Interface moment dynamics and its contribution to spin-transfer torque switching process in magnetic tunnel junctions

Abstract

A practical problem for memory applications involving perpendicularly magnetized magnetic tunnel junctions is the reliability of switching characteristics at high-bias voltage. Often it has been observed that at high-bias, additional error processes are present that cause a decrease in switching probability upon further increase of bias voltage. We identify the main cause of such error-rise process through examination of switching statistics as a function of bias voltage and applied field, and the junction switching dynamics in real time. These experiments show a coincidental onset of error-rise and the presence of a new low-frequency microwave emission well below that dictated by the anisotropy field. We show that in a few-macrospin coupled numerical model, this is consistent with an interface region with concentrated perpendicular anisotropy, and where the magnetic moment has limited exchange coupling to the rest of the layers. These results point to the important role high-frequency interface magnetic moment dynamics play in determining the switching characteristics of these tunnel junction devices.

Resolving Discrepancies in Spin-Torque Ferromagnetic Resonance Measurements: Lineshape vs. Linewidth Analyses

“…Additional evidence for an origin associated with the heavymetal/ferromagnet interface comes from the fact that the large-∆ linewith is very sensitive to applied DC current (see supplemental information), consistent with a very thin and/or low-moment region under the influence of the spin current generated by the heavy metal. Our observations might be related to recent findings from the IBM group of interfacial regions in CoFeB/MgO/CoFeB magnetic tunnel junctions whose dynamics can become partially decoupled from the bulk of the magnetic films [21,22]. The two experiments differ, however, in that the IBM work deduced a difference in effective magnetic anisotropy (compared to the bulk of the magnetic film) for the interfacial layers at CoFeB/MgO interfaces, while in our devices the large-∆ resonance corresponds to an increased damping near a Pt/ferromagnet interface without a large difference in anisotropy.…”

Back hopping in spin transfer torque switching of perpendicularly magnetized tunnel junctions

“…With these accurate torques, high bias was predicted to lead to undesired switching of the fixed layer, which then induces fast perpetual cyclic switching of both the FL and pinned layers, as indeed compatible with some experimental observations [12]. This survey of the studies of back hopping indicates two proposed origins of the phenomenon: the bias dependence of the torques acting on the FL [4][5][6], or the dynamics of the reference layer [9][10][11][12]. In this work, we discriminate between these two scenarios by measuring in a single-shot time-resolved manner the BH in several MTJs having different FLs but sharing a fixed system with layers of identical nominal compositions.…”

“…We therefore turn to the second proposed scenario [2,[9][10][11], which suggested that some "subsystem" within the fixed layers system is destabilized by STT once the FL has switched. This subsystem affects the MTJ resistance and must thus include a switching of at least the SPL.…”

“…Over the years, different generations of PMA-MTJ stacks of been studied: some showed clear BH [9,10,13], some others did not [14][15][16][17]. Since our objective is to understand the phenomenon of BH, we have selected the MTJ stacks where BH is clearly happening.…”

“…An alternative explanation was proposed more recently from time-resolved characterizations [9,10] or short pulseinduced characterizations [2] of the BH phenomenon on systems with perpendicular magnetic anisotropy (PMA). These measurements suggested that the BH was a dynamical process involving magnetic configurations in which the magnetizations of the fixed layers were also destabilized at high voltage biases.…”