Spin-transfer switching at ambient temperature in an antiferromagnetically coupled two-macrospin model system

Spin-torque switching of nanomagnets in tunnel junction based random access memory requires understanding of finite temperature statistics beyond simple macrospin-limit. The next simplest model beyond macrospin is a two-macrospin coupled system. We extend an earlier two-moment finite temperature study to a two-moment antiferromagnetically coupled model system. In a limited region of spin-current, antiferromagnetic coupling allows for faster spin-torque switching at lower threshold spin-current compared to the same moments coupled ferromagnetically. However, spin-torque switching in antiferromagnetically coupled case is more complex. There is only a narrow window in spin-current for reliable switching. At higher spin-current, a 2nd instability threshold emerges, above which the switching error-rate rises. This is associated with the forced alignment towards spin-polarization direction for both moments under spin-torque, due to precession induced dynamic coupling between the two moments. It results in occupation of higher energy intermediate states under spin-torque, resembling a pumped spin system’s “negative temperature”-like state-population inversion. These moment-near-parallel intermediate states upon spin-torque withdrawal relax back into antiferromagnetic alignment with a finite probability of settling into the “wrong” moment orientations. We discuss this behavior through finite temperature numerical modeling and by comparing numerical results with analytical estimates of linear instability thresholds. This simple model demonstrates the importance of internal degrees of freedom in coupled magnetic moment systems under spin-torque, especially in antiferromagnetic or non-collinear configurations. For fast switching speeds well below 10 ns to reach deep write-error floors, it may require higher exchange strength than demonstrated so far.