Magnetic stratigraphy of ODP Leg 133 sites

This study is a synthesis of paleomagnetic and mineral magnetic results for Sites 819 through 823 of Ocean Drilling Program (ODP) Leg 133, which lie on a transect from the outer edge of the Great Barrier Reef (GBR) down the continental slope to the bottom of the Queensland Trough. Because of viscous remagnetization and pervasive overprinting, few reversal boundaries can be identified in these extremely high-resolution Quaternary sequences. Some of the magnetic instability, and the differences in the quality of the paleomagnetic signal among sites, can be explained in terms of the dissolution of primary iron oxides in the high near-surface geochemical gradients. Well-defined changes in magnetic properties, notably susceptibility, reflect responses to glacio-eustatic sea-level fluctuations and changes in slope sedimentation processes resulting from formation of the GBR. Susceptibility can be used to correlate between adjacent holes at a given site to an accuracy of about 20 cm. Among-site correlation of susceptibility is also possible for certain parts of the sequences and permits (tentative) extension of the reversal chronology. The reversal boundaries that can be identified are generally compatible with the calcareous nannofossil biostratigraphy and demonstrate a high level of biostratigraphic consistency among sites. A revised chronology based on an optimum match with the susceptibility stratigraphy is presented. Throughout most of the sequences there is a strong inverse correlation both between magnetic susceptibility and calcium carbonate content, and between susceptibility and d18O. In the upper, post-GBR, sections a more complicated type of magnetic response occurs during glacial maxima and subsequent transgressions, resulting in a positive correlation between susceptibility and d18O. Prior to and during formation of the outer-reef barrier, the sediments have relatively uniform magnetic properties showing multidomain behavior and displaying cyclic variations in susceptibility related to sea-level change. The susceptibility oscillations are controlled more by carbonate dilution than by variation in terrigenous influx. Establishment of the outer reef between 1.01 and 0.76 Ma restricted the supply of sediment to the slope, causing a four-fold reduction in sedimentation rates and a transition from prograding to aggrading seismic geometries (see other chapters in this volume). The Brunhes/Matuyama boundary and the end of the transition period mark a change to lower and more subdued susceptibility oscillations with higher carbonate contents. The major change in magnetic properties comes at about 0.4 Ma in the aggrading sequence, which contains prominent sharp susceptibility peaks associated with glacial cycles, with distinctive single-domain magnetite and mixed single-domain/superparamagnetic characteristics. Bacterial magnetite has been found in the sediments, particularly where there are high susceptibility peaks, but its importance has not yet been assessed. A possible explanation for the characteristic pattern of magnetic properties in the post-GBR glacial cycles can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop when the shelf becomes exposed at low sea-level.

Anomalous magnetic susceptibility values and traces of subsurface microbial activity in carbonate banks on San Salvador Island, Bahamas

Pure limestones beneath the paleosols on San Salvador Island, Bahamas, contain strong positive magnetic susceptibility anomalies, although the iron content is generally very low. These magnetic phenomena differ from those associated with disconformities, which are marked by accumulation of paramagnetic airborne dust deposits with relatively high iron content. The strength and characters of the magnetic response in these subsurface zones correspond to the presence of magnetite, particularly small single-domain magnetite crystals of microbial origin. These crystals are not present elsewhere in the intergranular rock pores or microvugs. They are preferentially concentrated in capillary microborings, which developed concurrently with formation of calcite cements that have soil-related C and O isotope compositions. These magnetic zones occur several meters below the overlying soil horizons. Very thin and long linear microborings may be attributable to cyanobacterial microborers. The single-domain magnetites in these micrometer-size tunnels plugged by calcite appear to result from later occupation of these tiny holes by magnetotactic bacteria. Inorganic origin of the magnetite seems unlikely. Numerous traces that suggest subsurface microbial activity provide evidence that may be used to develop possible scenarios for subsequent biological studies of the precise bacteria involved.

