Size-dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions

TL;DR

This study investigates how the size of perpendicularly magnetized tunnel junctions influences nanosecond-scale spin-torque switching, revealing size-dependent dynamics, variability, and the role of non-uniform stray fields in switching behavior.

Contribution

It provides the first detailed analysis of size-dependent spin-torque switching dynamics in nanoscale tunnel junctions, highlighting the effects of non-uniform stray fields and switching asymmetry.

Findings

Smaller junctions switch faster, with 50 nm devices switching in 3 ns.

Switching pathways are complex and vary with size, showing disorder and multiple reversal paths.

Sub-1 ns switching events are observed in the smallest junctions.

Abstract

We time-resolve the spin-transfer-torque-induced switching in perpendicularly magnetized tunnel junctions of diameters from 50 to 250 nm in the thermally activated regime. When the field and the spin-torque concur to favor the P to AP transition, the reversal yields monotonic resistance ramps that can be interpreted as a domain wall propagation through the device at velocities of 17 to 30 nm/ns; smaller cells switch hence faster. When the field hinders the P to AP transition, the switching is preceded by repetitive switching attempts, during which the resistance transiently increases until successful reversal occurs. At 50 nm, the P to AP switching proceeds reproducibly in 3 ns, with a monotonic increase of the device resistance. In the reverse transition (AP to P), several reversal paths are possible even in the smallest junctions. Besides, the non uniform nature of the response seems still present at nanoscale, with sometimes electrical signatures of strong disorder during the reversal. The AP to P transition is preceded by a strong instability of the AP state in devices above 100 nm. The resistance becomes extremely agitated before switching to P in a path yielding a slow (20-50 ns) irregular increase of the conductance with variability. Unreversed bubbles of 60 nm can persist a few microseconds in the largest junctions. The complexity of the AP to P switching is reduced but not suppressed when the junctions are downsized below 60 nm. The instability of the initial AP state is no longer detected but the other features remain. In the smallest junctions (50 nm) we occasionally observe much faster (sub-1 ns) switching events. We discuss the origin of the switching asymmetry and its size dependence, with an emphasis on the role of the non uniformities of the stray field emanating from the reference layers, which affects the zones in which nucleation is favored.