Ab initio and tight-binding calculations of noncollinear magnetism in manganese clusters

Combining ab initio and tight-binding calculations, we have studied the noncollinear magnetism in manganese clusters. The oscillations in the per-atom moments observed experimentally are reproduced theoretically. The tendency of antiferromagnetic coupling between near neighbors leads to noncollinear coupling between atoms within the clusters. For clusters containing 12, 13, 15, 19, and 23 atoms, the geometrical structures were optimized from ab initio calculations with collinear coupled spin moments among different atomic sites. For larger clusters such as Mn-36 and Mn-55, the geometries are taken as portions of an fcc structure. Although the local atomic moments have high values close to 4 mu(B), the net moments lie in the range of 0.4-1.2 mu(B)/atom. Taking the noncollinear coupling into account brings the calculated magnetic moments much closer to the experimental results.

Orbital-polarization terms: From a phenomenological to a first-principles description of orbital magnetism in density-functional theory

Phenomenological orbital-polarizition (OP) terms have been repeatedly introduced in the single-particle equations of spin-density-functional theory, in order to improve the description of orbital magnetic moments in systems containing transition metal ions. Here we show that these ad hoc corrections can be interpreted as approximations to the exchange-correlation vector potential A(xc) of current-density functional theory (CDFT). This connection provides additional information on both approaches: phenomenological OP terms are connected to first-principles theory, leading to a rationale for their empirical success and a reassessment of their limitations and the approximations made in their derivation. Conversely, the connection of OP terms with CDFT leads to a set of simple approximations to the CDFT potential A(xc), with a number of desirable features that are absent from electron-gas-based functionals. (C) 2008 Wiley Periodicals, Inc.

Rock magnetism in soils of the Pampean plain. Buenos Aires province, Argentina. Linking climate and magnetic behaviour

Results of the study of Argiudolls in two localities (Zarate and Veronica) of the Pampean plain, Argentina, are presented in this contribution. This is a typical area covered by loess. The magnetic studies carried out allowed to determine the presence of detrital magnetite and titanomagnetite, as well as maghemite, pedogenic goethite and superparamagnetic particles (SP). In Veronica soils, a depletion of ferromagnetic minerals is recorded The dominant process in these soils has been the reductive loss of detrital magnetite and titanomagnetite. This is associated with a greater degree of evolution of the soil, which is determined by the concentration and type of detected clays. The higher clay concentration in these soils facilitated reducing conditions and a greater loss of detrital magnetic particles. The loss is reinforced under poor drainage conditions. In the poorly drained soil of Zarate, a concentration of magnetic particles is observed in the Bt horizon, which is associated with an illuviation process. The well drained soil of the same locality shows neoformation of SP particles. These particles would have an ephemeral life until a new wet period in the annual cycle occurs. Although some characteristics of the magnetic signal appear reinforced by the conditions of drainage, this aspect does not seem to be too significant, at least in the Pampean region with low topographic gradients.

Multi-scale rheological model for discontinuous phenomena in materials under deformation conditions

A study of possibilities given by the developed Cellular Automata–Finite Element (CAFE) multi-scale model for prediction of the initiation and propagation of micro-shear bands and shear bands in metallic materials subjected to plastic deformation is described in the paper. Particular emphasis in defining the criterion for initiation of micro-shear and shear bands, as well as in defining the transition rules for the cellular automata, is put on accounting for the physical aspects of those phenomena occurring in two different scales in the material. The proposed approach led to the creation of the real multi-scale model of strain localization. This model predicts material behavior in various thermo-mechanical processes. Selected examples of applications of the developed model to simulations of metal forming processes, which involve strain localization, are presented in the paper. An approach based on the Smoothed Particle Hydrodynamic, which allows to overcome difficulties with remeshing in the traditional CAFE method, is presented in the paper as well. In this approach remeshing becomes possible and mesh distortion, which limits application of the CAFE method to simple deformation processes, is eliminated.

Student opportunities in materials design and manufacture: Introducing a new manufacturing with composites course

The shift towards strong and lightweight fibre reinforced polymer-matrix composites for many high performance applications has resulted in an increasing need to expose students to composite design and manufacture courses in the undergraduate curriculum. In contrast, student exposure to composite materials is often still limited to a topic within a materials or manufacturing related course (unit). This paper presents the initial offering of a composite materials elective at Griffith University in Australia. The course also addresses environmental concerns through the inclusion of natural fibre composites. An evaluation of student perceptions is considered from Griffith’s Student Experience of Course (SEC) and separate Student Experience of Teaching (SET) surveys. These evaluations demonstrate the high level of student engagement with the course, but also highlighted areas for improvement, including the need to incorporate even more hands-on practical work. Interestingly, the inclusion of natural fibre composites and the related discussion surrounding environmental and societal issues are not focused on in student feedback.

