Altermagnets, a new magnetic phase

Rafael Gonzalez Hernandez is a professor at the Universidad del Norte, Colombia. He is the winner of the 2024 Walter Kohn Prize, awarded bi-annually by ICTP and the Quantum ESPRESSO Foundation to a young scientist for their outstanding contributions in the field of quantum-mechanical materials and molecular modeling, performed in a developing country or emerging economy.

Gonzalez was awarded the Walter Kohn Prize for his ground-breaking work on altermagnetism, an unconventional magnetic state of matter that has been highlighted as one of the most important discoveries of the past few years. In 2024, it was featured in The Economist and in Stephen Colbert’s Late Show. The journal Science included altermagnets in its runners up list of top scientific breakthroughs of 2024.

Gonzalez agreed to tell us more about this discovery in an interview he gave after the prize ceremony that took place on 9 January, during the 22nd International Workshop on Computational Physics and Materials Science: Total Energy and Force Methods. 

How would you describe altermagnetism in simple terms, and why is it so important?

Altermagnetism is a new phase of matter that combines the most important properties of the two other magnetic phases we know: the ferromagnetic and the antiferromagnetic phase. In order to understand the unique features of altermagnets, I will start by quickly describing these two other phases.

We all have a rough idea of what ferromagnets are as we are used to exploit their properties in our everyday life, for example to stick notes on our refrigerators at home. At the microscopic level, electrons in a ferromagnetic material respond to an external magnetic field by aligning their spins along the same direction, which gives rise to a macroscopic magnetic field generated by the ferromagnet itself. This phase was discovered about 120 years ago and it has very important applications. We use ferromagnets to store information in hard drives and credit cards, and among the key properties that make this possible is their ability to develop spin currents, ‘magnetic currents’ that generate when an external electric field is applied. Because they respond so easily to external magnetic and electric fields, however, ferromagnets can also lose the information stored in them, something that all of us have experienced when keeping a badge too close to a set of keys.

Antiferromagnets respond to an external magnetic field very differently from ferromagnets: at the microscopic scale, neighbouring electrons tend to align their spins in opposite directions, which does not result in any macroscopic magnetic field. In other words, you would not be able to stick your magnets to a fridge made of an antiferromagnetic material.  Antiferromagnets were discovered in the 1930s by French physicist Louis Néel, who in 1970 received the Nobel Prize after his theoretical predictions were supported by experiments. Néel described antiferromagnets as an interesting but useless class of materials, because their properties make it difficult to use them in applications: they do not develop spin currents, and it is hard to manage the opposite spin alignment of the electrons.  

Altermagnets combine the best properties of ferromagnets and antiferromagnets. At the microscopic level, they are very similar to antiferromagnets, in the sense that electrons in neighbouring atoms have opposite spins, so they do not generate a macroscopic magnetic field. However, like ferromagnets, they do develop spin currents.

This particular set of properties makes altermagnets particularly interesting for potential applications. In the future we will be able to use them to store information, as we do with ferromagnets but, very conveniently, there will be no risk to lose any information when an external magnetic field is applied. Another advantage of altermagnets is that information can be written and read about a thousand times faster than in ferromagnets.

One of the reasons why scientists are so excited about this new magnetic phase is that we have been studying magnetism for more than a century now and, to quote the Economist, altermagnets had been hiding in plain sight for more than 90 years. For a long time we were just unable to distinguish them from antiferromagnets. So much so that at the early stages of our study, we thought that this was just a small group of unconventional antiferromagnetic materials. Over time, however, it turned out that there are even more altermagnetic materials than there are ferromagnetic ones, which is really surprising!

How did you and your collaborators discover altermagnetism?

The discovery was the result of a collaboration born during the time I spent at the Johannes Gutenberg Universität Mainz between 2017 and 2018, thanks to the support of the Alexander Von Humboldt Foundation. That is when I met Libor Smejikal, a young theoretician working in the group of Jairo Sinova and collaborating with Tomas Jungwirth. Based on theoretical considerations, they realised that in principle there is nothing stopping collinear antiferromagnets from developing a very important property called anomalous Hall effect. However, that had never been observed and many people believed that it wasn’t possible.

I arrived in Mainz at the right time and, having spent the previous 15 years working on ab initio molecular dynamics simulations, I had the right skills to implement simulations to help us test Libor, Jairo and Tomas’s ideas. We developed density functional theory calculations and performed simulations on a wide range of materials, consistently observing the anomalous Hall effect where it was not expected. A first preprint focusing on RuO2 came out in 2019 and in 2020 the article was published in Science Advances.

