  Finding a conceptually new way to control the magnetic state of media with
the lowest possible production of heat and simultaneously at the fastest
possible time-scale is a new challenge in fundamental magnetism [1-4] as well
as an increasingly important issue in modern information technology [5]. Recent
results demonstrate that exclusively in metals it is possible to switch
magnetization between metastable states by femtosecond circularly polarized
laser pulses [6-8]. However, despite the record breaking speed of the
switching, the mechanisms in these materials are directly related to strong
optical absorption and laser-induced heating close to the Curie temperature
[9-12]. Here we report about ultrafast all-optical photo-magnetic recording in
transparent dielectrics. In ferrimagnetic garnet film a single linearly
polarized femtosecond laser pulse breaks the degeneracy between metastable
magnetic states and promotes switching of spins between them. Changing the
polarization of the laser pulse we deterministically steer the net
magnetization in the garnet, write "0" and "1" magnetic bits at will. This
mechanism operates at room temperature and allows ever fastest write-read
magnetic recording event (< 20 ps) accompanied by unprecedentedly low heat load
(< 6 J/cm3).
