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级的现象和材料结构，就无法开发用于量子计算和高容量存储器等应用的纳米磁性器件。这项工作的目的是提高能力的非侵入性的，高灵敏度的技术，扫描超导量子干涉仪（SQUID）的磁测量，纳米磁性材料，系统和现象的动态表征。主要研究者通过两项创新实现了这一目标。首先，自旋灵敏度从现有技术的每Hz 1/2几千个电子自旋提高到每Hz 1/2几十个自旋。这一改进是通过减少拾取环的尺寸在一个组合的光学和电子束光刻过程中，并通过利用超低噪声直流SQUID过程。其次，通过使用直流SQUID系列阵列放大器作为直流SQUID磁强计的前置放大器，带宽从当前技术水平的几十kHz增加到几十MHz。 该设备的特点是通量灵敏度和带宽在300 mK和20 mK。在300 mK的操作是必要的电子束定义的铝拾取回路成为超导，在20 mK的操作是必不可少的量子退相干机制的研究在电子系统中。本科生与合作者实验室的研究生一起工作，通过对单个钴纳米磁性球体进行成像来表征设备的自旋灵敏度及其在现实扫描条件下的性能，这些纳米磁性球体的可控直径范围为3 nm至10 nm，磁矩范围为数十至数万电子自旋。此外，使用一个SQUID辐射计对第二个相同的器件进行成像，以定量表征器件产生的噪声。该项目的科学重点是生产在纳米结构，新现象和量子控制领域应用的传感器，并使用这些传感器来成像单个钴纳米磁体的静态和动态特性。将高灵敏度、高带宽的SQUID温度计集成到扫描平台中，可显著增加可研究系统的数量和种类，减少周转时间，并增加单次实验中可研究的样品数量。因此，该项目有助于纳米材料，器件和系统架构的进步。该项目的另一个好处是，丹佛大学（PI的机构）和斯坦福大学（合作者的机构）的教育效益来自丹佛大学本科生和斯坦福大学研究生之间的合作。

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.