Measurement of H(sub c1) in a single crystal of YBa2Cu3O7 with low pinning

The measurement of H(sub c1) in barium yttrium copper oxide (BYCO) is often ambiguous because the presence of large pinning forces makes it difficult to discern exactly where the first deviation from linearity occurs. In addition there are complications because demagnetizing factors are often not well known. By utilizing a single crystal of YBCO with a nearly cubic shape, the uncertainty in the demagnetizing factor was minimized. In addition, the crystal used exhibited a very small amount of pinning with H applied perpendicular to the c axis, and a sharp break in the initial magnetization vs. field curve could be observed over a wide range of temperature. This allowed a precise determination of H(sub c1). The measured values of H(sub c1) could be well described by the Abrikosov relation with a Ginzburg-Landau parameter which varied linearly with temperature

Lower critical field measurements in YBa2Cu3O(6+x) single crystals

The temperature dependence of the lower critical field in YBa2Cu3O(6+x) single crystals was determined by magnetization measurements with the applied field parallel and perpendicular to the c-axis. Results are compared with data from the literature and fitted to Ginzberg-Landau equations by assuming a linear dependence of the parameter kappa on temperature. A value of 7 plus or minus 2 kOe was estimated for the thermodynamic critical field at T = O by comparison of calculated H (sub c2) values with experimental data from the literature

The NBS: Processing/Microstructure/Property Relationships in 2024 Aluminum Alloy Plates

As received plates of 2024 aluminum alloy were examined. Topics covered include: solidification segregation studies; microsegregation and macrosegregation in laboratory and commercially cast ingots; C-curves and nondestructive evaluation; time-temperature precipitation diagrams and the relationships between mechanical properties and NDE measurements; transmission electron microscopy studies; the relationship between microstructure and properties; ultrasonic characterization; eddy-current conductivity characterization; the study of aging process by means of dynamic eddy current measurements; and Heat flow-property predictions, property degradations due to improve quench from the solution heat treatment temperature

Studies of iron impurities in YxPr1-xBa2Cu3O7-delta

Pr is the only rare earth which, when substituted for Y in YBa2Cu3O7, significantly alters the superconducting transition temperature T(sub c) without changing the crystal structure. For YxPr1-xBa2Cu3O7-delta with delta approx. equal to 0, T(sub c) is reduced rapidly as x is increased, reaching zero for x about 0.5. For x above 0.5 the compound is antiferromagnetic with a Neel temperature that increases with increasing x, rising to above room temperature for x near 1. A similar behavior is observed when the oxygen deficit delta is increased from zero to 1 with x=0. For the case of Pr substitution, the drop in T(sub c) is believed due to magnetic interactions. For the case of varying delta with x=0, the drop can be attributed to a combination of magnetic interactions, band filling, and changes in crystal structure. To study these effects, the Mossbauer effect of 57 Fe atoms substituted for the Cu atoms has been observed as a function of delta, x, and temperature. The observed spectra are all well described by a two quadrupole-split pairs, a central singlet, and a six-line magnetic hyperfine field pattern. For several Pr compositions both delta and temperature were varied, and the results support the hypothesis that a magnetic interaction exists between the Fe in the Cu lattice and the substitutional Pr atoms

Processing Bi-Pb-Sr-Ca-Cu-O superconductors from amorphous state

The bismuth based high T sub c superconductors can be processed via an amorphous Bi-Pb-Sr-Ca-Cu oxide. The amorphous oxides were prepared by melting the constituent powders in an alumina crucible at 1200 C in air followed by pouring the liquid onto an aluminum plate, and rapidly pressing with a second plate. In the amorphous state, no crystalline phase was identified in the powder x ray diffraction pattern of the quenched materials. After heat treatment at high temperature the amorphous materials crystallized into a glass ceramic containing a large fraction of the Bi2Sr2Ca2Cu3O(x) phase T sub c = 110 K. The processing method, crystallization, and results of dc electrical resistivity and ac magnetic susceptibility measurements are discussed