Superconductivity and magnetism in the KxMoO2-delta

Coexistence between superconductivity and magnetism is reported for the KxMoO2-delta samples. Photoemission experiments show that the presence of Mo3+ ions is responsible for the weak ferromagnetic ordering observed in the KxMoO2-delta samples. Magnetic ordering temperature and superconducting critical temperature (T-C) ratio range from 7 to 18 in this compound. These are the highest ratios reported so far for a magnetic superconductor. T-C decreases with increasing potassium composition (x). For the first time, T-C near 10 K is reported in the K-Mo-O system. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757003]

First-principles studies of complex magnetism in Mn nanostructures on the Fe(001) surface

The magnetic properties of Mn nanostructures on the Fe(001) surface have been studied using the noncollinear first-principles real space-linear muffin-tin orbital-atomic sphere approximation method within density-functional theory. We have considered a variety of nanostructures such as adsorbed wires, pyramids, and flat and intermixed clusters of sizes varying from two to nine atoms. Our calculations of interatomic exchange interactions reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. We have found that the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion.

Opportunities in Materials Processing: Metal Casting

Revitalizing manufacturing in the US is a hot topic, and the $1B National Network for Manufacturing Innovation (NNMI) has three new centers focused on metal casting. For structural applications, magnesium casting, structural die-cast alloys, austempered ductile and compacted graphite cast iron, and high strength steel promise dramatic weight reduction and improved performance. Recent experiments and modeling at UAB focus on the development of such new materials and processing routes. A study of the rare earth content in an aerospace magnesium alloy is presented.

The use of mineral magnetism in the reconstruction of fire history: a case study from Lago di Origlio, Swiss Alps

The use of magnetic measurements in the detection of fire signals has been neglected since the work of Rummery et al., (1979), yet considerable developments have been made in the interpretation of magnetic measurements over the last 16 years. This paper presents a study of the fire history of Lago di Origlio in the southern Swiss Alps. The study utilises the technique of mineral magnetism alongside the stratigraphic pollen, spore and charcoal records. Correlation between the various proxy records indicates that a magnetic ‘fire’ record is present within the sediments for the last 4 ka. The magnetic fire record has a distinct mineralogical and magnetic grain size signature that can be recognised against the background sedimentary signal. The results suggest that magnetic measurements may be usefully employed in the reconstruction of fire history. Their application is rapid and non-destructive and the results may provide additional information in relation to the links between catchment fire events and the sedimentary record.

Vacancy-induced magnetism in graphene and graphene ribbons

We address the electronic structure and magnetic properties of vacancies and voids both in graphene and graphene ribbons. By using a mean-field Hubbard model, we study the appearance of magnetic textures associated with removing a single atom (vacancy) and multiple adjacent atoms (voids) as well as the magnetic interactions between them. A simple set of rules, based on the Lieb theorem, link the atomic structure and the spatial arrangement of the defects to the emerging magnetic order. The total spin S of a given defect depends on its sublattice imbalance, but some defects with S=0 can still have local magnetic moments. The sublattice imbalance also determines whether the defects interact ferromagnetically or antiferromagnetically with one another and the range of these magnetic interactions is studied in some simple cases. We find that in semiconducting armchair ribbons and two-dimensional graphene without global sublattice imbalance, there is a maximum defect density above which local magnetization disappears. Interestingly, the electronic properties of semiconducting graphene ribbons with uncoupled local moments are very similar to those of diluted magnetic semiconductors, presenting giant Zeeman splitting.

Magnetism in graphene nanoislands

We study the magnetic properties of nanometer-sized graphene structures with triangular and hexagonal shapes terminated by zigzag edges. We discuss how the shape of the island, the imbalance in the number of atoms belonging to the two graphene sublattices, the existence of zero-energy states, and the total and local magnetic moment are intimately related. We consider electronic interactions both in a mean-field approximation of the one-orbital Hubbard model and with density functional calculations. Both descriptions yield values for the ground state total spin S consistent with Lieb’s theorem for bipartite lattices. Triangles have a finite S for all sizes whereas hexagons have S=0 and develop local moments above a critical size of ≈1.5  nm.

Project Neos; a general study of processes for the realization of design configurations in materials

"Cornell Aeronautical Laboratory. Contract: AF 33(600)-42921. ASD project NR 7-867."