Luis E. Fong

High-resolution room-temperature sample scanning superconducting quantum interference device microscope configurable for geological and biomagnetic applications

We have developed a scanning superconducting quantum interference device (SQUID) microscope system with interchangeable sensor configurations for imaging magnetic fields of room-temperature (RT) samples with submillimeter resolution. The low-critical-temperature (Tc) niobium-based monolithic SQUID sensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25μm sapphire window separates the vacuum space from the RT sample. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. We achieved a sensor-to-sample spacing of 100μm, which could be maintained for periods of up to four weeks. Different SQUID sensor designs are necessary to achieve the best combination of spatial resolution and field sensitivity for a given source configuration. For imaging thin sections of geological samples, we used a custom-designed monolithic low-Tc niobium bare SQUID sensor, with an effective diameter of 80μm, and achieved a field sensitivity of 1.5...

Investigating impact demagnetization through laser impacts and SQUID microscopy

Understanding demagnetization by hypervelocity impacts is crucial for the interpreta- tion of planetary magnetic anomalies and remanent magnetization in meteorites. We de- scribe an innovative approach for investigating the effects of impacts on the remanent magnetization of geologic materials. It consists of the combination of pulsed laser impacts and Superconducting Quantum Interference Device (SQUID) microscopy. Laser impacts are nondestructive, create shocks with peak pressures as high as several hundred GPa, and allow well-calibrated modeling of shock wave propagation within the impacted sam- ples. High-resolution SQUID microscopy quantitatively maps the magnetic field of room- temperature samples with an unprecedented spatial resolution of ;100 mm. We present shock modeling and magnetic field data obtained for two laser impacts on a magnetite- bearing basalt sample. Magnetic measurements show a demagnetized area at the impact locations. We also show that high-resolution magnetic measurements combined with im- pact modeling provide a continuous relation between the demagnetization intensity and the peak pressure undergone by the sample. This promising technique will allow for the investigation of the demagnetization behavior of a variety of geological materials upon impacts, with implications for our understanding of the magnetization of extraterrestrial materials and of terrestrial impact structures.

Monolithic low-transition-temperature superconducting magnetometers for high resolution imaging magnetic fields of room temperature samples

We have developed a monolithic low-temperature superconducting quantum interference device (SQUID) magnetometer and incorporated the device in a scanning microscope for imaging magnetic fields of room temperature samples. The instrument has a ∼100 μm spatial resolution and a 1.4 pT/Hz1/2 field sensitivity above a few hertz. We discuss design constraints on and potential applications of the SQUID microscope.

Ultrafine-scale magnetostratigraphy of marine ferromanganese crust

Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years. Their secular records of chemical, mineralogical, and textural variations are archives of deep-sea environmental changes. However, environmental reconstruction requires reliable high-resolution age dating. Earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts, but have limitations on the millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-life. To overcome these limitations, we applied an alternative chronometric technique, magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) from the northwest Pacific. Submillimeter-scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high-resolution magnetometry technique called scanning SQUID (superconducting quantum interference device) microscopy. By correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that deduced from the 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study of a geologic sample to date. Ultrafine-scale magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale (1–40 k.y.).

High-resolution imaging of cardiac biomagnetic fields using a low-transition-temperature superconducting quantum interference device microscope

We have developed a multiloop low-temperature superconducting quantum interference device sensor with a field sensitivity of 450 fT/Hz−1/2 for imaging biomagnetic fields generated by action currents in cardiac tissue. The sensor has a diameter of 250 μm and can be brought to within 100 μm of a room-temperature sample. Magnetic fields generated by planar excitation waves are associated with a current component parallel to the wave front, in agreement with predictions of the bidomain model. Our findings provide a new basis for interpreting the magnetocardiogram.

LTS SQUID /spl radic/¿?? Microscopy: A Leap in sensitivity?.

Room-temperature (RT) sample scanning superconducting quantum interference device (SQUID) microscopy (SSM) is a very powerful and promising technique for imaging magnetic field distributions . A SQUID microscope with interchangeable sensor configurations for imaging magnetic fields of room-temperature (RT) samples with sub-millimeter resolution was developed. The sensors are mounted in the vacuum space of a cryostat and thermally anchored to the helium reservoir. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. Combining microfluidic based devices and SQUID microscopy allows us to identify a moving single micron sized ferromagnetic particles with a magnetic moment of 10-14 Am2 at a bandwidth of 2.5 KHz and a sensor to particle spacing of 95 mum. Although magnetic force microscopes (MFMs) have better moment sensitivities than SQUID microscopes, the current generation of SQUID microscopes have field sensitivities 105 times better than the MFM and do not suffer from the sample-instrument interactions that plague MFM-sensing of magnetically soft materials.