NanoPaleoMagnetism

As we approach the Christmas break, some positive news from PhD student James Bryson, who is currently enjoying a post AGU trip to the Advanced Light Source in Berkeley, California. In collaboration with our colleague Simon Redfern, James is attempting to test out a new form of lensless X-ray imaging to view the nanomagnetic structure of Fe-Ni meteorites. This new method, called ‘ptychography‘ (pronounced tie-cog-ra-fee!) allows images to be formed that go far beyond the current spatial resolution limit of X-ray imaging methods (normally around 30 nm at best).

Like holography, ptychography measures the phase and amplitude of a scattered wave. Unlike holography, however, it does not require a reference wave – the object itself acts as its own diffracting reference. An area of illumination (which in the case of X-ray imaging at ALS is a 60 nm X-ray spot formed by a zone plate) is used to form a diffraction pattern. The sample is then rastered across the beam in 50 nm steps, so that each diffraction pattern is obtained from an area of the specimen which overlaps with an area of specimen previously illuminated. This introduces a large degree of redundancy in the data, which is used to solve the phase problem (i.e. to calculate the phase of the diffraction patterns). This lensless method creates aberration-free absolute phase images at a resolution determined by the effective numerical aperture dictated by the angle subtended by the detector at the object plane.

(explanation of the method adapted from Ultimate Microscopy)

Preliminary news from ALS is that images with resolution around 7 nm are being obtained! If true (we’ll have to wait to the new year for a full analysis) this will allow us to image directly the magnetisation of the island and matrix phases of the cloudy zone – an exciting new step for Nanopaleomagnetism!

After a number of adventures, our new Lakeshore/PMC AGM-VSM system is finally up and running! We are proud to present our very first FORC diagram, measured on a shard of fire obsidian from Glass Buttes, Oregon, using the AGM configuration. This material was chosen on the basis of previous electron microscopy work by Ma et al. (2007) that showed particles of pure magnetite with sizes just inside the pseudo single domain (PSD) range. This is confirmed by the FORC diagram, which shows classic PSD features! This is an extremely weak sample, magnetically, so this image demonstrates not only the high sensitivity of the AGM but also the excellent new VARIFORC (Egli, 2013) smoothing option of FORCinel v. 2.0.

Final preparations are underway for the AGU conference in San Francisco next week. We will be presenting the first major results of the nanopaleomagnetism project – a talk by James on the use of pioneering methods to study paleomagnetic signals in meteoritic Fe-Ni at sub-micron length scales using X-ray photoemission electron microscopy, and a poster by me outlining the results of our electron holography study of meteoritic Fe-Ni (recently accepted in EPSL). To get a sneak peak of the poster, visit our publications page.

UPDATE: This paper is now available online via open access. Head over to our publications page to download it!

A major milestone today as the first NanoPaleoMag paper is accepted for publication in Earth and Planetary Science Letters! The paper entitled “Nanomagnetic intergrowths in Fe-Ni meteoritic metal: the potential for time-resolved records of planetesimal dynamo fields” by Bryson, Church, Kasama and Harrison describes the first nanoscale magnetic images to be made of the “cloudy zones” within Fe-Ni meteorites. These thin zones are only a few microns wide but contain thousands of nanoscale islands of an ultra-hard magnetic mineral called tetrataenite. We show that this unique nanostructure has the potential to record useful paleomagnetic information about magnetic fields on meteorite parent bodies – information that could eventually lead to a better understanding of the conditions that prevailed on these bodies shortly after the birth of the Solar System. Tetrataenite forms when Fe and Ni atoms organise themselves into alternating layers during slow cooling (of the order of a few degrees per million years). If this process happens in the presence of a magnetic field then paleomagnetic information is encoded into the nanostructure. Due to the phenomenal magnetic stability of tetrataenite, the information can survive the subsequent 4.5 billion years intact, allowing us to examine it with nanopaleomagnetic methods, such as electron holography (see image).

Experimental (left) and Simulated (right) Electron Holography Images of the Cloudy Zone