NanoPaleoMagnetism

We are pleased to announce the publication of our latest open access paper in Earth and Planetary Science Letters:

Bryson J, Herrero-Albillos J, Kronast F, Ghidini M, Redfern SAT, van der Laan G, and Harrison RJ (2014) Nanopaleomagnetism of meteoritic Fe–Ni studied using X-ray photoemission electron microscopy. Earth and Planetary Science Letters 396, 125–133.

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This paper is the second in our series on the Tazewell IIICD Fe-Ni meteorite. Our first paper (open access download) described the fundamental chemical and magnetic structure of the cloudy zone, as revealed by electron holography, and put forward the idea that cloudy zones could contain a continuous, time-resolved record of the magnetic field generated by asteroidal dynamos. Our latest paper takes a giant step towards this goal by demonstrating that nanoscale paleomagnetic information can be extracted from the cloudy zone using X-ray Photoemission Electron Microscopy (XPEEM). Our results reveal in unprecedented detail the magnetic structure of the Widmanstätten sequence (kamacite, tetrataenite rim, cloudy zone, plessite). Furthermore, we demonstrate a methodology for performing quantitative paleomagnetic analysis of XPEEM images, which shows the way forward for future paleomagnetic studies of Fe-Ni within meteorites (e.g. pallasites) that may have experienced a dynamo field on their parent body.

XPEEM works by illuminating a polished surface with a beam of circularly polarised X-rays, tuned to the absorption edge of the element of interest (Fe L2,3 in this case). Photo electrons emitted from the surface are accelerated through a series of lenses to form an image of the sample with 30-40 nm resolution over a field-of-view of 5-20 µm. Magnetic contrast in the image is caused by XMCD (differential absorption of left versus right circularly polarised light), which produces a signal that is directly proportional to the magnetisation of the sample projected along the X-ray beam direction.

Schematic illustration of the XPEEM method.

The images below show the complex nanoscale magnetic structure associated with the cloudy zone – a spinodal intergrowth of magnetically hard (FeNi) and soft (Fe3Ni) phases. Regions of red and blue correspond to islands of FeNi magnetised along the three possible <100> easy axis directions. The dashed boxes in (b) and (c) illustrate individual < 1µm wide regions of interest that can be analysed quantitatively to extract the proportions of islands along each axis. In this case, the easy axes were shown to be equally populated, consistent with the formation of the cloudy zone in zero field (the  IIICD meteorites are likely to have originated from the core or deep mantle of the parent body, and so any dynamo field would have long switched off by the time the metal cooled to the cloudy zone formation temperature).

The door is now open to performing nanopaleomagnetism of other Fe-Ni bearing meteorites. Watch this space!

XPEEM Reveals the Detailed Nanomagnetic Structure of the Widmanstätten microstructure in the Tazewell meteorite.

Our MTJ microscope – the centrepiece of the nanopaleomagnetism project – recently arrived from Micro Magnetics in two large crates. Before the instrument is installed inside our shielded room at the beginning of April, CEO of Micro Magnetics Gang Xiao sent us these test images demonstrating the capabilities of the instrument. The first image is an optical image showing a ~ 100 micrometer sized grain of synthetic ‘dusty olivine’ embedded in epoxy. Dusty olivine contains thousands of sub micron particle of metallic iron, and is thought to be a potential carrier of pre-accretionary remanence in chondritic meteorites. The second image shows the Z component of magnetic field emanating from the grain. The scan was performed over 230*200 (X-Y) microns using the “vibrating” mode, whereby the sample is vibrated in the Z-direction in order to increase the sensitivity and eliminate sensor artifacts. Vibrating frequency was 135 Hz. Lock-in sensitivity was set at 500 micoVolt with a time constant of 1 second. Scanning speed was 1 micron/sec. The magnitude of the signal measured here is of the order of 250 nT, with noise levels indicating sensitivity of the order of a few nT. Internal details of the magnetisation pattern are clearly visible, with spatial resolution on the order of 10 microns. The results demonstrate exactly the capabilities we require for nanopaleomagnetism – the ability to measure remanent fields from localised regions of interest, and distinguish them effectively from adjacent areas.

As soon as the microscope is up and running, we’ll be looking for interesting projects to work on. If you think your work could benefit from this kind of measurement, please get in touch!

Back in 2002, I was part of a team that proposed a radically new mechanism for generating magnetic remanence in the ilmenite-hematite system (Robinson et al. 2002). The so-called “lamellar magnetism” hypothesis was put forward to explain the unusually strong and stable magnetisation of slowly-cooled rocks containing intergrowths of ilmenite and hematite. The most puzzling feature of the rocks was that they were much more magnetic than expected. Hematite is an antiferromagnet, which means its magnetic structure consists of  two sublattices of opposing magnetic spins. In a perfect antiferromagnet, the opposing spins cancel each other out exactly, leading to zero net magnetic moment. In hematite, however, a slight misalignment (or canting) of its spins leads to a very weak net magnetic moment that points in a direction perpendicular to the sublattice spins (red arrow in figure below). Ilmenite is a paramagnet, which means its magnetic spins are thermally disordered and cannot contribute at all to the observed magnetisation. This mixture of weakly magnetic and non-magnetic minerals left us with the paradox of a rock who’s magnetisation was far greater than the sum of its parts!

Bright-field transmission electron micrograph of a natural intergrowth of nanometer-scale hematite precipitates (bright lines surrounded by dark shadows) within a matrix of ilmenite. A different crystal from the same sample was used for the neutron diffraction work.