Normative data for two visually evoked magnetic components

The practicality or recording normative data for two components of the visually evoked magnetic response (VEMR) (P100M and P2M) using a single channel dc-SQUID second order gradiometer in an unshielded environment was investigated. Latency norms of the P100M and P2M were more variable than the corresponding electrical P100 and P2 visual evoked potentials. Methods of improving the normative data for clinical use were discussed.

The effect of stimulus location on the major components of the visual evoked response

The topographical distribution of the pattern reversal Visual Evoked Response (VER) was recorded from a localised montage of 20 electrodes over the visual cortex. The response was recorded after stimulation with a black and white checkerboard stimulus. The effect of field location on the major components was investigated in 11 subjects (age range (23-55). The major components of the half field response were; a negative around 75ms (N75) followed by a positivity around 80ms (P80), then a positivity around 100ms (P100) followed by another positivity at around 120ms (P120) and a negativity at approximately 145ms (N145). No effect of field size could be demonstrated on either the amplitude or latency of the late negativity, N145. No significant effect of field size or location was shown on the latency of the P100 response. A delay previously shown in the upper half field response was therefore not substantiated. In contrast the amplitude of the major positivity, P100 was significantly affected by the field size and location. The amplitude of both P100 and N145 were significantly reduced following upper field stimulation when compared with the lower field response. No significant amplitude difference between the upper and lower field responses was demonstrated using electroretinography, the amplitude may therefore be reduced as a result of the ventral position of the upper field representation on the visual cortex. The lateral half field VEP was compared with the distribution of the visual evoked magnetic response (VEMR). The distribution of the VEMR supported the proposal that the paradoxical lateralisation of the VEP half field response is the result of the source being directed ipsilaterally. The morphology of the VEP following octant and double octant stimulation suggests that the response is generated in the striate cortex, with a reversal in response distribution following stimulation of the upper vertical and horizontal meridia.

Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi 5Ti 3Fe 0.7Co 0.3O 15 thin films

Aurivillius phase Bi 5Ti 3Fe 0.7Co 0.3O 15 (BTF7C3O) thin films on α-quartz substrates were fabricated by a chemical solution deposition method and the room temperature ferroelectric and magnetic properties of this candidate multiferroic were compared with those of thin films of Mn 3 substituted, Bi 5Ti 3Fe 0.7Mn 0.3O 15 (BTF7M3O). Vertical and lateral piezoresponse force microscopy (PFM) measurements of the films conclusively demonstrate that BTF7C3O and BTF7M3O thin films are piezoelectric and ferroelectric at room temperature, with the major polarization vector in the lateral plane of the films. No net magnetization was observed for the in-plane superconducting quantum interference device (SQUID) magnetometry measurements of BTF7M3O thin films. In contrast, SQUID measurements of the BTF7C3O films clearly demonstrated ferromagnetic behavior, with a remanent magnetization, B r, of 6.37 emu/cm 3 (or 804 memu/g), remanent moment 4.99 × 10 -5 emu. The BTF7C3O films were scrutinized by x-ray diffraction, high resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis mapping to assess the prospect of the observed multiferroic properties being intrinsic to the main phase. The results of extensive micro-structural phase analysis demonstrated that the BTF7C3O films comprised of a 3.95 Fe/Co-rich spinel phase, likely CoFe 2 - xTi xO 4, which would account for the observed magnetic moment in the films. Additionally, x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) imaging confirmed that the majority of magnetic response arises from the Fe sites of Fe/Co-rich spinel phase inclusions. While the magnetic contribution from the main phase could not be determined by the XMCD-PEEM images, these data however imply that the Bi 5Ti 3Fe 0.7Co 0.3O 15 thin films are likely not single phase multiferroics at room temperature. The PFM results presented demonstrate that the naturally 2D nanostructured Bi 5Ti 3Fe 0.7Co 0.3O 15 phase is a novel ferroelectric and has potential commercial applications in high temperature piezoelectric and ferroelectric memory technologies. The implications for the conclusive demonstration of ferroelectric and ferromagnetic properties in single-phase materials of this type are discussed.

