Development of a nano-hall probe scanning microscope
Adam Cluett, Dorothee Petit, Sherri Johnstone, David Wood, Russell Cowburn
Sponsor: EPSRC Collaboration between Durham University and Imperial College, London
Virtually all micro- and nano- scale magnetic systems possess non-uniform magnetisation. Indeed, it is the presence of domains and micromagnetic configurations which creates much of the intrigue and potential in nanoscale magnetism. A means of visualising magnetisation distributions with nanometre scale resolution is essential both for fundamental investigations and for improving hard disk media, read-write heads and magnetic memory chips. A number of techniques are currently available, including: Lorentz Transmission Electron Microscopy, Scanning Electron Microscopy with Polarisation Analysis (SEMPA) and Magnetic Force Microscopy (MFM). These techniques each have an important role to play, but all have limitations, the most serious being the requirement for special sample preparation. This precludes many samples of interest to both physics and engineering, not least technologically important systems such as hard disk media and read-write heads. Of these microscopies, the most versatile and widely used is MFM. MFM is a variant on the well established scanned probe technique of Atomic Force Microscopy (AFM) and uses a sharp, magnetised tip to scan over a magnetic surface, detecting magnetic domains through the interaction of the magnetised tip with stray magnetic fields. The technique is already very successful, and could be expanded even further if the following three limitations could be addressed successfully:
The tip interacts strongly with the sample and can badly perturb it. The observed domain pattern is therefore liable to have artefacts induced in it from the microscopy itself. This is particularly true in industrially important soft magnetic materials.
The magnetic interaction with the tip extends past its sharp end, thus limiting spatial resolution to around 50nm and making it difficult to analyse quantitatively the microscopy images.
Thin in-plane magnetised samples, such as those used in the emerging technology of magnetic random access memory, do not generate enough stray field to be able to use MFM routinely. In this project 50nm Bismuth Hall probes have been manufactured and the interconnects and circuitry required to interface with a commercial AFM have been developed and chracterised.