FridayAFM - The final frontier

Written by Héctor Corte-León | Dec 1, 2023 7:30:00 AM

Héctor here, your AFM expert at Nanosurf calling out for people to share their Friday afternoon experiments. Today I'll show you how to combine AFM with astrogeology.

Nanosurf has a long tradition with space (not meaning the x y z space, I mean the actual SPACE with big letters, the outer space). Back in 2008, the Phoenix spacecraft landed on Mars equipped with an AFM built by Nanosurf.

The AFM was equipped with 8 AFM probes and a rotating sample stage that allowed exposing a substrate to the martian dust. This allowed capturing the first AFM images on another planet, and was of vital importance for future Mars missions, as dust, its size, its shape and composition can degrade equipment, and the precautions taken to avoid it vary depending on the type of dust. For instance, there are tales about the "dust problem" the astronauts faced when walking on the moon: "After each outdoor mission, the astronauts’ suits had significant dust deposits in the outer material that could not be brushed off. Astronaut Harrison Schmitt reported that the inside of the Apollo 17 Lunar Module was temporarily “full of dust”, including the atmosphere that the astronauts breathed.". Including its adverse effects on human health: "In 1972, Harrison Schmitt suffered a brief sneezing attack, red eyes, an itchy throat, and congested sinuses in response to lunar dust." See Ref 1 for more information.

So, I thought myself, what a better way to honour that tradition than doing a space-themed fridayAFM? This is why I went shopping and got myself some meteorites (or so they claim).

The thing about meteorites is that they are very well cataloged, and each has a serial number that allows looking for them. The problem, mine don't have that serial number, they do have another serial number belonging to the Space Research Corporation. Unfortunately, this company closed long time ago and is not possible to access their records anymore. So, where do my meteorites come from? If I check the meteorite database, there isn't anything for Namibia/Botswana, but there is one entry for Gibeon.

According to this entry, my piece of the meteorite should have a large content of iron, so a quick test is to see if my samples are magnetic can already tell me if it is the same meteorite. As you can see below, the magnet attracts the whole plastic container as the meteorite pieces are fixed inside, so indeed my meteorites are magnetic, and that at least seems to be correct (usually meteorites have a high iron content, so a non magnetic meteorite should at the very least raise concerns).

Now, next step is how to mount them for AFM imaging? With such small pieces, any type of glue might contaminate the surface, and we want something that can be easily removed. The answer? Clock-makers clay, or clock-makers dough, a special paste that can hold small pieces without leaving residues.

By the way, in case you were wondering, here is the full list of meteorites from the 2001 advertisement campaign from Nestle. So... found on 1965, distributed in 2001, and AFM-imaged in 2023. With 58 years since its discovery, so far, is my "oldest" sample (anybody is interested in doing AFM on dinosaur fossil?).

Also, in case you were also wondering why the sample holder with such weird shape, it is so I can mounted in our variable magnetic field system and remains fixed while varying the applied magnetic field. It looks something like this.

Now that the samples are mounted, how they look like? Can we be sure they are meteorites? Well, let's first see the Botswana sample. This is the topography.

I think that the structures we see here are NiFe dendrites, more precisely, I think they are plessites, which are the tiny dendrites seen in between the larger "lamellae" in Widmanstätten patterns. Widmanstätten patterns are typical of metallic meteorites because of the way the metal has solidified, and are very rare in earth formations (maybe not even occurring naturally). You can read more about these small dendrites on meteorites here, in relation with an article called "SpacePearls") So, I'm confident that this is really part of a meteorite (cool!).

Now, what else can we learn from this sample? Can we do magnetic force microscopy for instance? Usually meteorites are made of magnetic materials, so maybe there is something interesting hidden underneath the surface.

MFM is prone to have topography and electrical cross-talk, that is why when analyzing phase images from standard MFM, one should be extremely careful and avoiding jumping to conclusions when the feature resembles the topography. In this case, even with a lift height of 600 nm during the second pass, we still see a lot of topography cross-talk. This is likely due to the high topography, or even trapped charges (some of the material is oxidized and non conductive). However, there are some features appearing in flat areas, these can be trusted. The fact that these features usually come as bright/dark pairs means that likely they belong to small objects that don't extend too much in the material. Overall, we see large domains, larger than the scan range, and tiny domains usually appearing in bright/dark pairs. All this combined with the fact that there is no clear topography indicating a different material, suggest to me that we are seeing inclusions of a different material (or phase), with high magnetic moment, that are prone to oxidize in air and lose their magnetic properties (and thus not present at the surface). So maybe a different ratio of Ni/Fe than the one we see at the surface.

What about the Gibeon sample?

In this case I couldn't see the dendritic formations, which is suspicious. I can think of three different possibilities to explain this:

Dendrites were damaged during the cutting procedure. If the cutting overheated the sample, it could have melted the surface.
Although both samples are magnetic, maybe their composition or history is quite different and there are no dendrites in this sample.
The dendrites seen on the other sample are not part of the Widmanstätten pattern and are the result of contamination (e.g. some bacterial residue).

Also, of course, there is the possibility that this sample is not from a meteorite.

In any case, we can discuss the magnetic behaviour, and if it turns out to be a meteorite, all the best. From the MFM images taken at different fields it is possible to see changes, indicating that indeed what we see is due to magnetic interaction. However, in this case it is not possible to see dark/bright pairs (or at least they are not very common), this can indicate that the magnetic material with high magnetic moment is not concentrated in nodules but rather distributed uniformly. In terms of 3D distribution, this magnetic material again is not seen in the surface, because the phase images are rather diffuse without defined edges that can indicate the presence of domain walls or pinning points.

If you are interested in doing variable magnetic field measurements, you can have a look at our application notes:

Study of Magnetization in Artificial Spin Ice at Variable Magnetic Fields

Magnetization of electrical steel in external magnetic fields

Let's recap. I couldn't find a lot of literature on AFM done on meteorites (a part from single molecule detection as in Ref 2 or topography as in Ref 3), likely because the surface tends to be quite rough, and only cutted faces could be smooth enough for AFM (canceling the main advantage of the AFM, characterizing surfaces). However, as seen here, even on cutted surfaces, MFM can be of interest because of its capabilities to see underneath the surface. Here we were able to see that underneath the surface there are aggregations (i.e. "nodules"), of material with high magnetic moment, and what is more important, we were able to see that images from two different samples look different, thus allowing MFM to identify/classify meteorites or even studying their magnetic history (e.g. using a domain wall, by cycling an external field identify how much field is needed to un-pin the domain wall, and thus providing hints of how much field the meteorite saw during its voyage through space or during its formation).

I hope you find it useful, entertaining, and try it yourselves. Please let me know if you use some of this, and as usual, if you have suggestions or requests, don't hesitate to contact me.