Research

Brief Highlights

• Double Exchange Mechanism and Colossal Magnetoresistance
• Exploring the Mechanism of High-Tc Superconductivity
• Extending the Phase Diagram of the Layered Manganites
• Grain Boundaries in High-Tc Superconductors
• Growth Strains in Protective Oxides
• Interface Roughness: Surfaces and Buried Interfaces
• Proximity Interactions: Strain at CMR Grain Boundaries
• Residual Strains in Aluminum Oxides
• Role of Reactive Elements on the Growth Mode of Oxides
• Seamless Joining of High-Tc Superconductors
• Structural Features Explain Tc in the Layered Manganites
• Tunneling Reveals Anomalies in High-Tc Superconductors

Extended research/highlights

• M. Grimsditch, D. Renusch, A. P. Paulikas, and B. W. Veal - Optical Determination of Strains in Aluminum Oxide Scales
• D. G. Hinks - Single Crystal Growth and Powder Synthesis
• D. J. Miller and V. R. Todt - A New Process for the Growth of Bicrystals of High Temperature Superconductors

Recent Highlights

Colossal Magnetoresistance

• Single crystals of layered manganites have been grown with the easy-magnetization-axis along the c-axis, as well as perpendicular to this axis. For the c-axis-orientated samples a state of antiferromagnetically coupled ferromagnetic layers was found at low temperature where the material is nominally "metallic."
• It was observed that hydrostatic pressure on a layered manganite crystal leads to a sign-reversal of the Mn-O bond compressibilities below the ferromagnetic transition at T_c. It was also found that the MnO₆ octahedra of the layered compound become more distorted in the "metallic" phase than the insulating phase. This result is contrary to the perovskites, showing the structural flexibility of the layered materials to changes in magnetic and/or electronic state.
• The detailed magnetic structure of a single crystal of La_1.2Sr_1.8Mn₂O₇ revealed the existence of 2D canted magnetic correlations above T_c between neighboring planes within each bilayer, which is attributed to the competition between double exchange and superexchange. The variation of average cant angle with magnetic field explains anomalies seen in the bulk magnetization.
• Measuring the anisotropic conductance in the layered colossal magnetoresistance (CMR) manganites was shown to be strongly affected by the location of current and voltage terminals. At least six terminals are required to obtain reliable values, while unusual artifacts can appear using traditional four-terminal measurements.
• The anisotropic conductance of the layered CMR manganites is shown to go as the square of the zero-field magnetization (from neutron scattering) over a broad temperature range below T_c. This result contradicts the simple double-exchange model and needs explanation.
• Synthesis and structural studies of polycrystalline (R,Sr)₃Mn₂O₇ (R = Nd, Pr) layered materials has begun in order to explore the role of P(O₂) on phase stability. It is found that the degree of octahedral distortion does not correlate with the appearance of a "metallic" ground state.
• Coherent structural intergrowths were identified in TEM micrographs of the two-layer manganites due to stacking faults associated with extra and/or missing SrO barrier layers in the structure. These intergrowths give rise to a plateau in the magnetization and interesting scaling behavior that had not been properly identified and addressed previously.
• Single crystals of La-Sr-Mn-O perovskite were grown to study the re-entrant Jahn-Teller distortion induced by onset of ferromagnetism.
• Sintered pellets of (La, Ca)MnO₃ perovskite were grown, and it was found that the low-temperature transport is well fit by variable-range hopping plus a field-dependent channel of free carriers.
• Transmission electron microscopy was used to identify intergrowths in the layered CMR and to show their density is consistent with low-field magnetization. Such intergrowths could cause the resistivity anomaly seen at high temperatures.

Superconductivity

• Spectacular crystals of Bi₂Sr₂CaCu₂O_x achieved the highest reported T_c of 95 K and improved metal-ion homogeneity, in particular the lack of antisite defects.
• The first evidence for coherent Josephson vortex flow for fields parallel to the Cu-O planes in these Bi₂Sr₂CaCu₂O_x single crystals has been obtained.
• Point-contact tunneling into oxygen doped and depleted single crystals of Bi₂Sr₂CaCu₂O_x has strongly suggested that the 'pseudogap' has superconductivity as its origin. Josephson coupling in underdoped samples is inconsistent with a charge- or spin-density wave origin of this behavior (with J. Zasadzinski).
• The simplest high temperature superconductor (HTS) structure, Tl₂Ba₂CuO_x, contains a single CuO plane and can be overdoped to reduce T_c from 90 K to near 0 K. We grew Tl₂Ba₂CuO_x single crystals with large dimensions along the c-axis using a safe, simple method at atmospheric pressure. These allowed measurements of l_c that casts doubt on the Anderson interlayer tunneling model as a mechanism for HTS.
• The interaction of an Abrikosov vortex lattice with randomly-placed pinning centers has been modeled with a simple yet powerful expression. Excellent high-temperature fits to the dissipation in ion-irradiated Tl₂Ba₂CaCu₂O_x are found.
• Voltage noise in YBa₂Cu₃O₇ shows that vortex bundles grow with the c-axis correlation length due to Josephson coupling. This further implies that a large c-axis correlation length is needed for vortex-lattice freezing.
• Our dual-seeded melt processing technique has been modified so that bicrystals of virtually any misorientation may be fabricated. These are confirmed to consist of long, straight facets free of second phases.
• Both 90¡ [100] tilt and 90¡ [100] twist planar grain boundaries have been isolated in YBa₂Cu₃O₇. Consistently higher critical current density, J_c, in the former proves that grain boundary planes affect J_c and thus that transport across meandering thin-film boundaries is complex.
• Thin-film and bulk [001] tilt, bicrystal grain boundaries in YBa₂Cu₃O₇ exhibit a strong dependence of J_c on misorientation angle, but the microscopically more-perfect bulk is thirty-times-lower in magnitude. Our model, based on differences of the pinning of Josephson vortices, explains this anomaly.
• Single crystals of YBa₂Cu₃O₇-type show an unexplained peak in J_c vs. field. We varied oxygen doping and growth rate, and found systematic changes showing that each plays a major role in vortex pinning (with H. Claus). We have synthesized 5 gram single-domain samples of GdBa₂Cu₃O_x, to study the effect on pinning of Gd substituted on the Ba site.
• Our melt-textured-growth techniques were refined to produce structurally-oriented samples (>100 grams) of HTS are now reliably produced for flywheel and advanced motor uses. The ability to make larger monolithic samples by multiple seeding has been demonstrated. The well-oriented domain-growth boundaries originating from adjacent seeds are under investigation (with U. Welp).

Reacted Surface Layers

• Transient phase formation was monitored during early-stage oxidation of FeCrAl alloys using a variety of spectroscopic techniques. Ruby fluorescence was used to investigate compressive strains, growth strains and substrate creep, which play a central role in scale failure.
• In-plane and out-of-plane strains were found from an elastic analysis of the fluorescence data. Both strains were confirmed by x-ray diffraction.
• Local strain concentrations and relaxation were studied to investigate the role of surface irregularities. Large strain variations were observed across wavy interfaces and strain relaxation was observed up to 20 microns from edges and corners. Both are in quantitative agreement with finite element modeling (R. Williamson and J. Wright at INEEL).
• A new capability for measuring roughness at buried interfaces (to 150 � depth) has been developed using refracted x-ray fluorescence.