Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces

研究复合功能氧化物材料的电磁性能之间的相互作用对国际凝聚态物理学界和未来电子器件的发展具有重要意义。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，表明在两种钙钛矿氧化物--钛酸锶和铝酸镧--之间的界面上可以获得高流动性的二维电子气，这两种氧化物都是绝缘的。在这项工作中，氧化物是以脉冲激光沉积(PLD)的方式逐层生长的，并具有原子级监测和控制。这项工作将可编程逻辑器件的能力推向了一个新的水平。自那以后，其他研究人员已经发现了这种类型的界面在~300mK以下的磁有序的间接证据，并最近在~200mK的二维电子气中检测到了超导转变。这项工作对新一代电子设备的潜力是巨大的，但到目前为止，关于这种二维电子气的性质，界面附近氧空位的作用，特别是磁有序的性质及其与超导状态的关系，还有许多悬而未决的问题。在目前的工作中，我们旨在通过开发新的纳米级传感器和测量技术来解决和回答这些关键问题，以探测氧化物异质界面上含有二维电子气的小介面的直流和交流磁化。通过将二维电子气限制在~200 nm x 200 nm的小区域内，我们将最大限度地减少与氧化物薄膜中的缺陷有关的问题。这种界面被很好地埋藏在氧化物结构中，无法通过扫描隧道显微镜等表面技术进行探测。取而代之的是，我们将开发基于纳米超导量子干涉器件(SQUID)的传感器，这种器件是非常敏感的磁通量探测器。它包括一个由两个薄弱环节(约瑟夫森元件)隔开的非常小的超导薄膜回路，这两个环节由聚焦离子束(FIB)形成的一个非常窄的磁道(约150 nm宽)组成。我们将设计和优化这样的器件，使其在4.2K到~100mK的温度下工作，并将它们与氧化物结构集成在一起。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和交流磁化率测量的关键工具。通过将设备缩小到纳米级，我们将能够测量磁化作用的小得多的变化，并有足够的分辨率对我们的氧化物样品中预期的相对较少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.