At the Department for Materials of the ETH in Zurich, Pietro Gambardella and his collaborators investigate tomorrow’s memory devices. They should be fast, retain data reliably for a long time and also be cheap. So-called magnetic “random access memories” (MRAM) achieve this quadrature of the circle by combining fast switching via electric currents with durable data storage in magnetic materials. A few years ago researchers could already show that a certain physical effect – the spin-orbit torque – makes particularly fast data storage possible. Now Gambardella’s group, together with the R&D-centre IMEC in Belgium, managed to temporally resolve the exact dynamics of a single such storage event – and to use a few tricks to make it even faster.
Magnetising with single spins
To store data magnetically, one has to invert the direction of magnetisation of a ferromagnetic (that is, permanently magnetic) material in order to represent the information as a logic value, 0 or 1. In older technologies, such as magnetic tapes or hard drives, this is achieved through magnetic fields produced inside current-carrying coils. Modern MRAM-memories, by contrast, directly use the spins of electrons, which are magnetic, much like small compass needles, and flow directly through a magnetic layer as an electric current. In Gambardella’s experiments, electrons with opposite spin directions are spatially separated by the spin-orbit interaction. This, in turn, creates an effective magnetic field, which can be used to invert the direction of magnetisation of a tiny metal dot.
“We know from earlier experiments, in which we stroboscopically scanned a single magnetic metal dot with X-rays, that the magnetisation reversal happens very fast, in about a nanosecond”, says Eva Grimaldi, a post-doc in Gambardella’s group. “However, those were mean values averaged over many reversal events. Now we wanted to know how exactly a single such event takes place and to show that it can work on an industry-compatible magnetic memory device.”
Time resolution through a tunnel junction