Magnetic hyperfine field structure of iron Urushibara type catalysts

We have utilized the Mössbauer effect to study the hyperfine field structure of iron Urushibara catalysts. The Mössbauer spectra of those catalysts prepared using zinc show that they consist of a mixture of magnetic and non‐magnetic Fe‐Zn alloys. Both the magnetic field distribution in the magnetic phase and the relative amounts of magnetic and non‐magnetic phases depend on the Fe‐Zn ratio used in the preparation of the catalyst. This dependence on Fe–Zn ratio is in contrast to iron Urushibara catalysts prepared using aluminum (and Raney iron type catalysts) in which the active phase is almost pure α‐fe, irrespective of the Fe‐Al ratio. The activity of zinc prepared iron Urushibara catalysts for certain hydrogentation reactions is known to be greater than that of aluminum prepared iron Urushibara catalysts and the above results suggest a relationship between activity and the modification of the iron catalyst by alloyed zinc. The alloying behavior of the Fe‐Zn particles may be analogous to that of the so‐called bimetallic clusters observed in other alloy systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87521/2/391_1.pd

Supertransferred Hyperfine Fields at Sb5+ in Insulating Ferrites: Effects of Local Order and Ion‐Specific Properties

The supertransferred hyperfine fields at Sb5+ in LiFe5O8LiFe5O8, CoFe2O4CoFe2O4, and YIG have been determined using Sb Mössbauer spectroscopy. In contrast to CoFe2O4CoFe2O4, NiFe2O4NiFe2O4, and YIG, the small, average hyperfine field of ∼100 kOe at Sb5+ in LiFe5O8LiFe5O8 requires the existence of significant local order and indicates that the clustering of Li1+ about Sb5+ is approximately ten times as large as that expected for a random intrasite cation distribution, in agreement with the known strong influence of Sb substitution in destroying the Li:Fe ordering. The decrement in the hyperfine field at Sb5+ due to an A‐site Co2+ is also found to be larger than that due to Ni2+.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87642/2/518_1.pd

Modeling of micromagnetic Barkhausen activity using a stochastic process extension to the theory of hysteresis

Recent work by Bertotti [IEEE Trans. Magn. MAG‐24, 621 (1988)] and others has shown that it is possible to model the micromagnetic Barkhausen discontinuities at the coercive point using a two‐parameter stochastic model. However, the present formulation of the model is restricted to limited regions of the hysteresis curve over which dM/dH is approximately constant and whendH/dt is held at a constant rate. A natural extension of this model is to take the basic result, in which the level of Barkhausen activity in one time period is related to the activity in the previous time period, and increment it by a small amount which is dependent on the differential permeability. The extension of the model proposed here uses the theory of ferromagnetichysteresis to determine the differential permeability at any point of the hysteresis loop. The Barkhausen activity is then assumed to vary in proportion to the differential permeability. The resulting model allows the Barkhausen sum of discontinuous changes in magnetization to be modelled around the entire hysteresis loop, leading to an important generalization of the basic model

Comprehensive analysis of Barkhausen emission spectra using pulse height analysis, frequency spectrum, and pulse wave form analysis

The dependence of magnetic Barkhausen emissions (MBE) upon both field excitation and detection frequencies and excitation wave form was studied in order to investigate two of several crucial factors which affect the emissions. Sinusoidal, triangular, and square wave forms were used to generate the MBE and the pulse height spectra, frequency spectra, and pulse wave forms of these signals were analyzed. The frequency spectra of sinusoidal and triangular alternating field excitations showed similar behavior but the spectrum under square wave excitation was different due to the existence of high frequency components during square wave switching. As yet, no common standard has been agreed upon for parameterization and representation of Barkhausen signals. It appears from this work that field excitation wave form and frequency should define the inputs, while detection frequency range, pulse height spectrum, frequency spectrum, and emitted pulse wave form analysis should be used to quantify the output

Intra Site Cation Ordering and Clustering in Natural MnZn Ferrites

Cooling rates of 10−3 K/yr. permit naturally occurring MnZn ferrites (franklinites) to be in thermodynamic equilibrium at ambient temperatures with respect to both electrostatic and elastic energies. The elastic strain energy resulting from the occupancy of the A sites by ions of differing sizes is minimized by clustering of Mn and Zn in different regions of a crystal. Using 57Fe NGR, these regions have been observed directly in franklinite with low Mn3+ contents and their structures elucidated. The Mn rich region has a local crystal chemistry very similar to that of MnFe2O4MnFe2O4 and the Zn rich region has a local structure similar to that of ZnFe2O4ZnFe2O4. The two regions are crystallographically coherent. Laboratory heat treatments destroy the MnZn ordering and alter the magnetic properties. The Fe spin configurations are also observed to be collinear. The discovery of the intrasite ordering in these materials proves that Good‐enough's criteria for square B‐H loops in ferrites can be realized. For samples containing Mn3+, the effects of the Jahn‐Teller distortions do not permit a direct observation of MnZn intrasite ordering.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87350/2/125_1.pd