摘要:

A systematic investigation of structure and intrinsic magnetic properties of the novel compoundsR3(Fe, T)29(R=Y,Ce, Nd, Sm, Gd, Tb, and Dy;T=Vand Cr) has been performed. The lattice constants and unit cell volume decrease with increasing the rare-earth atomic number fromR=Ndto Dy, except for Ce, reflecting the lanthanide contraction. The Curie temperature increases fromR=Ceto Gd and decreases from Gd to Dy, respectively, with increasing atomic number andGd3Fe29−xTxhas the highest Curie temperature for each series ofR3Fe29−xTx(T=Vor Cr) compounds. The saturation magnetization ofR3Fe29−xTxat 4.2 K decreases gradually fromR=Ndto Dy with increasing atomic number, except for Ce, in each series ofR3Fe29−xTx.It is suggested that the Ce ion inCe3Fe29−xTxis valence fluctuated which leads to the unusual magnetic properties. The spin reorientations of the easy magnetization direction ofR3Fe29−xTxare observed at around 230, 230, and 160 K forR=Nd,Sm, and Tb whenT=V,and at around 230 and 180 K forR=Ndand Tb whenT=Cr,respectively. First order magnetization processes are observed around 5.7 T forSm3Fe26.7V2.3and 4 T forSm3Fe24.0Cr5.0at 4.2 K, 2.0 T forTb3Fe28.0V1.0,and 2.3 T forTb3Fe28.0Cr1.0at room temperature. A phenomenological analysis shows that the saturation magnetization ofR3Fe29−xTxcompounds with a low T concentration can be roughly calculated based on a combination of those of the2:17Rand 1:12 units in a ratio of 1:1. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365287

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Ferromagnetic resonance measurements were made as a function of the static external field angle forc-plane disks of single crystal flux grown manganese substituted bariumM-type (Ba-M) and zincY-type (Zn-Y) hexagonal ferrites at 50 and 8.8 GHz, respectively. A shorted waveguide technique was used. Analysis of the FMR field versus angle results confirmed the operational assumption of a uniform mode response. For the easy-axis Ba-Mdisk, the linewidth was 69 Oe when the external field and magnetization vectors were perpendicular to the disk. The linewidth increased to a maximum measured value of 472 Oe when the magnetization was directed 45.4° from the sample normal. For the easy-plane Zn-Ydisk, the linewidth had a minimum value of 18 Oe when the field and magnetization vectors were in-plane. The maximum linewidth was 391 Oe when the magnetization was directed 25.2° from the disk normal. The linewidths were larger than predicted for reasonable values of the Landau–Lifshitz damping and showed angle dependences which indicated nonintrinsic contributions to the loss. A modified two magnon scattering calculation based on the model of Sparks, Loudon, and Kittel was used to investigate these linewidth differences. The calculation included anisotropy modifications to the spin wave band for each material. The angle dependences of the excess linewidths show qualitative agreement with the two magnon predictions, with inhomogeneity sizes on the order of 1 and 0.25 &mgr;m and volume fractions of 0.01 and 0.005 for the Ba-Mand Zn-Ydisks, respectively. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365288

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

New features in electron paramagnetic resonance (EPR) of porous silicon have been examined here. A new isotropic EPR center was observed atg=1.9995(1) atT=4.2 K, in bothp-type andn-type porous silicon. By comparing itsgvalue with those of shallow donors in bulk silicon, the center was identified due to the conduction-band (CB) electrons in silicon microcrystals. The CB signal, present in freshly preparedp-type andn-type samples, can be dramatically and surprisingly enhanced by the presence of a polar solvent on then-type porous silicon surface. Even though it was shown that most of the donor electrons in ann-type sample can be pulled into the porous layer from the substrate by solvent exposure of the porous layer, the possible electrochemical effects are not yet completely understood; to establish a reasonable model for them would require appropriately controlled experiments. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365289

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Crack propagation in a piezoelectric lead–zirconium–titanate (PZT) material under simultaneous mechanical loading and applied electric fields is studied using the Vickers indentation technique. It is demonstrated experimentally that electric fields can inhibit or enhance crack propagation in piezoelectric materials. Cracks introduced by indentation are observed to propagate less under a positive applied electric field (the polarity of the field was the same as that for poling), whereas under a negative applied electric field, crack propagation is enhanced. Such an effect is observed to be more profound with increasing electric-field strength and decreasing mechanical loading. Attempts are made to compare these experimental observations with the results of various theoretical analyses. A mechanism for the change in crack propagation behavior of the piezoelectric PZT material under applied electric fields is presented. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365290

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Scanning force microscopy (SFM) has been used for the determination of friction, phase transformation, piezoelectric behavior (in the contact mode), polarization state, and dielectric constant (in the noncontact mode) of nanometer regions of lead zirconate titanate (PZT) films. The use of the SFM tip in the contact mode, to polarize different nanoregions of the PZT film and to apply an oscillating field thereon, led to effective piezoelectric coefficients and piezoelectric loops. The measured effective piezoelectric coefficient was shown to depend appreciably on both the tip contact force and the quality of the tip-to-film electrical contact. In the noncontact mode, application of an ac signal (with a frequency &ohgr;) across the tip—PZT film—electrode system produced an oscillation of the tip at frequencies &ohgr; (fundamental or first harmonic) and 2&ohgr; (second harmonic). The signals at &ohgr; and 2&ohgr; were related to the state of polarization and the dielectric constant of the PZT film, respectively. Analysis of the combined contact, noncontact and friction force microscopic data provided insight into the structure and into the dielectric, ferroelectric, and piezoelectric properties of distinct nanoregions of the PZT film. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365350