The key to unlocking the mystery of the excess magnetisation was the realisation that the intergrowths of ilmenite and hematite occurred on extremely fine length scales. Transmission electron microscopy revealed that lamellae of ilmenite in hematite (and vice versa) continued right down to the unit cell scale (~ 1 nm). A key principle in nanotechnology is that the properties of materials can change fundamentally at the nanometre scale, partly due to the increasing dominance of surfaces and interfaces. Our hypothesis was that the excess magnetic moment originated from a local imbalance of magnetic moments at the interfaces between nanoscale ilmenite and hematite lamellae (blue arrow in figure below). This excess lamellar moment can be thought of as a form of  “defect” moment that is generated every time a nanoscale precipitate is placed inside its host. Crucially, this defect moment was predicted to be oriented parallel to the sublattice magnetisation of hematite, in contrast to the perpendicular orientation of a canted moment.

This prediction lead to a simple experimental test that could prove or disprove the lamellar magnetism hypothesis: take a single crystal sample containing a fine-scale intergrowth of ilmenite and hematite, apply a saturating magnetic field (i.e. one that is large enough to cause the net moment throughout the entire sample to align with the field) and measure the corresponding orientation of the sublattice spins with respect to the field. There are three possible outcomes of the experiment: 1) if the sublattices adopt an orientation parallel to the field then the net moment is generated by lamellar magnetism alone, 2) if the sublattices adopt an orientation perpendicular to the field than the net moment is generated by canting alone, 3) if the sublattices adopt an intermediate orientation, then both lamellar magnetism and canting contribute to the net moment.

By measuring the angle between the sublattice spins (black) and the net magnetisation (green) in a saturating field, the presence of both a canted moment (red) and lamellar moment (blue) can be detected.

Our paper published this week in Physical Review B used polarised neutron scattering to perform this test at last. Neutrons are a sensitive probe of magnetism since they themselves possess a spin, causing them to be scattered by the magnetic moments of the atoms in a sample. If the spin of an incoming beam of neutrons is oriented perpendicular to the atomic moments, then the scattered beam of neutrons will have its spin direction reversed (or ‘flipped’). If the spin of the incoming beam is oriented parallel to the atomic moments in the sample, then the scattered neutrons will keep their original spin direction. By measuring the relative intensity of the ‘spin flip’ versus ‘non spin flip’ signals, the orientation of the sublattice spins can be deduced. The results presented by Brok et al. (2014) demonstrate that the sublattices adopt an average angle of ~ 60° to a saturating field (i.e. option 3 of the possible outcomes described above). This simple (yet extremely hard-won) result is the most direct demonstration yet that lamellar magnetism contributes significantly to the magnetic properties of ilmenite-hematite intergrowths, and a fantastic example of nanopaleomagnetism in action.

Richard Harrsion

Robinson, P., Harrison, R.J., McEnroe, S.A., and Hargraves, R.B. (2002) Lamellar magnetism in the haematite-ilmenite series as an explanation for strong remanent magnetization. Nature, 418, 517–520.

Brok E, Sales M, Lefmann K, Theil Kuhn L, Schmidt WF, Roessli B, Robinson P, McEnroe SA and Harrison RJ (2014) Experimental evidence for lamellar magnetism in hemo-ilmenite by polarized neutron scattering. Physical Review B, 89, 054430.

Last week was a busy one for the group, as Nono and I headed over to the U.S.A. to visit two of the companies providing key equipment for the NanoPaleoMag lab. First, I went to Columbus, Ohio to visit the headquarters of Lakeshore Cryotronics Inc., builders of our recently installed VSM/AGM system. There I gave a talk about the application of FORC diagrams to rock magnetism, and discussed the possibility of developing our FORCinel software for current and future systems. The rock magnetic community is a new one for Lakeshore, and it is great to see their commitment to continuing to support our needs with future generations of their VSM/AGM system. This year’s IRM meeting in Santa Fe will include a one day workshop on FORC diagrams, which Lakeshore will attend. This will be an opportunity for the community to discuss with them future developments in their systems. For a copy of the FORC presentation see our Publications page.

Next we spent two days in Boston visiting Micro Magnetics, who are building the centrepiece of the NanoPaleoMag lab – a scanning magnetic microscope that will enable us to make highly localised remanence measurements in specific regions of interest. The microscope uses a Magneto Tunnel Junction (MTJ) sensor that is capable of measuring nanoTesla fields with a spatial resolution of around 10 µm. Our highly customised version of their CS-1000 microscope is optimised for the measurement of static magnetic fields above polished rock samples and then correlating them with high magnification optical images. Our visit gave us the chance to see the system operating for the first time and get some basic training before the unit ships at the end of the month. Our preliminary measurements left us in no doubt of the potential of this instrument to revolutionise the way we measure remanence at local levels in challenging samples. The images below show Nono together with MicroMagnetics CEO Gang Xiao and Chief Engineer Matt Carter admiring the first measurements of the magnetic field around a ~ 200 µm grain of synthetic dusty olivine. Based on these first results, the outlook for future NPM research is looking very positive. We can’t wait to get started!

I have posted a minor update to FORCinel (Version 2.01). This has added one useful feature: VARIFORC now remembers the last set of smoothing parameters used. These are stored in the wave “last_sf_used”. When choosing “change smoothing factor”, the previously used parameters are automatically entered into the dialogue box, making it easier to make adjustments to the smoothing factors. Don’t forget, you can leave comments on the FORCinel page if you have any comments/feature requests.

Rich

On January 8th and 9th 2014 the NPM group hosted the Magnetic Interactions 2014 meeting – the annual gathering of researchers from the UK (and beyond) working on all aspects of magnetism in the Earth sciences. Several members of our group, including 4th year undergraduates Claire Nichols and Roberts Blukis, PhD student James Bryson, post doc Nono Lascu, and even the PI presented talks and posters on our recent research (see publications to download PDFs of some of these). Click here for a full summary of the meeting.

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.