Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions

TL;DR

This paper theoretically analyzes the roles of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions, finding that the out-of-plane torque mainly affects precession but not switching efficiency.

Contribution

It provides a theoretical framework showing the out-of-plane torque's limited role in switching, supported by macrospin simulations and experimental data interpretation.

Findings

Out-of-plane torque influences precession frequency but not switching time or current.

In-plane torque primarily determines switching behavior.

Theoretical results align with experimental voltage-field switching diagrams.

Abstract

The relative contributions of in-plane (damping-like) and out-of-plane (field-like) spin-transfer-torques in the magnetization switching of out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically analyzed using the transformed Landau-Lifshitz (LL) equation with the STT terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics, the out-of-plane torque influences the precession frequency but it does not contribute significantly to the STT switching process (in particular to the switching time and switching current density), which is mostly determined by the in-plane STT contribution. This conclusion is confirmed by finite temperature and finite writing pulse macrospin simulations of the current-field switching diagrams. It contrasts with the case of STT-switching in in-plane magnetized MTJ in which the field-like term also influences the switching critical current. This theoretical analysis was successfully applied to the interpretation of voltage-field STT switching diagrams experimentally measured on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like behavior. The physical nonequivalence of Landau and Gilbert dissipation terms in presence of STT-induced dynamics is also discussed.