Superparamagnetic dwell times and tuning of switching rates in perpendicular CoFeB/MgO/CoFeB tunnel junctions

We investigated magnetic tunnel junctions with very thin magnetically perpendicular CoFeB electrode and MgO tunnel barrier. In particular the crossover to a superparamagnetic state with thermally activated switching rates is analyzed. The dwell times in the parallel and antiparallel state are evaluated for devices with 140 nm diameter, which is small enough to be apparently in the single domain regime. The depedence of the dwell times on magnetic or electric fields and of the temperature allowed to separate the impact of Zeman energy, spin-transfer-torque and voltage induced anisotropy change. While the dwell times statistics follows an Arrhenius law, their absolute values can be described only by taking entropic effects of multiple paths for magnetization switching into account. The results allow to estimate a magnetic activation volume with a typical lateral size of 35nm, i.e. considerably smaller than the devive diameter. This result supports the assumption of a granular magnetic structure of these ultrathin electrodes. In addition, we demonstrate a tuning of the superparamagnetic switching rates in a wide frequency range by combining magnetic and electric fields, which opens a path for their application in noisy neural networks.