Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces

研究复杂功能氧化物材料的电性能和磁性能之间的相互作用对国际凝聚态物理界和电子器件的未来发展至关重要。最近，东京大学和康奈尔大学的开创性工作点燃了这一领域，这些工作表明，有可能在两种钙钛矿氧化物SrTiO 3和LaAlO 3之间的界面处获得高度移动的二维电子气，这两种氧化物都是绝缘的。在该工作中，氧化物通过具有原子级监测和控制的脉冲激光沉积（PLD）以逐层方式生长。这项工作将PLD的能力推向了一个新的水平。其他研究人员已经发现了在低于~300 mK的这种类型的界面处磁有序的间接证据，并且最近在~ 200 mK的二维电子气中检测到超导转变。这项工作对于新一代电子器件的潜力是巨大的，但到目前为止，关于这种二维电子气的性质，靠近界面的氧空位的作用，特别是磁有序的性质以及它与超导态的关系，还有许多尚未解决的问题。在目前的工作中，我们的目标是解决和回答这些关键问题，通过开发新的纳米级传感器和测量技术，探测直流和交流磁化的小台面包含一个二维的电子气在氧化物异质界面。通过将二维电子气限制在约200 nm x 200 nm的小区域内，我们将最大限度地减少与氧化膜中的缺陷相关的问题。这个界面被埋在氧化物结构内部，不能通过表面技术如扫描隧道显微镜进行探测。相反，我们将开发基于纳米级超导量子干涉器件（SQUID）的传感器，这些器件是非常灵敏的磁通量探测器。这些由一个非常小的超导薄膜环组成，被两个弱连接（约瑟夫森元件）中断，弱连接由聚焦离子束（FIB）制成的非常窄的轨道（~150 nm宽）组成。我们将设计和优化这样的器件，使其在4.2K到~ 100 mK的温度下工作，并将其与氧化物结构集成。基于SQUID的仪器是许多实验室对含有大量磁矩的宏观样品进行直流磁化强度和交流磁化率测量的关键工具。通过将设备缩小到纳米尺度，我们将能够测量磁化强度的更小变化，并具有足够的分辨率，以便对我们的氧化物样品中预期的相对少量的磁偶极矩进行有用的测量。

The study of the interplay between the electronic and magnetic properties of complex functional oxide materials is of central importance to the international condensed matter physics community, and for the future development of electronic devices. Recently this field has been set alight by pioneering work at Tokyo and Cornell Universities that showed it is possible to obtain a highly mobile two dimensional electron gas at the interface between two perovskite oxides, SrTiO3 and LaAlO3, both of which are insulating. In that work the oxides were grown in a layer-by-layer manner by pulsed laser deposition (PLD) with atomic level monitoring and control. The work has pushed the capabilities of PLD to a new level. Other researchers have since found indirect evidence for magnetic ordering at this type of interface below ~300 mK and have recently detected a superconducting transition in the two dimensional electron gas at ~200mK. The potential of this work for a new generation of electronic devices is enormous, but so far there are many unresolved issues about the nature of this two-dimensional electron gas, the role of oxygen vacancies close to the interface, and especially the nature of the magnetic ordering and how it relates to the superconducting state. In the present work we aim to address and answer these key questions by developing new nano-scale sensors and measurement techniques to probe the dc and ac magnetisation of small mesas containing a two dimensional electron gas at an oxide heterointerface. By confining the two dimensional electron gas to a small area ~ 200nm x 200 nm we will minimise issues relating to defects in oxide films. This interface is buried well inside the oxide structure and cannot be probed by surface techniques such as scanning tunnelling microscopy. Instead we will develop sensors based on nano-scale superconducting quantum interference devices (SQUIDs) that are very sensitive detectors of magnetic flux. These consists of a very small loop of superconducting thin film interrupted by two weak links (Josephson elements) which consist of a very narrow track (~150 nm wide) made by a focussed ion beam (FIB). We will design and optimise such devices to operate at temperatures from 4.2K down to ~ 100mK, and integrate them with oxide structures. SQUID-based instruments are the key tool in many laboratories for performing dc magnetisation and ac susceptibility measurements on macroscopic samples containing a very large number of magnetic moments. By shrinking the devices to the nano-scale we will be able to measure much smaller changes in magnetisation and have sufficient resolution to make useful measurements on the relatively small number of magnetic dipole moments expected in our oxide samples.