Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology


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.


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Blois A (2017) Heat propagation models for superconducting nanobridges at millikelvin temperatures in Superconductor Science and Technology

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Patel T (2014) Investigating the Intrinsic Noise Limit of Dayem Bridge NanoSQUIDs in IEEE Transactions on Applied Superconductivity

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Romans E (2011) Noise Performance of Niobium Nano-SQUIDs in Applied Magnetic Fields in IEEE Transactions on Applied Superconductivity

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Rozhko S (2013) Study of Low-Frequency Noise Performance of Nanobridge-Based SQUIDs in External Magnetic Fields in IEEE Transactions on Applied Superconductivity

Description In this project we developed new types of nanoscale SQUIDs made from Ti/Au proximity bilayers. These are ultrasensitive magnetic flux sensors and incorporate thin film nanobridges as the weak links. The nanoscale size means that they have spin sensitivities approaching that required for single spin detection. However due to thermal effects, such devices made from conventional superconducting metals would be difficult to operate at miilikelvin temperatures which limits their application to many quantum measurements.We showed that for our devices the electrical properties were consistent with a theoretical model developed for heat flow in bilayers, and demonstrated that they enable magnetic measurements to be made on diamagnetic samples at system temperatures down to 60 mK.
Exploitation Route The devices/techniques developed could be used by both academic and non-academic researchers interested in the magnetic properties of nanoscale systems such as nanoparticles, biomolecules, or small solid state systems. The work was disseminated in journals, at major US, EU and UK conferences.

The work was exploited through interaction with colleagues at Imperial College, the project partner NPL and through a larger network of collaborators in the same field including PTB Berlin, Osaka University and CSIRO Australia.
Sectors Electronics

Description Materials Innovation Impact Acceleration Award (MIIIA)
Amount £40,000 (GBP)
Funding ID EP/K503745/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2014 
End 01/2015