Helium ion beam milling to create a nano-structured domain wall magnetoresistance spin valve.

We have fabricated and measured single domain wall magnetoresistance devices with sub-20 nm gap widths using a novel combination of electron beam lithography and helium ion beam milling. The measurement wires and external profile of the spin valve are fabricated by electron beam lithography and lift-off. The critical bridge structure is created using helium ion beam milling, enabling the formation of a thinner gap (and so a narrower domain wall) than that which is possible with electron beam techniques alone. Four-point probe resistance measurements and scanning electron microscopy are used to characterize the milled structures and optimize the He ion dose. Successful operation of the device as a spin valve is demonstrated, with a 0.2% resistance change as the external magnetic field is cycled. The helium ion beam milling efficiency as extracted from electrical resistance measurements is 0.044 atoms/ion, about half the theoretical value. The gap in the device is limited to a maximum of 20 nm with this technique due to sub-surface swelling caused by injected ions which can induce catastrophic failure in the device. The fine patterning capabilities of the helium ion microscope milling technique indicate that sub-5 nm constriction widths could be possible.

A quantitative evaluation of spurious results in the infrared spectroscopic measurement of CO2 isotope ratios.

The possible generation of spurious results, arising from the application of infrared spectroscopic techniques to the measurement of carbon isotope ratios in breath, due to coincident absorption bands has been re-examined. An earlier investigation, which approached the problem qualitatively, fulfilled its aspirations in providing an unambiguous assurance that 13C16O2/12C16O2 ratios can be confidently measured for isotopic breath tests using instruments based on infrared absorption. Although this conclusion still stands, subsequent quantitative investigation has revealed an important exception that necessitates a strict adherence to sample collection protocol. The results show that concentrations and decay rates of the coincident breath trace compounds acetonitrile and carbon monoxide, found in the breath sample of a heavy smoker, can produce spurious results. Hence, findings from this investigation justify the concern that breath trace compounds present a risk to the accurate measurement of carbon isotope ratios in breath when using broadband, non-dispersive, ground state absorption infrared spectroscopy. It provides recommendations on the length of smoking abstention required to avoid generation of spurious results and also reaffirms, through quantitative argument, the validity of using infrared absorption spectroscopy to measure CO2 isotope ratios in breath.

Evaluation of spurious results in the infrared measurement of CO2 isotope ratios due to spectral effects: a computer simulation study.

The application of infrared spectroscopy to the measurement of carbon isotope ratio breath tests is a promising alternative to conventional techniques, offering relative simplicity and lower costs. However, when designing such an instrument one should be conscious of several spectral effects that may be misinterpreted as changes in the isotope concentration and which therefore lead to spurious results. Through a series of computer simulations which model the behaviour of the CO2 absorption spectrum, the risk these effects pose to reliable measurement of 13CO2/12CO2 ratios and the measures required to eliminate them are evaluated. The computer model provides a flexible high-resolution spectrum of the four main isotopomer fundamental transitions and fifteen of their most significant hotband transitions. It is demonstrated that the infrared source, infrared windows and breath sample itself all exhibit strong temperature induced errors but pressure effects do not produce significant errors. We conclude that for reliable measurement of 13CO2/12CO2 ratios using infrared spectroscopy no pressure controls are required. window effects are eliminated using windows wedged at a minimum angle of 0.8-2.2 mrad, depending on the material, and the temperature sensitivity of source and gas cells necessitates stabilization to an accuracy of at least 0.2 K.

The application of infrared spectroscopy to breath CO2 isotope ratio measurements and the risk of spurious results.

Stable CO2 isotope breath tests are established as a valuable tool in diagnostic and investigative medicine with the potential to become more prominent in the future. However, their development and widespread clinical use is limited by the requirement of isotope ratio mass spectroscopic analysis. To overcome this restriction alternative analytical techniques have been developed; the most promising, offering relative simplicity and lower costs, are those instruments using infrared spectroscopy. Clinical investigations using such instruments show them to perform well but very little attention has been given to the possibility of interference from the infrared absorption spectrum of other compounds in the breath. To provide an unambiguous answer to this concern we have analysed literature on over 200 detected breath compounds and their infrared absorption spectra to identify any absorption bands coincident with the nu3 absorption band of CO2. It was found that only five breath trace compounds possess coincident fundamental absorption bands, none of which pose the risk of spurious results. We conclude that the 13C16O2/12C16O2 ratio can confidently be measured for isotopic breath tests using an infrared spectrometer, the position of the nu3 absorption band of CO2 in the infrared spectrum precluding any discernible risk of spurious measurements due to coincidental absorption bands.

Photosensitivity of lead germanate glass waveguides grown by pulsed laser deposition.

We report very large photoinduced refractive-index changes Dn, of the order of ~10(2), in lead germanate glass waveguides grown by pulsed-laser deposition. The magnitude of Dn was derived from measurements of diffraction efficiency for gratings written by exposure to 244-nm light through a phase mask, whereas the sign of Dn was determined from ellipsometric data. Results are shown for films grown under oxygen pressures ranging from 1 chi 10(-2) to 6 chi 10(-2)mbars (1.33mbars=1 Torr).