摘要:

Transmission electron microscopy is used to provide a detailed description of how magnetization reversals take place in NiFe/Cu/NiFe/FeMn spin‐valves. Direct observation is made of how both NiFe layers respond to an applied field. Marked differences from the behavior observed in single Permalloy layers of the same thickness are identified. Complex 360° wall structures frequently form and are studied in some detail. A description of their structure is given and a theory involving the compensation of charges from the biased layer is suggested to explain stability. The work has a direct bearing on the performance of spin‐valves as sensors as the way the magnetization changes under the influence of an applied field affects the noise characteristics. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359676

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Co‐Nb/Pd multilayers were prepared by a radio‐frequency sputtering method. As the thickness of Pd layers increases, the phases of Co‐Nb layers periodically alternate from an amorphous state to a crystalline state. The oscillation of the strain of Pd layers and that of the coherent length versus the thickness of Pd layers with a period of about 10 A˚ are obtained. The strain is caused mainly by the degree of lattice mismatching of Co‐Nb layers and Pd layers, but the oscillatory behavior of the strain may be related to the oscillatory interlayer coupling between Co‐Nb layers through Pd layers. The oscillation of the saturation magnetization is also obtained. It corresponds to the oscillatory polarization of Pd layers and the structural change in Co‐Nb/Pd multilayers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359677

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The magnetic properties of a series of Ce2Fe17−xSixsolid solutions withxequal to 0.0, 0.23, 0.4, 0.6, 0.8, 1.02, 1.98, and 3.20 have been studied by magnetic measurements, neutron diffraction, and Mo¨ssbauer spectroscopy. An x‐ray‐diffraction study indicates that the compounds adopt the rhombohedral Th2Zn17‐type structure. The substitution of silicon for iron in Ce2Fe17leads to a contraction of theaaxis by 0.2%, an expansion of thecaxis by 0.2%, and a consequent reduction of the unit‐cell volume by about 0.2% per substituted silicon. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17to 455 K for Ce2Fe14Si2. Powder neutron‐diffraction results, obtained at 295 K, indicate both that the silicon atoms preferentially occupy the 18hsites and that the iron moments increase with increasing silicon content, an increase which is related to the increase in Curie temperature. The Mo¨ssbauer spectra have been fit with a binomial distribution of the near‐neighbor environments in terms of a maximum hyperfine fieldHmaxfor an iron with zero silicon near neighbors, and a decremental field &Dgr;Hper silicon near neighbor. The compositional independence of both the weighted average maximum hyperfine field and of the decremental field indicates that the silicon acts as a magnetic hole, a hole which does not perturb the magnetic moments at the iron sites. The compositional dependence of the weighted average isomer shift is explained in terms of an interband mixing of the iron 4sand silicon 2pbands, due to the reduction of the iron 18hbond lengths. This interband mixing affects the charge but not the spin distribution at the iron sites. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359678

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

According to its congruency property, the Preisach model demands an equivalent shape of magnetic minor loops, the so‐called Rayleigh loops. We measured these loops with an inductive setup and noticed a different anomalous shape of Rayleigh loops which depends on the magnetic history. Special materials (particularly recording media) show a concave‐convex shape in contrast to the normal biconvex shape. This anomalous shape can be explained by combining the Preisach model with the Stoner–Wohlfarth model. It follows from this explanation that the degree of the anomaly depends on the material, especially in how far it fulfills the conditions of the Stoner–Wohlfarth model. The experiments show the effect that is expected according to the material. In this way the measurement of the anomalous Rayleigh loops can be used as a new method to test the Stoner–Wohlfarth properties of a material. This is more effective than using the Henkel plot [G. Bertotti and V. Basso, J. Appl. Phys.73, 5827 (1993)]. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359679

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In a previous article, we proved and characterized a phase transition of an order‐disorder type, below room temperature, in the Pb1−3X/2LaX&laplac;X/2(Zr0.95Ti0.05)O3(X=0.5–4 wt % La) ceramics. In the present article, the analysis of this order‐disorder transition is continued on the basis of the comparative study of Raman scattering, as a function of the Nb or La dopant. The parallel analysis and the interpretation of the Raman spectra of the nondoped, Nb‐doped, and La‐doped Pb(Zr0.95Ti0.05)O3ceramics lead to new information concerning the identification of the vibrational modes involved. In addition, the correlation between the phase transitions and the nature of the dopants is established. New information concerning the cell doubling, during the ferroelectric low temperature to ferroelectric high temperature phase transition, is obtained as well. The Raman study of the lead‐lanthanum zirconate‐titanate ceramics allowed us to identify for the first time the so‐called ‘‘antiferroelectric mode’’ and ‘‘ferroelectric mode’’ which had been theoretically predicted. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359680

