Circularly polarized femtosecond pulses are helping researchers in the Netherlands to write data to a hard drive significantly faster than conventional magnetic transfers. Based on a principle known as all-optical magnetization reversal, the technology could be used in future magnetic storage devices and even quantum computing. (Due to be published in Physical Review Letters July 27th issue)
"One of the main driving forces for our research has been the need for new technologies which can replace the relatively slow conventional magnetic data writers," Daniel Stanciu, a researcher at Radbound University Nijmegan, told optics.org. "The research area of magnetization dynamics and magnetization reversal is crucial for the speed of writing information in magnetic data storage devices."
Today's magnetic memory devices store logical bits (1 and 0) by reversing the state of the applied magnetic field. The team's idea was to use ultrafast pulses as an alternative to an applied field. What's more, right- and left-hand circularly polarized light acts as magnetic fields with opposite signs.
"Conventional magnetic data writers were believed to have a limit of about 2 picoseconds," commented Stanciu. "Our demonstration of magnetization reversal, without the aid of an applied magnetic field, leads to around two orders of magnitude higher recording speed."
The team used regeneratively amplified 40 femtosecond pulses from an 800 nm Ti:Sapphire laser with a repetition rate of 1 kHz. Circularly polarized pulses strike the surface at normal incidence and act as a magnetic field and reversing the polarity of the laser pulses produces the equivalent of either a 1 or a 0 on the disk. The team found that a fluence of 2.9 mJ/cm2 was required to switch the magnetization.
"The magnetic field is directed along the wave vector of light and its sign is controlled by the helicity of the light," explained Stanciu. "Changing the helicity from right-handed to left-handed changes the orientation of the magnetic field from up to down. In other words, the direction of the angular momentum of the photons."
This optically induced magnetization reversal is a result of the combination of femtosecond laser heating of the magnetic system and circularly polarized light acting as a magnetic field. "The demonstrated all-optical switching was a big step forward where we have observed for the first time that light can act non-thermally (via the angular momentum of the photons) on metallic magnets," commented Stanciu.
The team will continue to expand and improve its research in this field. "We are working on all-optical magnetization reversal in different materials and with different laser pulse lengths. We are also working on bringing laser induced switching of the domains down to 100 nm in size and beyond," concluded Stanciu. "The next challenge will be to find a relatively cheap laser technology that can fire pulses lasting less than 100 fs."
This work was carried out under the supervision of Professor Theo Rasing, researcher group leader at Radboud University Nijmegan.
