be 1.4(×106cm4/C2).Attempts to fit the bulk strain behavior on the same sample required lower values of the electrostrictive coefficientQ∼1, and generally yielded worse fits where the model predicted greater than the observed strain values at low electric fields. Subsequentin situXRD measurements on previously electric-field cycled samples yielded much lower crystallographic strain values with a significantly different electric field dependence. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365705

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We show that by measuring the separation between the Stokes and anti-Stokes peaks excited by two different laser lines we obtain a very precise determination of absolute phonon energies. The method is useful for measuring small changes of these energies with strain, temperature, laser power, etc. It doubles the changes and avoids the necessity of using the reference lines in the Raman spectra. The method can be applied for the determination of phonon deformation potentials, for the characterization of strained heteroepitaxial layers, and for micro-Raman analysis of strain in silicon integrated circuits. We give examples of phonon shifts in Si, Ge, GaAs, InAs, and GaP as a function of applied biaxial strain, laser power, and temperature. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366537

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An extensive investigation of the fluorescence dynamics of the(2H11/2,4S3/2),4F9/2,4I9/2,4I11/2,and4I13/2states was carried out for 0.1&percent;, 1&percent;, 2.5&percent;, and 10&percent;Er3+doped inYVO4.The decay profiles of the luminescence of the4S3/2level ofEr3+(1.0&percent;) obey the Inokuti–Hirayama model for energy transfer for an electric dipole–electric dipole interaction in the absence of diffusion among the donors. From the analysis we were able to calculate the critical distance,R0,and it was found to be 5.7 Å: The dipole–dipole coupling parameter was estimated to be2.9×10−51 m6 s−1and compared well to other well known systems. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365706

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this work, we have studied the spectroscopic and dynamic properties of the green(4S3/2→4I15/2)and blue(2P3/2→4I11/2)upconversions for a series ofEr3+dopedCa3Al2Ge3O12,induced by a red tunable laser excitation. The green emission is due to either an energy transfer involving two ions excited in the4I11/2multiplet or an excited state absorption from the4I13/2level, depending on whether the laser excitation wavelength is tuned on4I15/2→4F9/2or4I13/2→4F5/2.The blue emission mainly results from a three-step absorption process through the4I13/2and4S3/2multiplets. The temperature and concentration effects have been analyzed for the green and blue upconversions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365707

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Fluorescence from the1D2level ofPr3+-doped AsGaGeS glass is found to be strongly quenched with increasing temperature. A model for this quenching is proposed which involves energy transfer from thePr3+ions to near-band gap defect states in the host glass. Energy transfer from these defect states back toPr3+in the1G4state with an estimated efficiency of≈10−3is also observed. The occurrence of these energy transfer processes complicates quantum efficiency measurements made using a self-calibrating branching ratio technique. After accounting for this energy transfer, the quantum efficiency for the 1300 nm emission inPr3+doped AsGaGeS glass is found to be 0.48 rather than the previously obtained value of 0.55. No temperature quenching of the1G4level ofPr3+is observed when this level is pumped directly.Nd3+impurities in the same glass are much less strongly quenched thanPr3+when pumping around 600 nm. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365708

出版商:AIP    

年代:1997

数据来源: AIP