Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces

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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.