Researchers from all over the world are using the Swiss muon source at the Paul Scherrer Institute to explore the magnetic and electronic properties of new materials. Nowhere else are so many muons produced: exotic elementary particles that act as micro-probes to explore the processes within materials.

The complex instruments for the muon experiments were custom-developed by the PSI researchers themselves.

“We have instruments that are unique in the world”, says Elvezio Morenzoni, director of the laboratory where the muons are used to investigate the microscopic properties of materials. Muons are electrically charged particles. A negative muon is similar to an electron, but is about 200 times heavier. A positive muon behaves like a lightweight proton. Muons are formed in our surroundings when cosmic rays collide with air molecules in the atmosphere. At the Paul Scherrer Institute (PSI) in Villigen in the Canton of Aargau, they are produced using a particle accelerator.

Elvezio Morenzoni, Professor at the University of Zurich and head of the Laboratory for Muon Spin Spectroscopy (LMU) at PSI, explains about the labyrinth of machines, concrete blocks, wires and cables spread out below him as he stands on a gallery in  the large experimentation hall. In the opposite corner, behind a metre-thick shield, there is the centrepiece of the hall: the proton accelerator. The 40-year old cyclotron is still the world’s most powerful installation of this type and produces more protons than all the others. In the ring accelerator the positively charged atomic particles reach speeds of up to 80 % of the speed of light. From there the particles are sent to the muon source in the middle of the hall.

The expert uses a model to show what it looks like. Basically it consists of a rotating graphite ring. When the fast protons strike the atomic nucleii in the graphite, new particles are produced, the so-called pions. “These decay to form muons, which can be gathered to form a beam”, the physicist explains. “Because we have more protons available than the competitors, we can also produce more of the muons, and use them to perform experiments that are not possible elsewhere.”

A new type of magnetic material

Thus in summer 2015 an international team was able to show how they had managed to produce a magnetic material from a non-magnetic metal with the aid of the muon measurements. The researchers had applied a layer of special carbon molecules to thin copper strips, thereby altering the properties of the combined material so that it could be permanently magnetised. The measurements at PSI demonstrated that the boundary layer between the copper and the carbon is responsible for the magnetic behaviour.

“These experiments would not have been possible in any other facility in the world”, says Oscar Céspedes from the University of Leeds, who is directing the research project. “We were finding it difficult to determine the magnetic profile in our thin layers.” It was then that members of the group told him about the muon source in Switzerland. The methods at PSI were found to be ideal for studying the thin films from Leeds. Using the new type of material it may be possible to develop magnets that can be used in various new technologies of the future, for example for stor-ing data on hard disks, or for electrical generators, or medical equipment.

Slow muons for thin films

Prof. Elvezio Morenzoni, head of the Laboratory for Muon Spin Spectroscopy (LMU) at the PSI.

To study the copper-carbon films, the team use a method that Elvezio Morenzoni and his group started developing over twenty years ago. “We can produce slow muons with very specific energies, which come to rest in different layers”, the PSI researcher explains. “In this way thin films and multi-layered structures can be examined.” To do this, a trick is needed. This is because the ordinary muons produced from pion decay are so fast they penetrate about half a millimetre into the sample before they decay further. This means they can be used to investigate crystals, but not nanometre-thin layered structures. At PSI a portion of the muons is passed through a frozen noble gas, bringing them almost to a standstill, and then they are accelerated again just enough to achieve the desired very short depth of penetration.

Once they have arrived at the target, the positively charged muons tend to stay between the atoms. Here they behave like tiny compass needles. For the muons have what is known as spin, a quantum mechanical property, which one can imagine as a rotational axis. The freshly generated muons all have this spin aligned in the same direction; that is to say the particles are polarised. But the spin can change depending on the magnetic environment. The polarisation changes or expires in the course of time. If one measures the change in polarisation over a certain period of time, one can draw certain conclusions about the magnetic properties of the material. “The muons are magnetic micro-probes positioned up close to the atoms”, says Elvezio Morenzoni.

The installation that produces slow muons is one of six instruments that are supplied with muons from the muon source. Magnets guide the muons to the detector units shielded in concrete. To view one of the instruments, the physicist has the particle beam switched off, and each visitor has to take a key from a holder before he opens the door to the installation – as a safety measure. Only after everyone has left the detector section, and all the keys are back in their places, can the source be started up again.

Lowest temperatures, highest pressures

Passing along a narrow spiral staircase, one comes to the upper part of the two-floor facility, where the tiny sample is located right at the centre. “We have developed all the instruments ourselves”, says Elvezio Morenzoni. In each experiment the researchers carry out measurements on some ten million muons. The basic method is the same, but each of the six detector sections has a special function. “Here we can, for example, examine samples at very low temperatures”, explains the expert, referring to the figure of ten millikelvins – which is just ten thousandths of a degree above absolute zero. At another detector, the material can be subjected to very high pressures and powerful magnetic fields.

It is true there are similar instruments in England, Canada and Japan, the physicist notes, but it is not possible to achieve such low temperatures, strong magnetic fields and high pressures there. “Even where our facilities are not unique, as is the case with some of the instruments, we are better than the competition.” It is not surprising that in 2015 a scientist from China and a researcher from South Korea came to PSI for a long visit. Similar projects are planned in both those countries. At the PSI collaboration with guests from abroad is a daily event. Many of the muon experiments are performed by international teams, such as the investigations into magnets made from non-magnetic metals, where two PSI staff members, two doctoral students from Leeds University, and two other researchers were involved.

“The teamwork went really well”, says Oscar Céspedes positively. The PSI staff contributed with their considerable knowledge, and showed great commitment, even beyond the normal working hours, he says. “They were a tremendous help to us, first during the experiment, and later also during the data analysis.” Besides the international collaboration, the PSI researchers also perform their own experiments. “One particularly interesting field right now is the study of magnetic semi-conductors”, says Elvezio Morenzoni. The hope is that this will lead to methods of processing data more rapidly while also storing it more efficiently.

Samples from around the world

The magnetic semi-conductors are thin films. It is often not clear whether the whole sample is magnetic or only a part of it. “This is where we come in”, the PSI scientist says. “We can investigate the magnetic properties at the nanometre scale, and see if the sample is homogeneous magnetically.” Therefore the researchers in Villigen obtain the material from all over the world, which is often difficult to manufacture, and therefore highly in demand for experiments.

Besides the magnetic semi-conductors, there are also experiments to study unconventional superconductors made of iron, a category of materials that was only recently discovered in 2008. By examining these materials, the scientists essentially hope to gain greater insight into the phenomenon of superconductivity, where an electrical current is conducted without any losses. “With these iron-based and other unconventional superconductors, something new is found almost every day”, says Elvezio Morenzoni enthusiastically. “If we find something of interest we try to obtain more of the material for our experiments. Of course for such things it helps when you have a good reputation.”