Ncholu I. Manyala

Bio: Ncholu I. Manyala is an academic researcher from University of Pretoria. The author has contributed to research in topics: Supercapacitor & Graphene foam. The author has an hindex of 38, co-authored 177 publications receiving 4220 citations. Previous affiliations of Ncholu I. Manyala include National University of Lesotho & Louisiana State University.

Large anomalous Hall effect in a silicon-based magnetic semiconductor

Abstract: Magnetic semiconductors are attracting great interest because of their potential use for spintronics, a new technology that merges electronics with the manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology, and although their 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, which displays large magnetic-field effects on its electrical properties.

Magnetoresistance from quantum interference effects in ferromagnets

Abstract: The desire to maximize the sensitivity of read/write heads (and thus the information density) of magnetic storage devices has stimulated interest in the discovery and design of new magnetic materials exhibiting magnetoresistance Recent discoveries include the 'colossal' magnetoresistance in the manganites and the enhanced magnetoresistance in low-carrier-density ferromagnets An important feature of these systems is that the electrons involved in electrical conduction are different from those responsible for the magnetism The latter are localized and act as scattering sites for the mobile electrons, and it is the field tuning of the scattering strength that ultimately gives rise to the observed magnetoresistance Here we argue that magnetoresistance can arise by a different mechanism in certain ferromagnets--quantum interference effects rather than simple scattering The ferromagnets in question are disordered, low-carrier-density magnets where the same electrons are responsible for both the magnetic properties and electrical conduction The resulting magnetoresistance is positive (that is, the resistance increases in response to an applied magnetic field) and only weakly temperature-dependent below the Curie point

Magnetoresistance From Quantum Interference Effects in Ferromagnets

Abstract: The desire to maximize the sensitivity of read/write heads and thus the information density of magnetic storage devices has produced an intense interest in the magnetoresistance (MR) of magnetic materials. Recent discoveries include "colossal" MR of the manganites1-4 and the enhanced MR of low carrier density ferromagnets4-6. In the low carrier density systems investigated to date as well as the manganites, a key feature is that the electrical conduction is due to a different set of electrons than the localized electrons responsible for the magnetism. Here we propose a mechanism for ferromagnetic MR originating from quantum interference effects, rather than simple scattering. The new mechanism obtains in disordered low carrier density magnets where the magnetism as well as the electrical conduction are due to the same electrons. Here the MR is positive and only weakly temperature dependent below the Curie point. This is very different from the MR seen when conduction electrons and local moments can be treated separately, in which case the MR is negative and strongly peaked at the Curie point.

Renewable pine cone biomass derived carbon materials for supercapacitor application

Abstract: The environmental degradation and hazard to human life caused by the depletion of fossils fuels and the urgent need for sustainable energy sources to meet the rising demand in energy has led to the exploration of novel materials that are environmentally friendly, low cost and less hazardous to human life for energy storage application using the green chemistry approach. Herein, we report on the transformation of the readily abundant pine cone biomass into porous carbon via KOH activation and carbonization at 800 °C as electrode materials for supercapacitors. The porous carbon material exhibited a mesoporous framework with a specific surface area of 1515 m2 g−1, a high voltage window of 2.0 V, a gravimetric capacitance of 137 F g−1, energy density of 19 W h kg−1 and excellent cyclability in neutral 1 M Na2SO4 electrolyte for a symmetric carbon/carbon electrode cell. The result shows that the material is robust and shows great promise with neutral electrolytes in high-performance energy-storage devices.

Microwave-Assisted Synthesis of High-Voltage Nanostructured LiMn1.5Ni0.5O4 Spinel: Tuning the Mn3+ Content and Electrochemical Performance

Abstract: The LiMn1.5Ni0.5O4 spinel is an important lithium ion battery cathode material that has continued to receive major research attention because of its high operating voltage (∼4.8 V). This study interrogates the impact of microwave irradiation on the Mn3+ concentration and electrochemistry of the LiMn1.5Ni0.5O4 spinel. It is shown that microwave is capable of tuning the Mn3+ content of the spinel for enhanced electrochemical performance (high capacity, high capacity retention, excellent rate capability, and fast Li+ insertion/extraction kinetics). This finding promises to revolutionize the application of microwave irradiation for improved performance of the LiMn1.5Ni0.5O4 spinel, especially in high rate applications.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Anomalous hall effect

Abstract: We present a review of experimental and theoretical studies of the anomalous Hall effect (AHE), focusing on recent developments that have provided a more complete framework for understanding this subtle phenomenon and have, in many instances, replaced controversy by clarity. Synergy between experimental and theoretical work, both playing a crucial role, has been at the heart of these advances. On the theoretical front, the adoption of Berry-phase concepts has established a link between the AHE and the topological nature of the Hall currents which originate from spin-orbit coupling. On the experimental front, new experimental studies of the AHE in transition metals, transition-metal oxides, spinels, pyrochlores, and metallic dilute magnetic semiconductors, have more clearly established systematic trends. These two developments in concert with first-principles electronic structure calculations, strongly favor the dominance of an intrinsic Berry-phase-related AHE mechanism in metallic ferromagnets with moderate conductivity. The intrinsic AHE can be expressed in terms of Berry-phase curvatures and it is therefore an intrinsic quantum mechanical property of a perfect cyrstal. An extrinsic mechanism, skew scattering from disorder, tends to dominate the AHE in highly conductive ferromagnets. We review the full modern semiclassical treatment of the AHE together with the more rigorous quantum-mechanical treatments based on the Kubo and Keldysh formalisms, taking into account multiband effects, and demonstrate the equivalence of all three linear response theories in the metallic regime. Finally we discuss outstanding issues and avenues for future investigation.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.