Magnetic recording on hard drive disk remains the best choice for reading and writing large quantities of information because it combines high density and low cost. Storage densities are still increasing at a rate of 100% a year and densities of 400Gbit/in² are already obtained in laboratories. This increase is limited by the thermal instability of the information which appears as the grains of the magnetic layer become too small (the “superparamagnetic limit”). Different solutions have been studied to overcome this limitation.