Ultrafast Spin-Transfer-Torque Switching of Synthetic Ferrimagnets

The state-of-the-art switching speeds of spin-transfer-torque-driven free layers in magnetic tunnel junctions (MTJs) are of the order of a few hundred picoseconds, hindering their wide implementation in memory and logic devices. To significantly speed up the spin-torque-driven reversal time, we propose a composite free-layer structure using synthetic ferrimagnets (SFMs). It is commonly assumed that to achieve a given switching delay, the current must exceed the critical current by a certain factor; therefore a higher critical current implies a higher switching current. We show that this is not the case for SFM structures, and that they can provide significantly reduced switching delays for a given current density even though the critical current is increased. This non-intuitive result can be understood from the requirements of angular momentum conservation. We predict that a 20 nm diameter MTJ incorporating the proposed SFM free layer can be switched in tens of picoseconds, which could advance the state-of-the-art of sub-200 ps switching. We show that this speed can be achieved by experimentally demonstrated parameters using current perpendicular magnetic anisotropy materials.