D. J. Lam

Piezoelectrically‐induced switching of 90° domains in tetragonal BaTiO3 and PbTiO3 investigated by micro‐Raman spectroscopy

Domain switching of 90° ferroelectric domains in tetragonal BaTiO3 and PbTiO3 is induced by the application of stress along specific crystallographic axes. For BaTiO3, single‐domain crystals are obtained from twinned specimens by the application of ∼1.1 MPa of stress parallel to the a axis and twin boundaries are induced by application of ∼0.22 MPa of stress parallel to the c axis. Similar piezoelectrically‐induced domain switching was observed in PbTiO3 at elevated temperature. We observed the rotation of the crystallographic axes associated with domain switching via micro‐Raman spectroscopy. These results were consistent with optical microscope images of the domain switching which demonstrates the usefulness of micro‐Raman spectroscopy for the study of ferroelectric domain structures. A phenomenological treatment of domain switching in a piezoelectrically‐coupled system is described.

The Magnetic Susceptibility of Neptunium Oxide and Carbide between 4.2° and 350°K

The magnetic susceptibilities of the dioxide and monocarbide of neptunium have been measured from 4.2 to 350°K. The inverse susceptibility of neptunium dioxide is linear in temperature above 60°K with an effective paramagnetic moment of 2.95 μB. A peak in the susceptibility‐vs‐temperature curve is observed at 25°K, in agreement with the maximum observed in the specific heat of NpO2. The neptunium monocarbide becomes ferromagnetic below 200°K. A peak in the susceptibility‐vs‐temperature curve at 310°K indicates a transition from a magnetically ordered arrangement to a paramagnetic one. The effective moment per NpC unit for the ferromagnetic and paramagnetic states is 1.4 μB and 3.37 μB, respectively.

The fermi surface in YBa2Cu3O7−x BY 2D ACAR☆

Abstract Positron-annihilation two-dimensional angular correlation measurements (2D ACAR) have been performed on a YBa2Cu3O7−x single crystal with Tc = 93.6 K. Projections both parallel and perpendicular to the c-axis were obtained. On the basis of these experiments, we propose that the Fermi surface consists of four nearly cylindrical sheets, in substantial accord with band theory predictions. These are first Fermi surfaces measured directly via c-axis projection in a high-Tc superconductor.

NMR, MAGNETIC SUSCEPTIBILITY, AND ELECTRONIC SPECIFIC HEAT OF Nb AND Mo METALS AND Nb-Tc AND Nb-Mo ALLOYS

Magnetic susceptibility and nuclear magnetic resonance measurements have been made in Nb-Tc alloys. Calculations on the temperature variations of electronic specific heats and magnetic susceptibilities are performed for Nb and Mo metals and Nb-Mo alloys by making use of the density of states determined from low temperature specific heat data for 4 d transition metals and their alloys. Temperature variations of the electronic specific heats for Nb and Mo metals are well explained by the present numerical calculations. Temperature variations of the magnetic susceptibilities of these metals and their alloys are explained by taking account of the effect of the temperature independent orbital paramagnetism and the effects of the negative (for Nb metal) and the positive (for Nb 0.75 Mo 0.25 alloy) molecular fields. The Knight shifts of Nb 93 and Tc 99 in Nb-Tc and Nb-Mo alloys are discussed. It is shown that the main contribution to these Knight shifts appear to arise from the orbital paramagnetism of the d ele...

Hyperfine Interaction and Susceptibility in Some Actinide Metals and Intermetallic Compounds

Magnetic effects in a number of actinide metals and intermetallic compounds are being investigated utilizing the 59.6 keV Mossbauer resonance of 237Np in conjunction with bulk susceptibility measurements. The cubic Laves phase compound NpAl2 is found to be ferromagnetic. The paramagnetic moment is 2.30 μB, somewhat lower than in other cubic Np magnetic materials. The saturation magnetic hyperfine field is found to be (3155±50) kOe. Taking H(T)/H(0) = D[1−T/Tc]β, we find D = 1.48±0.10, Tc = 55.8±0.1°K and β = 0.355±0.017 for 0.994>T/Tc>0.83. At lower temperatures, departure from molecular field behavior is seen. In cubic actinide magnetic materials, an electric‐quadrupole hyperfine interaction will be observed due to alignment of the unquenched orbital momentum. In NpAl2 we find e2qQ = − (27±1) MHz. For NpC, which has the NaCl structure, we obtain the hyperfine parameters Hi = (4789±50) kOe and e2qQ = (577±58) MHz at 4.2°K. Metallic neptunium (orthorhombic) and americium (hexagonal) show no magnetic hyperf...