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The spatial distribution of the polarized cathodoluminescence (CL) emissions from thin ZnSe films grown by metalorganic vapor phase epitaxy method on GaAs(001) has been examined by a low-temperature polarized CL measurement system combined with a transmission electron microscope. It is found that theY0andY1emissions come from the regions near dislocation tangles and near individual dislocations. The polarized CL images of these emissions show changes in intensity distribution when the polarization condition is changed. The observations suggest that the polarization direction of these emissions is parallel to the dislocation line. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365441

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Mechanisms underlying the refractive index changes in germanosilica films deposited by hollow cathode plasma enhanced chemical vapor deposition and subjected to UV irradiation are proposed based on observed changes in film thickness, stress, and structure. An increase in refractive index after UV exposure is observed in films deposited under low ion bombardment conditions. This increase in refractive index is accompanied by a reduction in film thickness which is an order of magnitude larger than that expected from the Lorentz–Lorentz relation. This behavior is shown to result from: (i) a significant degree of porosity in the as-deposited material; (ii) oxygen deficiency of the as-deposited material. Upon UV irradiation, the porous structure is compacted, thus accounting for the large decrease in thickness, while the oxygen deficiency is reduced causing a decrease in the material polarizability and counteracting the effect of the thickness reduction. On the other hand, germanosilica deposited under high ion-bombardment conditions is of normal optical quality and exhibits a decrease in refractive index after exposure to UV. This refractive index reduction is shown to be the result of three processes: structural dilation and stress relief on one side; and an increase in material polarizability on the other, with structural dilation having the largest effect. Annealing of the exposed samples has shown that most of the polarizability increase is likely to be annealed out at 500 °C, while the refractive index change caused by structural dilation is stable up to 800 °C. Finally, it is shown that during plasma enhanced chemical vapor deposition, germanosilica is more prone to nucleation and columnar growth than pure silica and therefore a higher level of ion bombardment is required in the former case in order to obtain a high quality homogeneous material. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365291

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Polarized absorption and fluorescence spectra are reported forHo3+ (4f10)ions incorporated into single crystals of strontium fluorapatite,Sr5(PO4)3F,also known as SFAP. Site-selective excitation experiments indicate that, to within instrumental resolution, theHo3+ions occupy similar lattice sites havingCssymmetry. Spectra were obtained between 1940 and 400 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure[P63/m(C6h2)].Individual energy (Stark) levels for the fourteen lowest-energy multiplet manifolds ofHo3+ (4f10)were analyzed in detail. These manifolds include the5I8(ground state), the remaining5IJmanifolds,5FJ,5S2,3K8(2),5G6,and the3G5(2)manifolds. A Hamiltonian consisting of Coulombic, spin-orbit, and crystal-field terms was diagonalized over these manifolds in anLSJMJbasis set, which includes 146 crystal-quantum states, labeled either&Ggr;1or&Ggr;2,appropriate to the crystal-field symmetry ofCs.By varying the crystal-field parameters,Bnm,and the centroid for each manifold, we obtained agreement between 101 experimental and calculated Stark levels with a rms deviation of9 cm−1.An additional 15 Stark levels, not included in the original analyses, are predicted to within the rms deviation. The predicted symmetry label of each Stark level agrees with the experimental assignment based on the analysis of the polarized spectra. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365292

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Transient electroluminescence (EL) from organic bilayer light emitting diodes addressed by a rectangular voltage pulse often features an overshoot when the voltage is switched off. Experimental results are presented for a variety of hole transporting layers in contact with an oxadiazole layer simultaneously acting both as a blockade for holes and as an electron transport layer. The overshoot occurs in spin coated yet not in vapor deposited samples. A model is developed to rationalize charge recombination under the premise (i) of an interfacial layer of finite thickness between hole and electron transport layers in which both transport molecules coexist and (ii) of interfacial energy barriers impeding both hole and electron passage. It predicts the occurrence of an EL overshoot due to the recombination of stored electrons and holes under the action of their mutual space charge field when the external voltage is switched off. The temporal pattern of the predicted transient EL signal is in good agreement with experiment. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365293

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Organic nonlinear optical crystals have several advantages over inorganic crystals for frequency conversion and electro-optic modulation. In the present work, the effect of 100 MeVAg14+ion irradiation on the electrical and optical properties of organic nonlinear crystal para-hydroxy acetophenone (HAP) is investigated. It is observed that second harmonic generation efficiency is marginally affected. However, changes in refractive index and dielectric constants (13 times higher) are noticed in the irradiated HAP. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365294

出版商:AIP    

年代:1997

数据来源: AIP