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

如果没有更快、更灵敏的表征工具来直接探测纳米尺度的现象和材料结构，就无法开发用于量子计算和大容量存储器等应用的纳米磁器件。这项工作的目标是推进一种非侵入性、高灵敏度技术的能力，即扫描超导量子干涉器件（SQUID）电纳测量，用于纳米磁性材料、系统和现象的动态表征。首席研究员通过两项创新实现了这一目标。首先，自旋灵敏度从目前的技术水平（每Hz1/2有几千个电子自旋）提高到每Hz1/2有几十个自旋。这一改进是通过减少光学和电子束组合光刻工艺中的拾取环路尺寸以及利用超低噪声直流SQUID工艺来实现的。其次，通过使用直流SQUID系列阵列放大器作为直流SQUID电纳计的前置放大器，将带宽从目前最先进的数十kHz提高到数十MHz。该器件具有300 mK和20 mK的通量灵敏度和带宽。300 mK的工作是电子束定义的Al拾取回路成为超导所必需的，而20 mK的工作对于研究电子系统中的量子退相干机制是必不可少的。本科生和研究生在合作实验室工作，通过成像单个钴纳米磁球，表征设备的自旋灵敏度及其在实际扫描条件下的性能，这些球体的直径范围为3纳米至10纳米，磁矩范围为数十到数万个电子自旋。此外，使用一个SQUID电纳计对第二个相同的设备成像，以定量表征设备产生的噪声。该项目的科学重点是生产在纳米级结构、新现象和量子控制领域应用的传感器，并使用这些传感器对单个钴纳米磁铁的静态和动态特性进行成像。将高灵敏度、高带宽SQUID电纳计集成到扫描平台中，可以显著增加可研究系统的数量和种类，减少周转时间，并增加单次实验可研究的样品数量。因此，该项目有助于纳米材料、器件和系统架构的进步。这个项目的另一个好处是丹佛大学（PI所在机构）和斯坦福大学（合作机构）的教育效益，这是丹佛大学本科生和斯坦福大学研究生之间合作的结果。

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.