XPS study of LaCoO3

X‐ray photoelectron spectroscopy (XPS) measurements have been performed on LaCoO3 and related compounds including LaTiO3 and LaCrO3. The XPS spectra of LaCoO3 are analyzed to determine the spin states of the Co ions. Final state multiplets of the d6 electron configuration in the valence band region are apparent near the band edge. The predominately O 2p valence bands are corrected using XPS data from LaTiO3 and LaCrO3 where the 3d electron spectra are understood. The results are discussed in terms of the mixed spin‐state trivalent cobalt ion model.

Preparation and characterization of the 110 K superconducting phase in the Bi-Sr-Ca-Cu-O system☆

Abstract Superconducting properties of the Bi2SrχCayCu3Oz system are studied in the composition range 1.2⩽χ⩽2.2 and 2⩽y2.4. Both resistivity and low field magnetization data are presented. Both types of measurements show the development of the 110 K phase from a low-Tc phase as the Sr content is reduced from 2.2 to 1.2. Best results are obtained for the nominal composition Bi2Sr1.6Ca2Cu3Oz. The very similar character of the resistive and inductive transition for two phase samples is attributed to interlayering of the two phases.

Investigation of the Magnetic Behavior of Some Conducting Am Systems

A number of Am compounds have been investigated by bulk magnetic and nuclear gamma resonance (NGR) measurements. For the latter type of investigations, the compounds were used as a source for the 59.6‐keV resonance in 237Np. AmSb, Am3Se4, and Am3Te4 show no magnetic ordering down to 4.2 K, and the NGR isomer shifts show that the Am ions are present in the trivalent state. The susceptibility of AmSb is temperature independent. Similarly, no ordered magnetism is seen in Am2C3, although Np2C3 has a ferromagnetic transition at 109 K. These results show the tendency of conducting Am compounds to form a trivalent, nonmagnetic state. The results are discussed in terms of a crystal‐field calculation which includes J mixing caused by the large crystal‐field interaction present in actinide compounds.

Low field magnetic properties of superconducting Tl2Ba2Ca2Cu3Ox single crystals

Abstract Low field characterization of single crystals of Tl 2 Ba 2 Ca 2 Cu 3 O x using shielding, Meissner, and flux trapping measurements in fields of 0.5 to 40 G are presented. The highest diamagnetic onset temperature observed is 124 K. It is found that the diamagnetic transition is significantly broadened and the onset temperature is strongly suppressed with increasing fields. At low temperatures, the Meissner effect is found to be about 40% of the shielding effect. Within a few degrees of T c ideal superconducting behavior is observed, where the Meissner and shielding effects are equal and pinning effects are neglibile.

Magnetic Properties of Dilute Alloys of Scandium

Previous experiments have shown that small quantities of cerium will induce a spin‐compensated state in the d1 elements yttrium and lanthanum. The magnetic susceptibility of scandium alloys with 0.02–2‐at.% cerium has been measured over the temperature range of 4°–300°K. No evidence for a spin‐compensated state is found. Addition of gadolinium has been shown to polarize the scandium matrix. To study the influence of a heavy rare earth with nonzero orbital angular momentum on the polarization of the scandium matrix a series of dilute dysprosium alloys have been studied. The effect of dysprosium is found to be similar to that of gadolinium. Also the influence of the nonmagnetic alloying elements titanium and thorium on scandium has been investigated in order to separate the various factors affecting the magnetic properties of dilute scandium alloys.