Ultrafast magnetic switching induced by picosecond electric current pulses

The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales, including helicity-independent all-optical switching.  We have united these phenomena by using picosecond charge current pulses generated on-chip using an ultrafast photoconducting (Auston) switch to induce deterministic, repeatable ultrafast reversal of the magnetization of a ferromagnetic GdFeCo thin film. Using 9 ps duration current pulses, the magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. Furthermore, the energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm)³ cell.

 

We also show that 6-10 ps duration electric current pulses can be used to deterministically switch the out-of-plane magnetization of a ferromagnetic thin cobalt film via spin–orbit torque (SOT). The current pulses were applied to a heavy metal/ferromagnet thin film heterostructure in the presence of an in-plane symmetry breaking magnetic field.  Depending on the relative current pulse and in-plane magnetic field polarities, we observe either SOT switching or an ultrafast demagnetization and subsequent recovery, but no switching. The short current pulse induces the ultrafast demagnetization of close to 30% due to transient Joule heating. This heating plays a crucial role in promoting the SOT switching in the presence of the in-plane magnetic field. Nevertheless, we also project low energy (~fJ range) for switching of a (20 nm)³ cell using this mechanism.  We use a macro-magnetic simulation model coupled with an ultrafast heating model to analyze the effect of ultrafast thermal anisotropy torque (intimately related to the ultrafast demagnetization) and current-induced SOT in the observed dynamics. Good agreement between our experimental results and the macro-spin model shows that the switching dynamics are coherent rather than involving domain nucleation and growth, even though our device dimensions are as large as 4 X 5 um².