Physics

Spin Doctors

On December 10th, 2007, in Stockholm, Albert Fert was awarded the Nobel Prize in physics for his discovery of giant magnetoresistance, a phenomenon that has revolutionized hard disks. Though this discovery was made 20 years ago, his laboratory is still today at the forefront of cutting-edge research, busy developing tomorrow's electronics.

In the main hall of the steel and glass building, large posters are proudly displayed. “Albert, have you seen your ‘totem poles’ in the hall? How do you like your picture?” jokes Frédéric Nguyen Van Dau, director of the Unité Mixte de Physique CNRS/Thales¹ that moved into its new location in Palaiseau two years ago. The person to whom Nguyen Van Dau is speaking to is Albert Fert. And the posters that bear his portrait celebrate the 2007 Nobel Prize in physics that Fert has just been awarded.² Inundated with congratulatory messages, the prize-winner is still spending a lot of time explaining his discovery: the giant magnetoresistance effect (GMR), which makes it possible to control the motion of electrons in magnetic metals like iron or nickel, not by acting on their charge–as in conventional electronics–but on their spin. The spin is a kind of “compass needle” linked to the direction of rotation of the electron. “Basically, it’s a magnetic moment,” Fert explains, mimicking rotations with his hands. The whiteboard in his office is covered with drawings of magnetic multilayers called “sandwiches,” which are the essential requirement for GMR to work well. “It is the direction of an electron’s spin, whether it corresponds or not to the magnetic orientation of the layer, that determines if the electron will pass through the layer or not,” Fert continues.

First proposed as early as 1970 by the results of Fert’s doctoral thesis, it nonetheless took quite some time before the GMR effect was actually discovered. This only happened when the researchers managed to obtain layers that were sufficiently thin–i.e., just a few nanometers thick, the mere equivalent of ten atoms or so. Next door, Cyrile Deranlot, a CNRS engineer, shows us something that resembles a large stainless steel pressure cooker covered with pipes with viewing ports (see photo). “This is the machine we used to make them,” he proudly explains. It is the partnership between Fert’s lab and the company Thomson-CSF (now Thales), which had already mastered such nanometer-scale technologies, that led to the discovery of GMR in 1988. They managed to produce magnetic multilayers whose electrical resistance–their ability to transmit electric current–can be made to vary considerably depending on their magnetic configuration. This makes them objects that can detect very weak magnetic fields, such as those that are generated by the magnets encoding the information bits on a computer’s hard disks. Highly sensitive, yet taking up a minimum amount of space, GMR revolutionized hard disk storage capacity. “It increased a hundredfold thanks to this technology, which has been used since 1997, and is now found in every computer,” Fert points out.
This discovery launched the development of spintronics, a new kind of electronics, which has continued to break new ground ever since. For instance, Agnès Barthélémy, a professor at Paris-Sud University, has been working on other types of magnetic nanostructures in which some metallic layers are replaced by insulating ones. In these conditions, at the nanometer scale, classical mechanics gives way to quantum mechanics and its singular laws. “Actually, there is a non-zero probability that electrons can cross this insulating ‘wall’: This is the tunnel effect–in other words, the property of a quantum object that enables it to cross a potential barrier. This is impossible in classical mechanics, and we call these alternating layers a ‘tunnel junction,’” Barthélémy explains. Magnetoresistance records have been broken in the lab using tunnel junctions based on magnetic oxides.³ Unfortunately, this currently only works well at extremely low temperatures–around
-270°C. If it can be made to work at room temperature, this fundamental research could lead to the Holy Grail of tomorrow’s computer technology: a genuine nanometer-scale electrical switch. This may provide an elegant solution to compete with silicon transistors, which will reach their minimal size limit in less than a decade–thus putting an end to any further miniaturization and to the possibility of packing a greater number of them into microprocessors. 

A few doors down, Vincent Cros, a CNRS researcher, is tackling GMR from an entirely opposite angle, studying the so-called spin transfer phenomenon. “It can be seen as the opposite of GMR: We ‘inject’ electrons so that their spins change the orientation of a magnetic moment,” he explains. This kind of magnetization reversal without applying a magnetic field is no less than a new way of writing information to magnetic memories, whereas GMR is used to read it. “Spin transfer can also be used to emit electromagnetic radiation at very high frequencies,” Cros continues. This could potentially be used for manufacturing new types of microwave oscillators that could find applications in cell phones, navigation system, and other wireless communication devices. “Spin transfer oscillators will be smaller and more reliable, consume less energy, and be cheaper to make,” Cros predicts. Performances still need to be improved to increase power, “from nanowatts, currently possible, to milliwatts, which is what’s needed for practical applications.” An objective that is reachable within five years. “If this research leads to concrete results on emitted power, the benefits will be considerable given the size of the telecommunications market.”
And that’s not all on telecommunications. Javier Briatico, a CNRS researcher, and Jean-Claude Mage, a Thales engineer, are involved in signal processing. They are studying and developing superconductor materials whose electrical resistance falls to zero below a certain so-called critical temperature–in order to improve the quality of signal control and reception in tomorrow’s cell phones, radars, and other telecommunication devices. “We’re studying how they improve the selectivity and the signal encodings according to their frequency range,” Mage explains. “Filters made of superconductors are better in signal processing than current filters,” adds Briatico. Moreover, they allow for more “frequency-agile” receivers: They make it possible to easily change the frequency band that is to be filtered. The hope is to produce a filter that could be tuned across the entire frequency spectrum solely via digital controls. The gain in compactness and ergonomics would be considerable, especially for satellites, which would become easy to reconfigure remotely. Developing such filters will no doubt require a lot more shuttling back and forth between materials test rooms and research departments. Something made possible by the double affiliation, public and private, of the lab. “We do fundamental research, use our results on practical or even industrial applications, then return to the bench to improve our results and push forward the limits,” enthuses Briatico.
Charline Zeitoun