It was incredible to see how fast experimental groups in the US, Europe and China supported our results with their experiments. The first experimental results were published only two years after our paper came out. This is an incredibly short time and it probably means that we will soon see some applications of altermagnets.

Why did it take so long for scientists to discover altermagnetism?

Discovering this new phase required us to be able to look beyond the surface and investigate the electronic structure of matter. As I said earlier, altermagnets appear to be very similar to antiferromagnets and for a long time everyone thought that there were only two possible strong magnetic phases. Finding altermagnets required sophisticated simulations accounting for the quantum mechanical properties that are involved in this effect, which we simply did not have until a few years ago. 

It is in particular the ab initio simulations on altermagnetic materials that earned you the Walter Kohn Prize. Could you give us an idea of what this method implies and how it helped you study these materials?

You are probably familiar with classical molecular dynamics simulations, which are used to study the evolution of classical systems. By knowing exactly what forces act on the particles, there one can compute their trajectories.

Ab initio molecular dynamics simulations describe the evolution of electrons in a system of many atoms and molecules, taking into account their quantum nature. In quantum mechanics, instead of studying how the velocity and the position of the particles change in time, we look at the evolution of their wave function, described by a series of complex equations called Kohn-Sham equations. 

Solving these equations analytically for real materials is an impossible task and computer simulations are a necessary and extraordinary tool to do that and compare results with experiments. They are a fantastic bridge between theory and experiments and help us connect our understanding of the microscopic structure of matter with its macroscopic behaviour. That of ab initio simulations is a field that is still developing very fast, as the many discussions taking place these days at the “Total Energy” conference hosted by ICTP this week show.  

What does the Walter Kohn Prize mean to you and to your career?

It has been an incredible honour to receive this prize. Having spent all of my career in Colombia, with the only exception of the time I spent in Mainz, I am used to the many challenges that scientists in developing countries experience. It is easy to get discouraged, but this achievement shows me, and hopefully others as well, that we too can work at the forefront of science and contribute to pushing the boundaries of knowledge. We too are part of the global scientific community. I hope that this will inspire future generations of scientists in developing countries, and especially in South America, as for now I have been the only winner from that region.

I also very much admire Walter Kohn, he is one of my heroes in science. His work was essential to develop ab initio simulations and it continues to inspire me, which is another reason why I am extremely happy to have won this prize.

Your research field is computational physics. Why did you choose numerical techniques as your preferred way to explore physics over theoretical or experimental methods? 

That is a choice that I made just after finishing my bachelor’s degree at Universidad Pedagogica and Tecnologica, in Colombia. I had chosen to work on an experimental project, but I quickly realized that it was very complicated to move forward in that direction while living in a developing country. In order to do experiments one needs specific materials and tools that often need to be imported either from the US or from Europe, which quickly make them become expensive and hard-to-get. That is why I decided to work on simulations, which to me looked like the closest affordable alternative to experiments and the best way to connect theory with the real world. I started working on ab initio simulations during my master’s degree. I was even able to buy a computer and work on my simulations at home. In my PhD I continued working along the same path and now I realise that this was a great choice.

The Walter Kohn Prize is awarded jointly by ICTP and the Quantum ESPRESSO Foundation. How are ICTP and Quantum ESPRESSO related to your work?

I used Quantum ESPRESSO for my PhD project and I learnt how to use it at two events that I attended thanks to the support of ICTP and the Quantum ESPRESSO Foundation. One is a school that took place in Chile in 2009. It was the first international event that I attended as a PhD student. The second is a workshop that took place here in Trieste in 2010 as a side event to the “Total Energy” conference of that year. That training was absolutely essential for me to understand what ab initio simulations do and how they work. The workshop in particular has had an immense impact on my work. I remember meeting Paolo Giannozzi. Stefano Baroni, Sandro Scandolo, Nicola Marzari and Stefano de Gironcoli, who all have contributed to Quantum ESPRESSO, and asking them many questions. Thanks to that workshop I was able to solve all the doubts that I had until then about the work I was doing for my PhD.

I really hope that similar tutorials continue to take place, because they are amazing occasions for master’s and PhD students, especially those coming from developing countries, to learn ab initio simulations. Also, it would have been impossible for me to come to Trieste all the way from Colombia only for the three-day Total Energy conference. The workshop made the journey worthwhile and I was able to attend two very high level events.

Pictures of the 2024 Walter Kohn Prize Ceremony can be found on ICTP's Filckr page.