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Our studies of piezoelectric properties of bidomain (‘‘head‐to‐head’’)Z‐cut plates of LiNbO3formed by diffusion treatment show that the spectrum of excited vibrations differs from that of monodomain crystals. We found that the length‐ and width‐longitudinal vibrations are suppressed and bending vibrations appear to be maximal for an inverted layer thickness equal to one‐half of the sample thickness. The amplitude of the fundamental frequency and its overtones for the thickness vibrations depend on the thickness of the inverted layer. A model explaining these properties is proposed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359681

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This article reports systematic results of corona poling measurements obtained on biaxially stretched polyvinylidene fluoride films charged in dry air. Charging was performed using the constant current corona triode. The dependence of the poling process on the sample thickness, charging current, and successive charging processes was investigated. Phase transitions from &agr; to &dgr; and to &bgr; phases were observed when virgin samples were corona charged. The thermal pulse technique showed that the polarization profiles during charging can be made consistently almost uniform and that the ferroelectric reorientation can be associated with the rising plateau region displayed on potential buildup curves. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359682

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Epitaxial thin films of BaTiO3have been prepared using radio‐frequency magnetron sputter deposition on MgO(100) substrates. Various x‐ray‐diffraction techniques were employed to characterize the crystal structure of the films. The films were fully tetragonal and consisted ofcdomains. Their tetragonality has been shown to be 1.013, which is almost the same as the bulk value of BaTiO3. However, the cross‐sectional transmission electron microscopic micrograph showed that a very small amount ofadomains coexists forming 90° domain boundaries in the matrix ofcdomains. In spite of the negligible strain caused by the phase transformation, it seems to be inevitable to form a certain small amount ofadomains in the BaTiO3film on MgO system. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360696

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The temperature dependence of the optical‐absorption edge (Urbach edge) of GaAs is measured in semi‐insulating andn‐type GaAs (n=2×1018cm−3) over the temperature range from room temperature to 700 °C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 °C, for semi‐insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi‐insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359683

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Er‐doped GaAs shows a sharp and simple intra‐4f‐shell luminescence spectrum of Er3+ions under host photoexcitation, when the crystal is grown with deliberate oxygen codoping by metalorganic chemical‐vapor deposition. Photoluminescence‐excitation measurements by direct intra‐4f‐shell excitation, however, revealed that there are many kinds of Er3+centers in the same crystal. To clarify the differences between these Er centers, photoluminescence was measured under both host photoexcitation and direct intra‐4f‐shell photoexcitation. It was found that there are three types of Er centers with distinctly different characteristics. The first type of Er center shows a sharp and simple photoluminescence spectrum with a high intensity under host photoexcitation. This center had been assigned as an Er atom coupled with two oxygen atoms (ErGa–2O center). The second type of Er center show sharp and simple spectra but only under direct intra‐4f‐shell photoexcitation and not under host photoexcitation. In a sample with Er concentration of 1.4×1018cm−3, several such centers with distinctly different atomic configurations were found at substantial concentrations, probably exceeding that of the ErGa–2O center. As there is no intra‐4f‐shell luminescence for this type of center under host photoexcitation, no energy transfer path exists between the host and the 4fshells of this type of Er center. The third type of Er center shows complicated spectra even under intra‐4f‐shell photoexcitation with a specific photon energy. Analysis of photoluminescence spectra from this type of Er center revealed evidence of energy migration among Er3+ions, suggesting that Er‐rich aggregates are the origin of this type of PL spectra. Under host photoexcitation, this type of Er center shows luminescence but with a much lower intensity than the ErGa–2O center. The luminescence from Er centers similar to the third type probably is dominant in the PL spectrum of Er‐doped GaAs grown without deliberate oxygen codoping, which does not contain an appreciable concentration of the ErGa–2O centers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359684

出版商:AIP    

年代:1995

数据来源: AIP