Controlling magnetic memory with an electric current

Researchers led by former Hamburg University physicist Matthias Bode, now at the Argonne National Laboratory, have found a neat way of switching the magnetisation of nanoparticles that will in future be used in hard disk drives.

Bode, along with study lead author Stefan Krause and three other Hamburg physicists, employed a scanning tunnelling microscope (STM) tip to force an electric current through magnetic nanoparticles.

Future computers are likely to include high density storage in the form of magneto-resistive random access memory, or MRAM. In place of the electric charge held in capacitors in conventional dynamic RAM, data in MRAM chips are stored in magnetic elements that consist of two ferromagnetic layers separated by a non-magnetic spacer.

Magneto-resistive memory elements can be accurately controlled with electric currents carried by probes scanning over the surface

MRAM switching is normally controlled by an external magnetic field, but the non-local character of magnetic fields means that flipping one bit may affect nearby elements. An alternative approach is to switch the magnetisation by passing an electric current through a fine tip that scans over the surface of the device. The changes are then highly localised.

“We have shown that a scanning tip can be used to not only read out information at a lateral resolution down to the atomic scale, but also to control the magnetism at the same scale,” says Krause. “Since you can use the same tip to read and write information with spin-polarised tunnelling electrons, the electrical circuits could be drastically simplified.”

It will not be easy to turn the principle into practice, however. For one thing, you need stable magnetic materials that do not require cooling to 50 K. Another challenge is to overcome the existing requirement for ultra-high vacuum environments.

Krause believes that the problems are solvable: “We have made a big step on the way to a new type of hard disk by demonstrating that current-induced magnetisation switching is possible with an STM, but there are still some things to do for a commercial application of this new technique.”

Further reading: Current-Induced Magnetization Switching with a Spin-Polarized Scanning Tunneling Microscope, Krause et al., Science 317, 1537 (2007).

Figure: Magneto-resistive random access memory elements can be accurately controlled with electric currents carried by fine probes scanning over the device surface. (source: Stefan Krause/Universität Hamburg).

Article first published in Nanomaterials News.