Vincent Cros: in the spintronics saga

It was curiosity that led Vincent Cros to study physics at Paris 7 University (now Université Paris-Cité) after his baccalaureate. During his DEA in materials science (the equivalent of a 2nd year Master's degree today), he made an encounter that would prove decisive for his career: "I was extremely lucky to have Albert Fert as a professor, a few years after his discovery of the giant magnetoresistance effect in 1988, which won him the Nobel Prize in Physics in 2007," he recalls. Motivated by his professor's passion for the new field of research that this discovery gave rise to, which would later be known as spintronics, Vincent Cros embarked on a PhD on giant magnetoresistance in the physicist's laboratory at the time ‒ the Laboratory of Solid-State Physics at Université Paris-Sud. During this period, he also spent a year in Jülich, Germany, working on exchange coupling in magnetic multilayers with Peter Grünberg, co-winner of the 2007 Nobel Prize in Physics.

After a post-doctoral fellowship in surface physics in Madrid, Vincent Cros joined the CNRS in 1998 and returned to work with Albert Fert in his new laboratory, the CNRS-Thales joint physics unit, set up in 1995. He became CNRS director of research in 2010 and, in 2020, took over as deputy director of the laboratory, which changed its name at the end of 2023 to the Albert Fert Laboratory (LAB ‒ Univ. Paris-Saclay/CNRS/Thales).
 

Spintronics, from fundamental research to applications

Vincent Cros devoted his research to spintronics and nano-magnetism, i.e. "studying, understanding and trying to control information transport properties via the spin of electrons, and not just via their charge," explains the researcher. Spintronics is where electronics and magnetism meet. While traditional electronics manipulates the negative charge of electrons, spintronics manipulates another property of these elementary particles: the spin. It is a quantum quantity with an up or down state, and is responsible for magnetisation.

At the start of his career, Vincent Cros worked on the phenomenon of giant magnetoresistance, which occurs in ultra-thin multilayer structures – in the order of a few atoms – alternating ferromagnetic and non-magnetic materials, and through which a current of electrons (electric current) flows. This effect is based on the mobility of electrons through a magnetic layer, which depends on the spin direction of these electrons with respect to the magnetisation of the layer encountered. By applying a magnetic field, it is possible to modify the relative direction of the magnetisation of the various magnetic layers of the heterostructure and, ultimately, to strongly vary the electrical resistance of the multilayer.

This initially very basic research topic rapidly gave rise to major applications in information storage and sensors, for example. "That's part of the laboratory's DNA: not to stay disconnected from the potential uses of the highly complex fundamental effects we're studying. As a result, 80% of the researchers in my laboratory have now filed a patent," emphasises Vincent Cros. He himself has filed more than twenty patents.
 

Magnetisation dynamics by spin transfer, towards efficient digital technology

For the past twenty years, Vincent Cros has been focusing on magnetisation dynamics using spin-transfer torque in spintronic nanodevices. The researcher explains: "What we call spin-transfer torque is the use of a spin-polarised current to write or act on a magnetisation, without applying a magnetic field." This physical effect is used today in non-volatile magnetic memories (MRAM or Magnetic Random Access Memory) to write information.

Combining magnetism and electronics, as in these MRAMs, offers the hope of developing energy-efficient digital technology. "A magnet sticks to a fridge without plugging it into an electrical socket. That's the advantage of magnetism: it's non-volatile," illustrates Vincent Cros.  It is therefore possible to write a magnetisation thanks to spin transfer, which will be preserved without the need for a power supply.

Skyrmions, structures of interest for the future of spintronics

Another subject that has occupied Vincent Cros' research over the last ten years is that of topological magnetic textures such as magnetic skyrmions. These are magnetic configurations of a few nanometres in size, in which the orientation of the spins together forms a spiral, each spin being weakly rotated with respect to its neighbour. These structures, about ten nanometres in diameter, are stable at room temperature thanks to their topological nature. They can be generated either intrinsically, by controlling material properties, or by current injection. Vincent Cros and his team are interested in this second aspect: "We'd like to manipulate the spins one by one and use them, for example, as information carriers. So, we're trying to create them, detect them or move them."

The Albert Fert Laboratory has recently integrated aspects of research in neuromorphic physics to develop low-energy AI components. Skyrmions could provide an original approach. This field of physics suggests drawing inspiration from biology, in particular the nervous system, to recreate this structure in computer systems. In such a system, skyrmions would represent packets of neurotransmitters which, at a synapse, would transmit information between two neurons.

PEPR SPIN, an opportunity for spintronics in France

Since 2023, Vincent Cros has been co-director, alongside Lucian Prejbeanu of the CEA, of the Priority Program and Equipment for Exploratory Research (PEPR) SPIN, focusing on spintronics and its applications. Launched in 2021 and operated by the French National Research Agency (ANR), PEPRs are part of the French government's France 2030 investment plan, which aims to reduce France's lag behind other countries in certain industrial and technological sectors and to support the ecological transition. Their purpose is to fund major areas of fundamental research. While some of these programmes – the PEPRs related to national acceleration strategies – have been defined in advance by the government, a budget has also been set aside for exploratory programmes selected following a call for proposals. "With a number of colleagues in France, we thought that spintronics would give us a chance," recalls Vincent Cros. The country has real expertise in this emerging field, which promises the development of efficient digital technology. Selected following a call for proposals, the SPIN programme, co-directed by CNRS and the CEA, has been awarded 38 million euros in funding over six years. "The entire French spintronics community is involved. In addition to the two supervisory bodies leading the programme, PEPR SPIN also has three partner universities, including Université Paris-Saclay," enthuses the programme's co-director. PEPR SPIN supports targeted projects defined when the programme was set up and, more recently, projects resulting from an initial call for proposals. It will also fund cutting-edge equipment, theses and post-doctoral fellowships. The aim is both to make new discoveries in spintronics and to develop the most innovative and emerging aspects of the field until they are mature.

Juggling all his different roles, Vincent Cros now has less time to devote to his own research. But "these commitments to leadership roles are part of my mission," he says. "I get the impression ‒ and I also hope ‒ that what we're doing with PEPR SPIN and the Albert Fert Laboratory is serving spintronics research in an effective and direct way," he concludes.