Magnetic properties of planets

Materials World magazine
28 Oct 2019

Institut Laue-Langevin D20 Instrument Specialist, Thomas Hansen, shares how research into iron could help us understand the magnetic properties of planets.

Scientists have explored the magnetic behaviour of iron by reaching high pressures and low temperatures for neutron diffraction at 25 GPa. The findings were published in the paper, Epsilon iron as a spin-smectic state, at Proceedings of the National Academy of Sciences, in September 2019.

Researchers have struggled to reach more than 10GPa of pressure in these experimental environments, as the small surface area of neutrons makes it hard to reach high pressures while probing into the material.

Observing how iron maintains a stable state in such intense environments could help us better understand what happens in the cores of small rocky planets. The centres of these planets have high pressures and, unlike Earth, are cool at their cores.

In the experiment, scientists did not find total magnetic disorder unlike spin liquids.

Different directions were observed, but the orientation in this new state was preserved and stable. Scientists are now keen to understand why this configuration exists. Hansen, one of the scientists involved in the research, shares his research.

What do you research and how does it affect space?

I’m a chemist at the Institut Laue–Langevin, France, and I do neutron diffraction. One part is about magnetism, one part is about structure, texture, and micro structure of materials, particularly crystalline materials, but sometimes amorphous materials, physo-soft materials and so on.

Sometimes it is important to know the structure of the material to know the behaviour, or the other way round in order to understand why things happen in specific ways, for example on other planets.

Let’s take Saturn’s moon Titan, where there are different temperatures and pressures on the celestial bodies. How does magnetism tectonic movements work with a different set of materials as compared to Earth?

Tell me about your particular instrument and its purpose.

My particular instrument is a neutron powder deflectometre. When people use powder diffraction to solve their structured chemical problems to understand the magnetic structure, they may come to a point where they cannot go further with laboratory methods and they need neutrons for some specific question, when it is about magnetism. Neutrons are very good for the structure and alignment in a magnetic spin.

In this test, how did you analyse the iron sample?

We have this sphere of iron and put it in an envelope of titanium zirconium. We squeezed it between the centre diamond anvils of a Paris-Edinburgh press and we pushed on it with 1,500 bars of helium on both sides. The whole thing sat in a cryo-stat and then we cooled it to liquid nitrogen temperature. We cooled it down further to about 5K. We then poured helium on it and pumped on the helium lake, a bit beneath the sample, in order to get the temperature even lower to 1.8K.

In the meantime, we were performing neutron powder dissection, and we looked in particular at the neutrons derived by the diffraction of the sample. While this was cooling and was staying cold, we were counting in order to get enough statistics. They are particularly important if you have a zero result.

If you have such a result, sufficient statistics are needed to confirm this as if you look for a few seconds, you might not see anything.

If you count for hours, you have enough numbers to say, ‘no, there is no Bragg peak, which would be an indication for long-range magnetic order. There is simply nothing. Or the peaks I see come either from the diamond anvil, or from the original iron structure, which gives diffraction as well. But there is no diffraction from antiferromagnetic ordering’.

Why was it so impressive to be able to achieve the 25GPa?

To get to high pressure, you have a small sample and you push on it. If you have a big sample and you push on it with a certain weight, you have much less pressure. The Paris–Edinburgh press was introduced to help achieve the high pressures required for neutron diffraction. This allowed us to to achieve 10GPa on samples of 50mg – an unprecedented sample size. Due to the fact that the instrument was focusing really on intensity, we were able to go for the small samples, allowing us to go to 10GPa.

Paper author, Stefan Klotz, designed a new anvil, which involved a slightly smaller sample of about 2–3mm. The new design involved syntho diamonds. If you exert pressure, you have a big piston, which has a certain diametre, let’s say 10cm. On this 10cm, you push with something like 1,000 bars of helium, which corresponds to about one tonne. You push on the big piston, and your anvil reduces the surface of the piston from 10cm2 down to a few mm2, so all that the load is exerting on a small surface.

