Parametric optimization for terabit perpendicular recording

The design of media for ultrahigh-density perpendicular recording is discussed in depth. Analytical and semianalytical models are developed to determine the constraints upon the media to fulfill requirements of writability and thermal stability, and the effect of intergranular exchange coupling is examined. The role of vector fields during the write process is examined, and it is shown that one-dimensional models of perpendicular recording have significant deficiencies. A micromagnetic model is described and the results of simulations of recording undertaken with the model are presented. The paper demonstrates that there is no physical reason why perpendicular recording should not be possible at or above 1 Tb/in(2)

Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia

Published online: 28 Mar 2021The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ34Spy) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ34Spy (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous.P. A. Hall, D. M. McKirdy, G. P. Halverson, J. B. Jago and A. S. Collin

A second type of highly asphaltic crude oil seepage stranded on the South Australian coastline

Strandings of semi-solid to solid asphaltic bitumen along the coastline of South Australia have been reported as far back as the late 1800s. Hitherto only a single variety, now referred to as asphaltite, has been attributed to seepage from the nearby Bight Basin. The geochemistry of the asphaltites suggest they were derived from a marine source rock deposited under anoxic or euxinic conditions, most likely a Cretaceous ocean anoxic event, and were generated within the early/main oil window. Here we identify a new type of semi-solid asphaltic bitumen collected following a severe storm event in 2016. Termed asphaltic tars, these viscous oils bear a strong geochemical resemblance to the asphaltites. Both oil types have high asphaltene contents, identical n-alkane carbon isotopic profiles and near identical source-specific sterane distributions. However, several notable geochemical variations can distinguish these new strandings from the asphaltites. The most notable of these differences include heavier bulk sulphur isotopic composition, extremely high abundances of Re and Os with distinct 187Re/188Os and 187Os/188Os values and thermal maturity parameters consistent with generation in the late oil window. The differences in sulphur isotopic composition and Re-Os systematics could be considered evidence that despite their other source-specific similarities, the asphaltic tars originated from a different source rock. However, alteration of these two parameters can occur due to thermochemical sulphate reduction. Conclusive identification of this alteration process typically relies on further diagnostic parameters which are unfortunately not available in the case of coastal oil strandings. This introduces uncertainty to the correlation of these two types of asphaltic oil. In either scenario, the similarities between these two types of oil suggest their source rock(s) contained highly comparable organic matter inputs. We therefore attribute the origin of these new asphaltic tar strandings to natural seepage from the offshore Bight Basin.Alexander J. Corrick, Philip A. Hall, Se Gong, David M. McKirdy, David Selby, Christine Trefry, Andrew S. Ros

Integrated low-temperature superconductor SQUID gradiometers for nondestructive evaluation

We describe an eddy current nondestructive evaluation system using a low-temperature superconductor magnetic field sensor in an electromagnetically unshielded environment. The sensor comprises a niobium dc superconducting quantum interference device (SQUID) integrated with a first-order gradiometric pickup coil to reject spatially uniform interference fields but remain sensitive to flaw induced fields. We demonstrate its use in locating and mapping subsurface flaws in a multilayered aluminum structure using an eddy current excitation field at a frequency of 70 Hz, with no magnetic shielding around the specimen or cryostat