Blocking phenomena in granular magnetic alloys through magnetization, Hall effect, and magnetoresistance experiments

Magnetization and magnetotransport were measured in CoxAg1-x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution. (C) 2003 American Institute of Physics.82576376

Large Anomalous Hall effect in a silicon-based magnetic semiconductor

Magnetic semiconductors are attracting high interest because of their potential use for spintronics, a new technology which merges electronics and manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology. While Curie temperatures are rising towards room temperature, these materials can only be fabricated in thin film form, are heavily defective, and are not obviously compatible with Si. We show here that it is productive to consider transition metal monosilicides as potential alternatives. In particular, we report the discovery that the bulk metallic magnets derived from doping the narrow gap insulator FeSi with Co share the very high anomalous Hall conductance of (GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens up a new arena for spintronics, involving a bulk material based only on transition metals and Si, and which we have proven to display a variety of large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure

Discerning Tumor Status from Unstructured MRI Reports—Completeness of Information in Existing Reports and Utility of Automated Natural Language Processing

Information in electronic medical records is often in an unstructured free-text format. This format presents challenges for expedient data retrieval and may fail to convey important findings. Natural language processing (NLP) is an emerging technique for rapid and efficient clinical data retrieval. While proven in disease detection, the utility of NLP in discerning disease progression from free-text reports is untested. We aimed to (1) assess whether unstructured radiology reports contained sufficient information for tumor status classification; (2) develop an NLP-based data extraction tool to determine tumor status from unstructured reports; and (3) compare NLP and human tumor status classification outcomes. Consecutive follow-up brain tumor magnetic resonance imaging reports (2000–­2007) from a tertiary center were manually annotated using consensus guidelines on tumor status. Reports were randomized to NLP training (70%) or testing (30%) groups. The NLP tool utilized a support vector machines model with statistical and rule-based outcomes. Most reports had sufficient information for tumor status classification, although 0.8% did not describe status despite reference to prior examinations. Tumor size was unreported in 68.7% of documents, while 50.3% lacked data on change magnitude when there was detectable progression or regression. Using retrospective human classification as the gold standard, NLP achieved 80.6% sensitivity and 91.6% specificity for tumor status determination (mean positive predictive value, 82.4%; negative predictive value, 92.0%). In conclusion, most reports contained sufficient information for tumor status determination, though variable features were used to describe status. NLP demonstrated good accuracy for tumor status classification and may have novel application for automated disease status classification from electronic databases

Resistivity and hall resistivity in percolating (NiFe)-SiO2 films

We found that in granular (NiFe)(x)(SiO2)(1-x), films, the ordinary Hall effect increases with decreasing x, in a way similar to the extraordinary Hall effect, with critical exponents greater than the theoretical value, while the resistivity increases with decreasing temperature as -logT for x slightly above the percolation threshold x(c), and as T-0.2 for x similar to x(c). We suggest that the giant Hall effect results from a reduced effective carrier density due to weak localization at x similar to x(c). Copyright (C) 1996 Published by Elsevier Science Ltd

Critical behavior of resistivity and Hall resistivity in percolating fellomagnetic metal-insulator films

Resistivity rho(xx) and Hall resistivity rho(xy) were measured as functions of the magnetic field, temperature, and the metal volume fraction x, on a series of percolating Ni-SiO2 granular films. At 5 K, the data for the metallic samples were fitted to power law dependencies of the forms, rho(xx) proportional to [(x - x(c))/x(c)](-1) and rho(xy) proportional to [(x - x(c))/x(c)](-g), with x(c) = 0.6. The critical exponents of the resistivity t = 2.7 +/- 0.2, the extraordinary Hall resistivity g(s) = 2.0 +/- 0.2 and the ordinaIy Hall resistivity g(0) = 1.8 +/- 0.3 were found to be notably larger than the predictions for discrete percolation models (t approximate to 2 and g approximate to 0.4-0.5). We suggest that a percolation model incorporating localization can explain that contradiction. In the region of transition from metallic conduction to thermally activated tunneling, 0.53 < x < 0.6, the extraordinary Hall resistivity rho(xys) was found to be saturated at a value > 100 mu Omega cm, which is almost four orders of magnitude greater than that of pure Ni

Observation of giant Hall effect in granular magnetic films

The Hall effect in granular co-sputtered ferromagnetic metal-insulator films was found to increase dramatically as the magnetic volume fraction decreases toward the metal-insulator transition. The saturated Hall resistivity is up to 160 mu Omega cm at T=5 K, that is almost four orders of magnitude greater than that in a pure magnetic metal sample. Close to the metal-insulator transition, both magnetoresistivity and the saturated Hall resistivity decrease with increasing temperature. Correlations of the Hall resistivity with resistivity and magnetoresistivity are discussed. (C) 1996 American Institute of Physics

Numerical study of conductance distribution in granular metal films

We study the shape of distribution F(G) for the conductance G between a point on the surface of a metal-insulator nanocomposite film and the conducting substrate. Random resistor networks with both metallic and tunneling bonds included are used to model nanocomposite films. Our simulation results show explicitly that the shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By applying our results to the available experimental data on granular NiFe-SiO2, we find important implications for the understanding of microscopic conduction mechanisms near the metal-insulator transition. (C) 2000 American Institute of Physics. [S0021-8979(00)02903-0]

Anisotropic magnetoresistance and planar Hall effect in magnetic metal-insulator composite films

We studied anisotropic magnetoresistivity (AMR) and planar Hall effect of granular Ni-rich NiFe-SiO2 and Fe-SiO2 films for various metallic volume fraction. Planar Hall resistivity was found to be the same as the magnetoresistivity (MR) difference between the longitudinal and the transverse geometry. As metallic volume fraction decreases, we found that the MR evolves from the AMR domination in the metallic conduction region, through an intermediate conduction region with mixed AMR and negative MR behavior, to the isotropic negative MR in the tunneling conduction region. Plausible explanations to this complicated evolution are discussed. (C) 1997 American Institute of Physics

Observation of giant Hall effect in non-magnetic cermets

We study the Hall coefficient over a wide temperature range in co-sputtered Cu:SiO2 samples with metal volume fraction in the range of 0.45 less than or equal to x less than or equal to 0.8. With decreasing metal volume fraction x, the samples change from metal to insulator and the Hall coefficient R increases drastically. A maximum has been observed at x = 0.51, which is similar to 700 times larger than that at x = 0.8. This enhancement far exceeds the factor of 20, which can be derived from the classical percolation (finite size scaling) theory. Based on these observations, we conclude that the non-magnetic GHE arises most probably from quantum interference effects on the mesoscopic scale. (C) 2000 Elsevier Science B.V. All rights reserved

GIANT HALL-EFFECT IN PERCOLATING FERROMAGNETIC GRANULAR METAL-INSULATOR FILMS

We studied both the resistivity and the Hall resistivity of cosputtered granular Ni-SiO2 films with the metal volume fraction x in the range of 0.5-1.0. Near the metal-insulator transition, or x of about 0.53-0.61, the saturated value of the Hall resistivity was up to 2 X 10(-4) Omega cm. This value is almost 4 orders of magnitude greater than that of pure nickel. Both the resistivity and the Hall resistivity varied weekly with temperature, throughout the range of 5-300 K. We suggest that the percolating ferromagnetic granular metal films can be an alternative candidate material for high sensitivity Hall sensors. (C) 1995 American Institute of Physics