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Posted: Aug 04, 2011
New bounds in magnetic writing
(Nanowerk News) The computer files we use every day for both work and leisure are nothing more than streams of digital data made-up of zeros and ones. These zeros and ones are found on a thin magnetic layer on a computer's hard disk, where magnetic domains pointing upwards represent a one and magnetic domains pointing down the way represent a zero.
The size of these magnetic domains has now reached a few tens of nanometres, meaning we can squeeze a terabyte of data into the space of just four square centimetres. Whilst it sounds impressive, this 'miniaturisation' poses numerous problems for physicists and engineers. The fast-paced and ever-expanding information technology industry now demands that each piece of information is written on these tiny magnetic bits one at a time, and as quickly and energy efficiently as possible.
Help could be here in the form of a new method of magnetic data writing, as developed by a team of EU-funded scientists from Spain and France. Writing in the journal Nature ("Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection"), the team outline how their method could help tackle these problems as well as meet changing market requirements. The study received a European Research Council (ERC) Starting Grant worth EUR some 1.5 million as part of the 'Nanoscale magnetization dynamics' (NOMAD) project, funded under the 'Ideas' Theme of the Seventh Framework Programme (FP7).
The current method, which uses magnetic fields produced by wires and coils, suffers from severe limitations in scalability and energy efficiency. The team's new technique eliminates the need for cumbersome magnetic fields and provides extremely simple and reversible writing of memory elements, by injecting an electric current parallel to the plane of a magnetic bit. The key to this effect lies in engineering asymmetric interfaces at the top and bottom of the magnetic layer, thereby inducing an electric field across the material, in this case a cobalt film less than one nanometre thick sandwiched between platinum and aluminium oxide.
Due to subtle relativistic effects, electrons crossing the magnetic layer effectively see the material's electric field as a magnetic field, which in turn affects their magnetisation. Depending on the intensity of the current and the direction of the magnetisation, one can induce an effective magnetic field intrinsic to the material that is strong enough to reverse the magnetisation.
The research has implications for the development of magnetic random access memories, or 'MRAMs'. If these MRAMs were to replace standard RAMs, which need to be refreshed every few milliseconds, it would mean a computer could be instantly powered up and a substantial amount of energy saved.
The team showed in their study that the method works reliably at room temperature.
An additional advantage to this discovery is that current-induced magnetic writing is more efficient in 'hard' magnetic layers than in 'soft' ones. This is somehow counterintuitive, as soft magnetic materials are by definition easier to switch using external magnetic fields; however, it is very practical since hard magnets can be miniaturised to nanometre dimensions without losing their magnetic properties. This could allow the information storage density to be increased without compromising the ability to write it.
The overall aim of the NOMAD project, which will run until 2013, is to develop frontier approaches to control the magnetodynamic properties of nanometre-sized molecular and metallic elements.