Metrological measurements of small magnetic fields and moments
We have developed a range of experimental techniques for measurements and visualisation of small magnetic fields/moments. Our research is focused on:
Advanced and Quantitative nanoscale magnetic measurements
The ability to quantifiably measure nanoscale magnetic field distributions currently eludes the field of metrology. Industries, including: magnetic sensing, information technology, bio-medicine and consumer electronics, would benefit from traceable and reliable measurements of magnetic fields and flux densities on the nanoscale. Magnetic force microscopy (MFM) is a popular technique for nanoscale magnetic analysis due to its nanoscale spatial resolution and ease of use in standard conditions. Advanced nanoscale magnetic measurements by MFM, are conducted within the Nanomagnetism group using specialised scanning probe methods including magnetic scanning gate microscopy, in situ field MFM and electrostatically-compensated MFM. Quantitative measurements are routinely conducted at NPL by calibrating MFM probes from different methods; such as Graphene Hall sensors, and calibrated stray-field reference materials (including continuous films, patterned structures and current-carrying nanowires). Understanding the probe’s magnetic profile allows us to acquire quantitative magnetic measurements from unknown samples within a specified uncertainty budget.
Graphene Hall sensors for calibrating nano-objects
Calibration of individual nano-objects (including MFM probes, microbeads and nanoparticles) can be achieved in the Nanomagnetism group by using epitaxial graphene Hall sensors and scanning gate microscopy. The high sensitivity of Hall sensors makes them ideal for measuring such fields. From the catalogue of scanning probe techniques at our disposal, we can overcome common sources of errors associated with imaging current-biased devices in order to accurately determine stray magnetic fields at the nanoscale.
Metrology for Topological spin structures
Smaller, faster, and more efficient electronic devices are a vital part of Europe’s economic growth and industrial innovation. Spintronics, which uses a fundamental property – intrinsic spin – of electrons to process information in a way that is analogous to charge in traditional electronics, holds potential to meet such aims. Researchers have begun exploring materials that have geometric structure which protect the configuration of electrons’ spin. These are known as topologically-protected spin structures (TSS). However, easily accessible laboratory based tools which can differentiate between such a TSS and a trivial spin state are still missing. Our group is uses quantitative magnetic force microscopy as a means to differentiate trivial spin structures from TSS. This project aims to progress TSS towards standardisation and support Europe’s continuing expertise and competitiveness in electronic device manufacturing.
Standardisation of magnetic nanoparticle manufacturing and applications
We are working to improve standardisation within the field of magnetic nanoparticles (MNP) manufacturing and measurement. In a European collaborative project in association with other NMIs, SMEs and academies we aim to draft international standards to guide the industry and to increase the level of harmonisation between laboratories conducting new or rare measurements techniques. Furthermore, we are involved in the work of technical committee ISO/TC 229 contributing to the drafting of a new ISO Technical Specification (ISO 19807) on the characterisation techniques for suspensions of magnetic nanoparticles.
Single magnetic nanoparticle characterisation
Significant developments in MNPs has primed them for biomedical applications. In particular, the possibility to manipulate and detect individual MNPs is of great interest due to their potential for Lab-On-a-Chip applications. NPL's Nanomagnetism Group works towards single MNP characterization and detection. NPL’s ability to isolate MNPs to characterise their intrinsic magnetic properties by e.g. magnetic scanning gate microscopy has allowed NPL to test a new generation of single MNP detectors. These devices, based on magnetic domain wall (DW) dynamics, have the capability of acting both as a sensor and as a means to manipulate MNPs.
Domain wall sensing
Individual DW manipulation has the potential to enable a wide range of new devices. From logic circuits that use DWs for information transfer, requiring only a fraction of the energy current computers use; to chips to manipulate individual MNPs and perform biological analysis such drug testing or cell manipulation. The Nanomagnetism Group combines electromagnetic manipulation/sensing of DWs with MFM studies to help develop DW-based technologies. Our electromagnetic laboratory enables measurement of electrical effects associated with changes in magnetization (e.g. magnetoresistance, anomalous Hall effect, anomalous Nernst effect, etc.), individual DW detection, and electrical manipulation of DW using current pulses. The MFM studies, which can be customised to allow simultaneous electrical measurements, has enabled imaging of DW dynamics including DW movement through electrically driven spin torque.