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

Abstract

We have developed a scanning superconducting quantum interference device ͑SQUID͒ microscope system with interchangeable sensor configurations for imaging magnetic fields of room-temperature ͑RT͒ samples with submillimeter resolution. The low-critical-temperature ͑T c ͒ niobium-based monolithic SQUID sensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25 m 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 100 m, 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-T c niobium bare SQUID sensor, with an effective diameter of 80 m, and achieved a field sensitivity of 1.5 pT/ Hz 1/2 and a magnetic moment sensitivity of 5.4ϫ 10 −18 A m 2 /Hz 1/2 at a sensor-to-sample spacing of 100 m in the white noise region for frequencies above 100 Hz. Imaging action currents in cardiac tissue requires a higher field sensitivity, which can only be achieved by compromising spatial resolution. We developed a monolithic low-T c niobium multiloop SQUID sensor, with sensor sizes ranging from 250 m to 1 mm, and achieved sensitivities of 480-180 fT/ Hz 1/2 in the white noise region for frequencies above 100 Hz, respectively. For all sensor configurations, the spatial resolution was comparable to the effective diameter and limited by the sensor-to-sample spacing. Spatial registration allowed us to compare high-resolution images of magnetic fields associated with action currents and optical recordings of transmembrane potentials to study the bidomain nature of cardiac tissue or to match petrography to magnetic field maps in thin sections of geological samples.

Scanning SQUID microscope system for geological samples: system integration and initial evaluation

“…A rigid GFRP shaft that passes through the hollow center directly connects a micrometer spindle attached to the top flange of the cryostat to a copper rod placed beneath the He reservoir via thermal anchors. Instead, Fong et al (2005) used a lever mechanism for sensor-to-sample distance adjustment, which is manipulated from outside of the cryostat. A SQUID chip is mounted on the conical top of a sapphire rod, which is tightly connected to the copper rod.…”

“…Basically, current collection of scanning SQUID microscopes used for paleomagnetic studies on geological samples were developed based on the model described in Fong et al (2005). In order to realize extensive application of ultrafine-scale magnetostratigraphy of geological samples such as ferromanganese crusts for dating with a scanning SQUID microscope and pursue the possibility to improve for better resolution, sensitivity, convenience, and maintainability, we have decided to construct a scanning SQUID microscope with some new ideas.…”

“…It has been used increasingly in the study of meteorites (Weiss et al 2000;Fu et al 2014), volcanic rocks (Weiss et al 2007b), impacted rocks using laboratory laser experiments (Gattacceca et al 2006), and sulfur-bearing metamorphic rocks (Fischer et al 2014). Different techniques use different magnetic sensors, including the SQUID microscope (e.g., Fong et al 2005;Weiss et al 2007a, b), the magnetic tunnel junction microscope (e.g., Lima et al 2014), and the magnetic microscope with nitrogen vacancy (NV) quantum diamond (Fu et al 2014). SQUID microscopy is a powerful technique for imaging weak magnetic field distributions with the highest field sensitivity.…”

“…A SQUID microscope allows samples of about 100 µm to be scanned at room temperature (Kirtley and Wikswo 1999;Chatraphorn et al 2000;Ono and Ishiyama 2004;Fong et al 2005). An important use of this technique is the study of geological samples (Fong et al 2005;Baudenbacher et al 2002Baudenbacher et al , 2003Wang et al 2014;Weiss et al 2000Weiss et al , 2007aOda et al 2011). In a recent work, SQUID microscope made it possible to use magnetostratigraphic dating as a way to image ultrafine-scale magnetic stripes related to geomagnetic reversals preserved in ferromanganese crust samples (Oda et al 2011).…”

Fast inversion of magnetic field maps of unidirectional planar geological magnetization

“…[14] This is a realistic model for SMM, in which doubly polished 30 mm thin sections of geological samples are typically measured at distances greater than three times the sample thickness [Fong et al, 2005;Weiss et al, 2007a]. We assume that the magnetic field is measured on the plane z = h parallel to a sample located on the x-y horizontal plane, where h is the sensor-to-sample distance or liftoff (Figure 1).…”