Isothermal Loops Research Articles

Surface Barrier and Bulk Pinning in MgB2 Superconductor

We present a modified method of preparation of the new superconductor MgB2. The polycrystalline samples were characterized using X-ray and magnetic measurements. The surface barriers control the isothermal magnetization loops in powder samples. In bulk as prepared samples we always observed symmetric magnetization loops indicative of the presence of a bulk pinning mechanism. Magnetic relaxation measurements in the bulk sample reveal a crossover of surface barrier to bulk pinning.

The effect of microstructure on the temperature dependence of the interlayer coupling in Co/Cu multilayers

Three classes of giant magnetoresistance Co(1 nm)/Cu(2.1 nm) multilayers were sputter grown with different microstructures in respect to grain size and interface roughness, depending on deposition conditions. Magnetization and current in-plane giant-magnetoresistance (GMR) isothermal loops reveal an unusually high increase of coercivity from 280 down to 5 K. In addition, a systematic variation was observed in the temperature dependence of the indirect exchange coupling as the Co–Cu layering is modified in the three classes of Co/Cu multilayers. Specifically, the temperature dependence of the saturation (switching) field in the GMR-loops, and the indirect coupling strength, vary as (T/T0)/sinh(T/T0) whereas the spin-blocking temperature T0 is found equal to 84(4), 96(11), and 105(10) K for class A, B, and C multilayers, respectively. These results indicate that the desirable low hysteresis appears in the GMR loops at room temperature because the spin structure becomes unstable above the obtained T0 due to domain wall fluctuations. Such magnetic fluctuations define a short–range order state above T0 that depends on Co–Cu intermixing and geometric factors of the grains.

Mechanical shear measurements on a ferroelastic multidomain state

The mechanical properties of the improper ferroelastic crystal calomel ${\mathrm{Hg}}_{2}{\mathrm{Cl}}_{2}$ have been investigated with a torque-twist setup. Three methods have been applied: low-amplitude ac measurements on the real and imaginary part of the shear compliance in the ${\mathrm{Hz}--10}^{2}\mathrm{}\mathrm{Hz}$ range, measurements of isothermal torque-twist hysteresis loops, and studies of the twist angle for different torque-temperature histories. The work gives insight into the response of the ferroelastic multidomain state to an external torque.

Flux pinning by columnar defects in Bi2Sr2CaCu2O8+d single crystals and annealing effects

Pinning properties of Bi2212 single crystals irradiated with 340 MeV Xe18+-ions before and after annealing are investigated and compared. Isothermal magnetic hysteresis loops are recorded and ac susceptibility measurements are also carried out. Inhomogeneities forming during the annealing process show higher critical current density, jc, at fields ≳ the matching field, Bφ. A strong effect of collective flux pinning in the high magnetic field regime is proposed.

Modeling of cyclic stress–strain behavior and damage mechanisms under thermomechanical fatigue conditions

A multi-component model was applied to predict the cyclic stress–strain response of different alloys under thermomechanical fatigue conditions based upon isothermal hysteresis loops. A ductile AISI 304 L-type stainless steel and two high strength alloys, the near-α titanium alloy IMI 834 and the nickel-base superalloy IN 100, were chosen as test materials. These represent alloys with rather different dislocation slip modes, stress–strain characteristics and damage mechanisms. Model predictions are compared with experiments and the differences in cyclic stress–strain response and damage mechanisms under isothermal and thermomechanical fatigue conditions, respectively, are discussed based upon microstructural observations.

Internal loops in pseudoelastic behaviour of Ti-Ni shape memory alloys: Experiment and modelling

The stress-strain isothermal hysteresis loops due to the incomplete martensitic transformation are analysed for Ti-Ni shape memory alloys. Experiments show the existence of two distinct yield lines for phase transition; one for the forward transformation austenite→martensite (A→M), the other for the reverse transformation M→A. The tensile behaviour of single crystals with only one yield line (A↔M) [1] can be considered as an ‘ideal’ case. An extension of a thermodynamic model for pseudoelasticity [2] allows these two yield lines to be taken into account.

