Development of Wideband Scanning Superconducting Quantum Interference Device Susceptometers for Nanomagnetic Materials Research and Education

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Nanomagnetic devices for applications such as quantum computing and high-capacity memory cannot be developed without faster, more sensitive characterization tools to directly probe phenomena and material structures at the nanoscale. The objective of this work is to advance the capability of a non-invasive, high-sensitivity technique, scanning superconducting quantum interference device (SQUID) susceptometry, for the dynamic characterization of nanomagnetic materials, systems, and phenomena. The principal investigator achieves this objective through two innovations. First, spin sensitivity is improved from the current state of the art, a few thousand electron spins per Hz1/2 , to a few tens of spins per Hz1/2. This improvement is accomplished by reduction of pickup loop dimensions in a combined optical and electron-beam lithography process and by utilizing an ultra-low-noise dc SQUID process. Second, bandwidth is increased from the current state of the art, tens of kHz, to tens of MHz by the use of dc SQUID series array amplifiers as preamplifiers for the dc SQUID susceptometer. The devices are characterized for flux sensitivity and bandwidth at 300 mK and 20 mK. Operation at 300 mK is necessary for the e-beam defined Al pick-up loops to become superconducting, and operation at 20 mK is essential for the study of quantum decoherence mechanisms in electronic systems. Undergraduates, working with graduate students in the collaborator's laboratory, characterize the spin sensitivity of the devices and their performance under realistic scanning conditions by imaging individual cobalt nanomagnetic spheres of controlled diameters ranging from 3 nm to 10 nm and magnetic moments ranging from tens to tens of thousands of electron spins. Further, one SQUID susceptometer is used to image a second, identical device to quantitatively characterize the noise generated by the devices. The scientific emphasis of this project is to produce sensors that have applications in the area of nanoscale structures, novel phenomena, and quantum control and to use these sensors to image both static and dynamic properties of individual cobalt nanomagnets. Integration of a high-sensitivity, high-bandwidth SQUID susceptometer into a scanning platform significantly increases the number and kind of systems that can be studied, decreases the turn-around time, and increases the number of samples that can be studied in a single experiment. Thus, this project contributes to advances in nanoscale materials, devices and system architecture. An added benefit of this project is the educational benefits at both CU-Denver (PI's institution) and Stanford University (collaborator's institution) resulting from the collaboration between CU-Denver undergraduate students and Stanford graduate students.