Smart macroporous structures for the purification of viral particles

Dissertação para obtenção do Grau de Mestre em Engenharia Química e Bioquímica

One-step in situ synthesis of polyamide microcapsules with inorganic payload and their transformation into responsive thermoplastic composite materials

Well-dispersed loads of finely powdered metals, metal oxides, several carbon allotropes or nanoclays are incorporated into highly porous polyamide 6 microcapsules in controllable amounts via an original one-step in situ fabrication technique. It is based on activated anionic polymerization (AAP) of ε-caprolactam in a hydrocarbon solvent performed in the presence of the respective micro- or nanosized loads. The forming microcapsules with typical diameters of 25-50 µm entrap up to 40 wt% of load. Their melt processing produces hybrid thermoplastic composites. Mechanical, electric conductivity and magnetic response measurements show that transforming of in situ loaded microcapsules into composites by melt processing (MP) is a facile and rapid method to fabricate materials with high mechanical resistance and electro-magnetic characteristics sufficient for many industrial applications. This novel concept requires low polymerization temperatures, no functionalization or compatibilization of the loads and it is easy to scale up at industrial production levels.

Electric-field control of exchange bias in multiferroic epitaxial heterostructures

The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices.

Synthesis of L1(0) Fe-Pd films by electrodeposition and thermal annealing

Fe-Pd alloy films have been prepared by electrochemical deposition from an alkaline electrolyte containing Fe sulfate, Pd chloride and 5-sulfosalicylic acid onto polycrystalline titanium substrates. The as-deposited films were nanocrystalline and magnetically soft (coercivity similar to 25 Oe). L1(0) Fe-Pd films with a (111) preferred orientation were obtained by post-deposition thermal annealing of films with composition about 37 at% Fe in an (Ar + 5% H-2) gas flow at 500 degrees C. Such films exhibit hard magnetic properties, with a coercivity up to 1880 Oe, and a slightly anisotropic magnetic response, with a larger in-plane remanence. Preliminary magnetic investigations support magnetization switching through pinning of domain walls. (c) 2008 Elsevier B.V. All rights reserved.

Estudo da síntese e propriedade magnética de ferritas de manganês/cobalto dopadas com níquel

For the chemical method of synthesis of co-precipitation were produced ferrite powders manganese-cobalt equal stoichiometric formula Mn (1-x) Co (x) Fe2O4, for 0 < x < 1, first reagent element using as the hydroxide ammonium and second time using sodium hydroxide. The obtained powders were calcined at 400 ° C, 650 ° C, 900 ° C and 1150 ° C in a conventional oven type furnace with an air atmosphere for a period of 240 minutes. Other samples were calcined at a temperature of 900 ° C in a controlled atmosphere of argon, to evaluate the possible influence of the atmosphere on the final results the structure and morphology. The samples were also calcined in a microwave oven at 400 ° C and 650 ° C for a period of 45 minutes possible to evaluate the performance of this type of heat treatment furnace. It was successfully tested the ability of this group include isomorphic ferrite with the inclusion of nickel cations in order to evaluate the occurrence of disorder in the crystalline structures and their changes in magnetic characteristics.To identify the structural, morphological, chemical composition and proportions, as well as their magnetic characteristics were performed characterization tests of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), thermogravimetric (TG), vibrating sample magnetometry (MAV) and Mössbauer spectroscopy. These tests revealed the occurrence of distortion in the crystal lattice, changes in magnetic response, occurrence of nanosized particles and superparamagnetism

Chemical deposition of La0.7Ca0.3MnO3±δ films on ceramic substrates

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Solid-liquid transition in Nb powder determined by third harmonic susceptibility