The trouble with this size is you need a very hard material, such as diamond, to help push on the small sphere. The sample is not just naked between the angles, it needs a so-called gasket around which you can squeeze to make sure the pressure is exercised everywhere on it.

We have this small sphere of iron and inside this diamond anvil and we could do diffraction patterns on it. We cannot avoid that the neutrons also hit the diamond anvil, which is close in contact with the sample. But it’s not all bad, because diamond is a pure, crystalline material, which only gives very little Bragg peaks.

There are peaks visible that come from the anvil, which are limited and still allow us to look in our diffraction pattern. This is an angle-dependent, intensive distribution of the diffracted neutrons that still allows us to see the pictures of our material, between the anvils. This enabled us to build our 25GPa, roughly, which was the main technical breakthrough.

What we found is there was no magnetic diffraction pattern, no dips, which means there was no long-range magnetic ordering. This was the most interesting observation in the frame.

So, you found there was no magnetism at that point?

There was no long-range magnetic order observable. What we do is Bragg diffraction. We expect to see a so-called Bragg peak at a certain angle, neutrons which are derived by diffraction, due to Braggs equation. According to this, they are scattered on a certain lattice plane. But such diffraction requires that you have so-called long-range order that you have a nearly infinite extent of three-dimensionally ordered things, like ordered atoms or, in this case, ordered magnetic spin.

If you have a typical ferromagnet, where all the spins are in a row and pointing in the same direction, it is clear that this is long-range ordered. This is because thousands of atoms further, they are still in the same order of up and down, and this is not the case here.

In Bragg diffraction, you have to have a certain extent of this order. If you have one atom and you cannot foresee what 10 atoms further is going on, then this is not long-range order. You would expect you can predict, this atom is this orientation, so 100 atoms further you will have that orientation.

How could we use these findings in the practical world? What have we learned from this?

To material development, there’s superconducting, and with even lower temperatures, high pressure iron. I still don’t think that there is anything like the room temperature superconductor. I think we have to move to another planet, where temperatures are around 100K, in order to live in a world dominated by superconducting environment. Only we will have a problem to cool and make ourselves be comfortable. Iron is one of our basic construction materials. Everything which makes us understand iron better is important somehow. It will eventually make us understand better the properties of iron.

What can you see?

Under these relatively exotic conditions, one of the potential implications is it could eventually explain the magnetic behaviour on celestial bodies, which are very cold. High pressure is not a problem in geology, but the temperature is more of a problem. On Earth, for instance, we have rather high temperatures in the core of our planet. Then there are cold planets, plus moons and so on, which have no tectonic action anymore because they have cooled out.

In other celestial bodies you may have very low temperatures, below 40K. That would not seem unrealistic to me, and iron is one of the most common elements in the universe, so it is possible that cold and rocky planets have an iron core.

Then you may wonder whether it is possible to have a magnetism on these planets or not, because the magnetism could not work the same way as on Earth, where it works because the planet is warm on the inside. There is a possibility of a rotating core and of having a magnetic field. But on other planets, you may not have this possibility, so the question beckons, is there any chance to have a magnetic field on other celestial bodies? Since we discovered these exoplanets, people have been thinking what does an exoplanet need to have the possibility of Earth-like life on it? A magnetic field is definitely something crucial to life as we have it on Earth because otherwise, the radiation from the sun would have blown our atmosphere away without a magnetic field. We would not have an atmosphere and would have a high radiation level. I would say a biology which is not used to high level of cosmic radiation would get adapted to it.

Life would be imaginable with a higher level of radiation, but, without an atmosphere, it would be a different form of life. So an atmosphere is important, and I think an atmosphere is likely present only with the presence of a magnetic field is as well. There is a little link between extra planetary research and the rather fundamental outcome of this paper, that there is this spin-smectic state in iron.

How else could people apply this knowledge in their own field of work?

They could go lower in temperature in order to go to the superconducting phase. You could imagine if you want to go with superconducting phase, from where we are, by applying a dilution cryo-stat. But this is quite a technical development. My approach as a chemist would be to look in nature if there are other systems which behave the same way.