Ultrafast nonthermal photo-magnetic recording in a transparent medium

Ultrafast photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet has very low heat load and is much faster than existing alternatives. A cool feat of short-term memory The use of magnetic materials for the recording and storage of information is a mature technology, yet the search is still on for a means to both further accelerate the switching process and further reduce the detrimental production of heat that occurs during switching. Andrzej Stupakiewicz and colleagues describe a potential route to achieving this goal. They show that the optical stimulation of a ferromagnetic garnet film—using a laser pulse carefully tuned to a specific electronic transition associated with the dopant ions responsible for the magnetic properties of the material—can be used to write magnetic information at ultrafast speeds (less than 20 picoseconds), without the laser-induced heating that normally accompanies optically driven magnetic transitions. Discovering ways to control the magnetic state of media with the lowest possible production of heat and at the fastest possible speeds is important in the study of fundamental magnetism 1 , 2 , 3 , 4 , 5 , with clear practical potential. In metals, it is possible to switch the magnetization between two stable states (and thus to record magnetic bits) using femtosecond circularly polarized laser pulses 6 , 7 , 8 . However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature 9 , 10 , 11 , 12 . Although several possible routes for achieving all-optical switching in magnetic dielectrics have been discussed 13 , 14 , no recording has hitherto been demonstrated. Here we describe ultrafast all-optical photo-magnetic recording in transparent films of the dielectric cobalt-substituted garnet. A single linearly polarized femtosecond laser pulse resonantly pumps specific d − d transitions in the cobalt ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing ‘0’ and ‘1’ magnetic bits at will. This mechanism outperforms existing alternatives in terms of the speed of the write–read magnetic recording event (less than 20 picoseconds) and the unprecedentedly low heat load (less than 6 joules per cubic centimetre).