Measurements of the third harmonic of the AC-susceptibility were employed to determine the boundaries of the linear regime of the magnetic response of Nb powder. Non-linear contributions to the magnetic response reveal the occurrence of a structured phase, disappearing as the vortex lattice melts to the liquid state. A systematic study of the third harmonic was conducted to determine how its onset temperature depends on experimental parameters, such as the frequency and amplitude of the excitation field. The melting line (ML) has been extracted from the onset temperature measured at low-frequencies and low-excitation fields in the presence of DC magnetic fields. The study indicates that the ML can be described by a 3D vortex-glass model, except at lower fields, where the system experiences a depinning crossover, and the best description of the experimental data is provided by a 3D Bose-glass model. (c) 2008 Elsevier B. V. All rights reserved.

Estruturação da seqüência vulcano-sedimentar Rio Mata Cavalo (Morro do Pilar, MG, Brasil) baseada na análise qualitativa de dados geofísicos

Geofísica terrestre (magnetometria e radiometria) foi utilizada como ferramenta de apoio ao mapeamento geológico em uma área de pesquisa mineral da Companhia Mineradora de Minas Gerais (COMIG), denominada Alvo Bueiro, localizada a sudeste da cidade de Morro do Pilar, na Serra do Espinhaço, Estado de Minas Gerais. A integração de informações de geologia de superfície com os dados geofísicos correspondentes permitiu definir quatro zonas: A, B, C e D. A Zona A, caracterizada por altos valores de susceptibilidade magnética e baixos valores de radiometria, é composta por itabiritos do Grupo Serra da Serpentina. A Zona B, pertencente ao mesmo grupo, é constituída por filitos cinza e metassiltitos e mostra baixa susceptibilidade magnética; interrupções nas linhas de contorno foram interpretadas como decorrentes de falhas de empurrão. A Zona C, relacionada à Seqüência Vulcano-Sedimentar Rio Mata Cavalo, é caracterizada por uma expressiva anomalia do campo magnético total em xistos máficos e ultramáficos com lentes de formações ferríferas e filitos carbonosos negros; esta zona hospeda as mais importantes anomalias geoquímicas de ouro conhecidas na área em estudo. A Zona D, correspondente a ortognaisses cisalhados do Complexo Dona Rita, mostra os mais elevados valores de radiometria do Alvo Bueiro e baixas respostas magnéticas.

Chelating agent effects in the synthesis of mesoscopic-size superconducting particles

Several routes and procedures have been used in these last years as an effort to achieve single-phase mesoscopic-size superconducting samples. In this paper, the effects of using citric acid (CA), tartaric acid (TA) and ethylenediaminetetraacetic acid (EDTA) as chelating agents and ethylene-glycol (EG) as polyhydroxy alcohol were studied in order to establish conditions to avoid the occurrence of BaCO(3) undesirable secondary phase in YBa(2)Cu(3)O(7-delta) (YBCO). Thermal evolution of intermediate compounds formed during the calcinations process by the use of different chelating agents was traced using thermogravimetric and spectroscopic methods. The obtained results indicated that the polymer breakdown of samples prepared using EDTA occurs at higher temperatures than others chelating agents and also reduces the occurrence of BaCO(3) secondary phase as studied by X-ray diffraction measurements. Furthermore, the magnetic response of the mesoscopic-size YBCO specimens obtained was verified showing that samples present different superconducting response.

Oxygen stoichiometry effects in spinel-type NiMn2O4-delta samples

We report the effects of oxygen stoichiometry in the structure and magnetic response of spinel-type NiMn2O4-delta polycrystalline samples controlled by heat-treatments in different atmospheric conditions.The post-annealed samples were analyzed by Scanning Electron Microscopy associated to X-ray Energy Dispersion Spectrometry, X-ray Photoelectron Spectroscopy and AC/DC magnetic measurements.Results indicate that the oxygen stoichiometry highly influences the magnetic interactions between the ferromagnetic, and antiferromagnetic sublattices in these compounds due to the presence of manganese in three possible valence states. (c) 2005 Elsevier Ltd. All rights reserved.