The Nobel citation said Dr. Fert and Dr. Grünberg’s work also heralded the advent of a new, even smaller and denser type of memory storage called spintronics, in which information is stored and processed by manipulating the spins of electrons.

Engineers have been recording information magnetically and reading it out electrically since the dawn of the computer age, but as they have endeavored to pack more and more data onto their machines, they have been forced to use smaller and fainter magnetic inscriptions and thus more and more sensitive readout devices.

It has long been known that magnetic fields can affect the electrical resistance of magnetic materials like iron. Current flows more easily along field lines than across them. The effect was useful for sensing magnetic fields, and in heads that read magnetic disks. But it amounted to only a small change in resistance, and physicists did not think there were many prospects for improvement.

So it was a surprise in 1988 when groups led by Dr. Fert at the Laboratoire de Physique des Solides and by Dr. Grünberg found that super-slim sandwiches of iron and chromium showed enhanced sensitivity to magnetic fields — “giant magnetoresistance,” as Dr. Fert called it. The name stuck.

The reason for the effect has to do with what physicists call the spin of electrons. When the magnetic layers of the sandwich have their fields pointing in the same direction, electrons whose spin points along that direction can migrate freely through the sandwich, but electrons that point in another direction get scattered.

Image Peter Grünberg Credit... Ina Fassbender/Reuters

If, however, one of the magnetic layers is perturbed, by, say, reading a small signal, it can flip its direction so that its field runs opposite to the other one. In that case, no matter which way an electron points, it will be scattered and hindered from moving through the layers, greatly increasing the electrical resistance of the sandwich.