Critical current density of a sample of melt grown Y1.2Ba1.8Cu2.4O x

Melt grown samples of Y1.2Ba1.8Cu2.4Ox have been prepared and studied for their current carrying capacity. The composition was chosen to include Y2BaCuO5 (211) particles in the YBa2Cu3Ox (123) phase. The critical current density (Jc) of these samples was studied as a function of magnetic field using magnetization technique. The micrographic investigation shows well aligned grains in this material. The magnetic hysteresis measurements were done using a MPMS SQUID magnetometer up to the fields of 5.5 T. TheJc was estimated from the remanent magnetization using Bean model. Isothermal magnetization hysteresis loops at low fields reveal the presence of only one kind of hysteresis loops (corresponding to intragrain magnetizations). This is a valid proof that the weak links are greatly eliminated in these samples prepared by MG process. TheJc behaviour as a function of magnetic field has two components, a rapidly decaying exponential function of field and the other component that predominates at higher fields. This could be explained if we assume that the sample contains two phases of superconductors, one having a lowHc2 becoming normal at fairly medium fields of the order of a few kilogauss will act as pinning centres for the other phase having higherHc2 and hence higherJc at high fields.

The decay of the 1973 August 9 flare

The state and evolution of the decay-phase plasma from a compact solar flare that occurred on August 9, 1973, are investigated on the basis of XUV and X-ray observations from Skylab and Solrad 9. Density-sensitive line ratios are used to determine the electron density over the temperature range from 30,000 to 5 million K, and the differential emission measure in the temperature interval from 30,000 to 20 million K is derived for several times in the decay phase. The morphology of the flare is discussed, including its relation to the observed photospheric magnetic field. The sequential formation of new loops during the decay phase is emphasized as an essential element for understanding the decay phase of the flare. This idea is developed further by comparing the observed differential emission measure with that predicted by a semiempirical model which considers the sequential formation of isothermal loops that cool by radiation and thermal conduction.

Emission-line spectra of two active regions on the solar limb - 1175-1940 A

Skylab emission-line spectra (1175 to 1940 A) of two active regions on the solar limb are examined. Electron densities for the active regions are derived using the intensity ratios of selected intersystem and allowed lines. The intensity ratios in the active regions are compared with those observed previously in quiet-sun and coronal-hole regions. The behavior of coronal forbidden lines relative to transition-zone lines is discussed along with the solar continuum intensities near and above the limb. These continuum intensities are then compared with those at similar slit positions for the quiet sun and the previously studied coronal hole. The active-region data are found to be consistent with either (1) multithermal loops with the high-temperature plasma occupying the tops of the loops or (2) nearly isothermal loops with the hotter loops extending to higher altitudes.

OFFICIAL REPORTS ON CORROSION

High‐temperature aluminium alloys. This report is a compilation of corrosion studies performed during the period February 1955 to October 1957 to evaluate aluminium alloys developed for use as cladding material for fuel elements in pressurised water‐cooled reactors at temperatures ranging from 500 to 600°F. The alloy compositions included basic 2S aluminium containing nickel and iron, and special alloys of aluminium and copper. Specimens of each alloy were evaluated in semi‐static and in dynamic streams in isothermal test loops at the American Naval Laboratory. These loops were designed to simulate operating conditions of interest (exclusive of radiation) to the pressurised‐water reactor programme using water conditioning compatible with the most advanced technology for crude and corrosion control for pressurised water systems. Studies of the most promising alloys of aluminium and nickel were extended to include corrosion‐irradiation environments in the test loops at M.T.R. and boiling heat‐transfer conditions in heat throughput loops at A.N.L. The data include supplementary information published in reactor engineering division quarterly reports Nos. 1 and 2, internal memoranda, and reports to the U.S. Navy Task Group on Aluminium Alloys. Subsequent studies are to be described in another report. Referenced ANXL 5546, this 72‐page report has 47 illustrations and is priced $2.