Jan 29, 2009
Nanomagnets switch for less
Devices known as spin valves are currently used for applications like magnetic random access memories (MRAMs) but these memories need to be made more competitive by building smaller devices that have higher density. This can be achieved by reversing the magnetisation of nanoscale magnets using a polarised current (also known as spin transfer) but the problem is that the currents required to reverse nanomagnets remain fairly high. Now, researchers in France and the US have lowered this current down to just 120 microamps at room temperature for a device that measures 45 nm across.
“Our result was expected from modelling but had not been shown experimentally until now,” team member Stéphane Mangin from Nancy University told nanotechweb.org.
Spin valves are spintronic devices that exploit the spin of electrons as well as their charge. They usually consist of a thin layer of metal sandwiched between two ferromagnetic electrodes.
Spin transfer is when the spin angular momentum of charge carriers (usually electrons) in a material is transferred from one place to another. This phenomenon leads to several important and observable physical effects.
In the most well known, spin-polarised current passing into a nanoscale magnet tends to deposit some of its spin angular momentum into the magnet, thus applying a large torque to the magnetisation. This allows the magnetism of a material to be locally manipulated far more efficiently than with magnetic fields alone, especially as devices get smaller. In the MRAM industry, the effect might also help to significantly reduce power consumption. But there is a snag: current densities demonstrated so far to switch magnetisation are higher than those needed to make robust devices. And integrating these spin valve devices with CMOS technology will also require a lower current.
Mangin and colleagues may now have overcome this problem by fabricating 45 nm diameter spin valves based on cobalt-nickel multilayer elements. Because these devices exhibit perpendicular anisotropy, they are thermally stable and require currents as low as 120 microamps for spin transfer switching without any applied magnetic field.
Mangin says that the work will motivate researchers to search for other materials that show a similar effect. Ultimately, his team would like to combine its devices with tunnel junctions to achieve an even lower critical current.
The work was published in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org