Nonuniform switching of the perpendicular magnetization in a spin-torque-driven magnetic nanopillar

Abstract

International audienceTime-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. Spin-polarized current-induced magnetization switching (CIMS) has now been reported in many experimental works involving a wide variety of geometries including point contacts, nanopillars (spin valves or tunnel junctions), and nanowires with or without notches. 1 These systems are extensively studied, in part, because they hold the potential for applications in spin-transfer magnetic random access memory. 2 Interest in materials with perpendicular magnetic anisotropy (PMA) has grown considerably as a pathway for lowering the critical current required to switch the magnetization while maintaining thermal stability as compared with in-plane systems. 3,4 This interest has, to a large degree, stemmed from calculations of the switching behavior using a macrospin approximation.

State diagram of nanopillar spin valves with perpendicular magnetic anisotropy

“…Indeed it was shown that the formation of domains and domain walls could be observed [11-13, 92, 93] and that it was influencing the slow magnetization switching dynamics. It influences fast dynamics as well as [54,55,91,92].…”

Time-resolved spin-torque switching in MgO-based perpendicularly magnetized tunnel junctions

“…After the discovery of STT, it was soon realized 4,5 that the cylindrical symmetry of the magnetic properties in Perpendicular Magnetic Anisotropy (PMA) systems and the resilience to thermal fluctuations that the anisotropy provides would make PMA systems ideal playgrounds to explore STT-induced dynamics. However MTJs with relevant properties became available only a decade after 6 and relied on ultrathin systems where strong interfacial effects can be present 7 ; besides, efficient spin-torque generation requires complex embedding stacks 8,9 in which each additional layers can be a fluctuator strongly coupled to the layer of main interest in a non uniform 8,10 and non local 11 manner. As a consequence the STT-induced magnetization switching in PMA MTJ systems exhibits rich features 12,13 that deserve to be studied, especially as it opens opportunities in information technologies.…”

Time-resolved magnetic relaxation of a nanomagnet on subnanosecond time scales

“…In fitting our data to this model we obtain a dimensionless energy barrier of ξ = 450, which is within 25% of that expected based on the volume, magnetization and anisotropy of the nanomagnet (ξ = M s H k V /(2kT ) = 360). Our previous results, as well as those of many other groups, have found energy barriers to reversal for long field [20][21][22][23] and long current pulses (> 100 ns) 9,10 to be only a small fraction of that expected for a macrospin, suggesting the reversal proceeds by subvolume nucleation 24,25 . For similar samples, we found that the energy barrier to reversal under long current pulses to be ≃ 60 k B T 9 , 1/6 the macrospin value.…”