An artist's impression of a laser pulse changing a magnetic bit

Writing data with lasers overcomes speed limits

A new ultra-fast method of writing information to hard drives using lasers could help overcome looming limits to write speed.

Hard drive stores bits in the form of tiny magnetic domains, with the directions of the magnetic north and south poles of these domains – referred to as the magnetisation – determining whether the domain represents a binary 0 or 1.

Data is stored by changing the direction of the magnetisation of the associated bits, which is currently done using a write head to create a local magnetic field to make a bit change direction.

The stronger the field, the faster the switch takes place, but this is subject to a physical limit that has now almost been reached by current information storage technology.

"The number of bits has been growing rapidly for many years, but the write speed has hardly increased. There's a need for a new data storage technology", said Eindhoven University of Technology researcher Sjors Schellekens, the lead author of a paper in Nature Communications that presents an alternative technology.

Led by Professor Bert Koopmans, the team used ultra-fast laser pulses to generate a flow of electrons in a material which all have the same spin – a quantum-mechanical property that acts like a kind of internal compass in the electron. The resulting 'spin current' is used to change the magnetic properties of the material but at a much faster rate than current technology.

Two layers with opposite magnetisation are separated by a neutral layer. When a laser pulse strikes electrons in the upper layer it causes them to move through the material, in the direction of the second layer.

The spin of these electrons exerts a force on the spin of the electrons in the lower layer to make them rotate in the same direction, altering their magnetisation.

"The change in the magnetisation is of the order of 100 femtoseconds, which is a factor 1,000 faster than what is possible with today's technology", said Schellekens. "There was discussion among physicists about whether the generated spin current is actually able to cause the change in magnetisation. We now definitely show that this is really the case."

The method is a step towards future optical computer chips, which TU Eindhoven is currently working on – in December the university received a Dutch government grant of almost €20m to integrate photonics in computer systems.

"Our technology allows optical data to be stored in the form of magnetic bits. That offers unprecedented opportunities if you want to use light as information carrier", said Schellekens.

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