Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.