Search

The influence of microstructure on magnetoresistive properties of Cu80Fe5Ni15 (at. %) melt-spun and annealed ribbons was studied. The microstructure was characterized by means of energy filtered transmission electron microscopy, small angle neutron scattering and atom probe tomography. Giant magnetoresistance (GMR) measurements were made at low temperature. The samples were annealed for 2 h at different temperatures from 400 °C to 600 °C in order to modify their microstructure and thus their GMR properties. The magnetic behavior was correlated to the structural data to explain the GMR properties that are observed. A precise structural characterization, using atom probe tomography, small angle neutron scattering and energy filtered transmission electron microscopy, is presented. It reveals that no precipitates or clusters are present into the core grains of the as spun state; thus the sample is a solid solution. The magnetic properties measurements indicate that the ribbon exhibits a superparamagnetic behavior and presents relatively high GMR properties. These results imply that GMR can be obtained from a solid solution. Therefore, interface scattering is not the only precursor for GMR. The presence of diluted magnetic atoms leads to the formation of magnetic clusters that behave like superparamagnetic particles which are interacting and creating GMR. [ABSTRACT FROM AUTHOR]

Magnetic and magnetotransport properties measurements of Cu80Fe10Ni10 (at. %) melt spun and annealed ribbons were combined to study the magnetic interactions present in this system. Those ribbons are composed of magnetic FeNi rich precipitates embedded in a nonmagnetic Cu rich matrix. When the precipitates are small enough, they have a superparamagnetic behavior. Upon annealing the precipitates get larger and progressively turn ferromagnetic. The relatively high magnetoresistive properties are attributed to the presence of those superparamagnetic precipitates. Using Mössbauer spectrometry, Zero Field Cooled/Field Cooled and magnetization curves, the presence of interactions among the precipitates was evidenced, which degrades the magnetoresistance properties. Using Allia model, the magnetic coherence length Rm between the precipitates was calculated and compared with the mean free path of the electrons λ, with the precipitates size and the mean distance between them. Three different regimes were observed. At high fields, where Rm<λ, meaning that the precipitates are not interacting, at smaller fields, where Rm>λ and the precipitates are in the so called “interacting superparamagnetic regime,” and at low fields, and at 5 K, Rm>10λ, the magnetic interactions among the precipitates are strong and give rise to a hysteresis on the magnetization curves. [ABSTRACT FROM AUTHOR]

Magnetic and magnetotransport properties measurements of [Cu.sub.80][Fe.sub.10][Ni.sub.10] melt spun and annealed ribbons are combined to study the magnetic interactions. The ribbons are composed of magnetic FeNi rich precipitates embedded in a nonmagnetic Cu rich matrix and the high magnetoresistive properties are attributed to the presence of those superparamagnetic precipitates.

Microstructure, magnetic and transport properties of magnetoresistive CuFeNi ( x = 5, 10 and 15 at.%) melt-spun ribbons were studied for the as-spun and annealed for 2 h at 400 to 600 °C samples. The microstructure was characterised by means of X-ray diffraction, transmission electron microscopy, energy filtered transmission electron microscopy and Mössbauer spectrometry (MS). Magnetic and transport measurements such as magnetization, zero field cooled/field cooled and giant magnetoresistance (GMR) measurements at 5 and 300 K were performed. The aim of this work is to study the influence of the microstructure of as-spun and annealed ribbons on magnetic and transport properties at 5 and 300 K. We have precisely determined the nature of precipitation of nanoparticles, the relative proportion and the magnetic properties of each phase in such a system using MS at both temperatures. We report the conclusive observation of the magnetic behaviour of each phase which was correlated to the structural data to explain the GMR properties that are observed. The influence of magnetic interactions between nanoparticles on magnetic and magnetoresistive properties of these ribbons was determined. We have clearly shown the existence and the nature of the magnetic interactions that reduce the GMR. [ABSTRACT FROM AUTHOR]

Over the years, numerous gold and silver artifacts have been excavated from the tombs of the Tang dynasty, which give evidence of the sophisticated metalworking techniques at that time. Few of the artifacts were thoroughly studied and their manufacturing processes were barely known. The present investigation concerns a metal headgear from a newly excavated tomb of a female in Xi'an of the Tang dynasty (618-907 A.D.), using advanced techniques in a complementary way, especially performing a detailed analysis of the corrosion products and alloying processes. The combined state-of-the-art methods and instrumentation used for the corrosion study included spectroscopy, diffraction, electron microscopy, synchrotron and their versions for specific measurements and sample preparation. The investigated headgear metal consists of a copper-based core, which is gilded by a thin gold layer, consisting of an Au-Hg alloy with a thin layer of about 400 nm. The technique used for shaping and hammered embellishments led to the creation of nanosized grains on the side that would eventually be the interior of the headgear. It was gilded using the mercury-amalgamation process, and the liquid diffusion caused the development of intermetallic compounds. This is the first recorded instance of these nano-scale and eutectic phases being observed on objects from an archaeological context. The crystallographic analysis offered valuable insights into the formation of needle-like malachite crystals growing on a layer of cuprite found on the surface of the corroded piece. The results highlight that the artisans utilized advanced methods in the creation of funerary items during the Tang dynasty.

The formation of a white etching layer (WEL), a very hard and brittle phase on the rail surface, is associated with a progressive transformation of the pearlitic grain to very fragmented grains due to the cumulative passage of trains. Its formation is associated with a complex thermomechanical coupling. To predict the exact conditions of WEL formation, a thermomechanical model previously proposed by the authors needs to be validated. In this study, monotonic and cyclic shear tests using hat-shaped specimens were conducted in the temperature range of 20 °C to 400 °C to reproduce the WEL formation. The tests showed a strong sensitivity of the material to temperature, which does not necessarily favor WEL formation. For the monotonic tests, no WELs were produced; however, a localization of the plastic deformation was observed for tests performed at 200 °C and 300 °C. In this temperature range, the material was less ductile than at room temperature, leading to failure before WEL formation. At 400 °C, the material exhibited a much more ductile behavior, and nanograins close to WEL stages were visible. For the cyclic tests, a WEL zone was successfully reproduced at room temperature only and confirmed the effect of shear in WEL formation. The same cyclic tests conducted at 200 °C and 300 °C yielded results consistent with those of the monotonic tests; the deformation was much more localized and did not lead to WEL formation.

The microstructural evolution of a 2101 lean duplex stainless steel (DSS) during isothermal aging from room temperature to 470 °C was investigated using thermoelectric power (TEP) measurements to follow the kinetics, atom probe tomography, and transmission electron microscopy. Despite the low Ni, Cr, and Mo contents, the lean DSS was sensitive to α-α' phase separation and Ni-Mn-Si-Al-Cu clustering at intermediate temperatures. The time-temperature pairs characteristic of the early stages of ferrite decomposition were determined from the TEP kinetics. Considering their composition and locations, the clusters are most likely G phase precursors.