Pietro Gambardella, Professor at the Department of Materials Science at ETH Zurich, and his colleagues at the Department of Physics and at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, have now shown that magnetic storage can be carried out by means of a new process in a very fast and energy-saving manner; the disadvantages of conventional magnetic-based storage media such as hard disks are avoided.
In traditional magnetic storage technologies, tape or disk media coated with a cobalt alloy are used. A magnetic field is generated by means of a coil through which an electric current is sent. This changes the direction of magnetization in a small area of the data carrier. The process is rather slow, measured by the speeds of modern processors, and the electrical resistance of the coils also causes energy losses. It would therefore be much better to change the direction of magnetization without using a magnet coil.
As early as 2011, Gambardella and his colleagues demonstrated a process that did just that: Electric current flowing through a specially coated semiconductor film reversed magnetization in a tiny metal dot. This is made possible by a physical effect called spin track torque. The flow of an electrical current in a conductor leads to an accumulation of electrons with opposite magnetic moments (spin) at both ends of the conductor. The spins of the electrons in turn create a magnetic field that leads to the alignment of the magnetic moments of the atoms of a magnetic material in the immediate vicinity. Now the scientists have carried out a new study to investigate how this process works in detail and how fast it is. The results were recently published in the journal Nature Nanotechnology.
In their experiment, the researchers reversed the magnetization of a cobalt dot 500 with a diameter of just nanometers with electrical current pulses flowing through a neighboring platinum wire. Meanwhile, they exposed the cobalt dot to highly focused x-rays, which scanned the point with a spatial resolution of 25 nanometers. The degree to which the point absorbed the X-rays at a certain point depended on the direction in which the metal was magnetized there.
“Thus, we obtained a two-dimensional image of the magnetization of the cobalt dot and were able to see how the current pulse gradually changed this,” explains Manuel Baumgartner, the first author of the study and doctoral student in Gambardella’s research group.
The researchers were thus able to establish that the reversal of magnetization took place in less than one nanosecond – significantly faster than in other methods that have been investigated recently. In addition, based on the parameters of the experiment, they were able to predict when and where the magnetization reversal starts and ends. In other processes, the reversal is also driven by electrical current, but it is triggered by thermal fluctuations in the material, which means that the time of reversal is subject to considerable fluctuations.
The researchers sent up to a trillion reversal pulses with a frequency of 20 megahertz through the cobalt dot without compromising the quality of the magnetization reversal. “This gives us hope that our technology is suitable for applications in magnetic RAM,” says Gambardella’s former post-doctoral fellow Kevin Garello, also one of the first authors of the study. Garello is now working on the commercial implementation of the process at the Imec research centre in Leuven, Belgium.
As a first step in this direction, the researchers want to optimize their materials in such a way that the reversal works even faster and with lower currents. Among other things, this is to be achieved by an improved form of cobalt dots. Presently these dots have a circular shape, but other shapes such as ellipses or diamonds could make magnetization reversal more efficient, the scientists believe. These findings allow the development of magnetic RAMs to be significantly closer.
Magnetic RAM would, among other things, make it unnecessary to charge the operating system when a computer is started up, as all programs and data would then remain stored in memory without power supply.