Magnetic microscopy of metallic meteorites: probing the magnetic state of the early solar system
Authors: Harrison Richard , Bryson James , Nichols Claire , Herrero‐Albillos Julia , Kronast Florian , Einsle Josh , Midgley Paul 
Published: December 2016

Meteorites are fragments of asteroids. They represent the oldest and most primitive materials in the solar system – rubble left over after the planets formed over four and half billion years ago. Information about the magnetic state of asteroids during the early solar system can, in principle, be recovered from meteorites. The paleomagnetic potential of meteoritic metal has been overlooked in the past, due to the fact that they are mostly comprised of kamacite (a soft magnet that makes a notoriously poor paleomagnetic recorder). Recent research, however, has uncovered regions buried within the Widmanstätten pattern that can capture reliable records of magnetic activity on asteroid bodies
1
. This discovery, combined with the advent of high‐resolution magnetic imaging methods, has allowed us to decipher the magnetic signals encoded within meteoritic metal for the very first time
2,3
.

Our attention has focussed on the ‘cloudy zone’: a region just a few microns wide lying next to the kamacite lamellae that define the Widmanstätten pattern. The cloudy zone consists of a tightly packed array of nanoscale islands of tetrataenite, an ordered Fe
0.5
Ni
0.5
phase that forms by the diffusive rearrangement of Fe and Ni atoms into alternating layers during slow cooling. The high intrinsic coercivity (> 2 Tesla) of tetrataenite makes it an excellent permanent magnet. Each island is ~100 nm or less in diameter, and is uniformly magnetised in one of six crystallographically defined directions. The proportions of islands magnetised in each direction are biased by interaction with the magnetic field of the asteroid. This bias is detected using X‐ray photo‐emission electron microscopy (XPEEM). Magnetic contrast is obtained using circularly polarised X‐rays incident at a glancing angle to the surface, exploiting the resulting X‐ray magnetic circular dichroism (XMCD) signal to reveal intricate nanoscale domains within the cloudy zone (Fig. 1). Combined with extensive image simulations, quantitative information about the magnetic state of the asteroid can be extracted from the data.

In this talk I will present an overview of nanopaleomagnetic XPEEM studies we have performed on metal from a range of meteorites that have experienced very different cooling rates on their parent bodies, from the most slowly cooled (<< 1 K/Myr) mesosiderites to the most rapidly cooled (>> 1000 K/Myr) IVA. The cooling rate is shown to have a dramatic effect on the length scale of the developing microstructures, with correspondingly dramatic effects on the magnetic properties. In the most extreme cases, six zones of distinct magnetic response are resolved over a 5 µm field‐of‐view, each zone defined by the properties of the underlying nanostructures. Combined with new insights into the chemical and crystallographic nature of the cloudy zone, as revealed by EDX tomography, atom probe tomography and scanning precession electron diffraction, we are discovering ever more about the underlying physics of these unique materials, and beginning to piece together the processes that helped shape some of the earliest objects in the solar system.