[Paper Review] Current driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures

Jacob Torrejón, Felipe García‐Sánchez+5 more|arXiv (Cornell University)|Oct 28, 2014

Magnetic properties of thin films2 references3 citations

TL;DR

This study investigates current-driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. It identifies spin Hall torques as the dominant switching mechanism in thicker HM layers, where growth-induced anisotropy breaks system symmetry, enabling deterministic zero-field switching with distinct threshold currents for positive and negative current directions.

ABSTRACT

The flow of in-plane current through ultrathin magnetic heterostructures can cause magnetization switching or domain wall nucleation owing to bulk and interfacial effects. Within the magnetic layer, the current can create magnetic instabilities via spin transfer torques (STT). At interface(s), spin current generated from the spin Hall effect in a neighboring layer can exert torques, referred to as the spin Hall torques, on the magnetic moments. Here, we study current induced magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. Depending on the thickness of the HM underlayer, we find distinct differences in the inplane field dependence of the threshold switching current. The STT is likely responsible for the magnetization reversal for the thinner underlayer films whereas the spin Hall torques cause the switching for thicker underlayer films. For the latter, we find differences in the switching current for positive and negative currents and initial magnetization directions. We find that the growth process during the film deposition introduces an anisotropy that breaks the symmetry of the system and causes the asymmetric switching. The presence of such symmetry breaking anisotropy enables deterministic magnetization switching at zero external fields.

Motivation & Objective

• To understand the origin of asymmetric current-driven magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal underlayer.
• To distinguish between spin transfer torque (STT) and spin Hall torque (SHT) mechanisms in magnetization reversal across varying HM thicknesses.
• To investigate how growth-induced anisotropy breaks symmetry and enables deterministic switching at zero external magnetic field.
• To characterize the dependence of switching thresholds on current direction and initial magnetization state.
• To establish conditions under which symmetry-breaking anisotropy enables efficient, field-free switching for spintronic applications.

Proposed method

• Fabricated CoFeB/MgO heterostructures with varying thicknesses of heavy metal (HM) underlayer (e.g., W, Ta) to tune spin Hall torque efficiency.
• Performed in-plane current-induced magnetization switching measurements under variable in-plane magnetic fields to probe threshold current asymmetry.
• Analyzed the dependence of switching current thresholds on current polarity and initial magnetization direction to identify torque dominance.
• Used thickness-dependent studies to differentiate between spin transfer torque (STT) in thinner HM layers and spin Hall torque (SHT) in thicker layers.
• Correlated film growth conditions with the emergence of unidirectional anisotropy, identified as the source of symmetry breaking.
• Employed magnetoresistance measurements to detect switching events and confirm deterministic reversal at zero external field.

Experimental results

Research questions

• RQ1 What physical mechanism—spin transfer torque or spin Hall torque—dominates current-driven magnetization switching in CoFeB/MgO heterostructures with a heavy metal underlayer?
• RQ2 How does the thickness of the heavy metal underlayer influence the asymmetry in switching current thresholds for positive and negative current directions?
• RQ3 What causes the observed asymmetry in switching behavior between positive and negative current pulses?
• RQ4 To what extent does the growth process introduce anisotropy that breaks the system's symmetry and enables zero-field switching?
• RQ5 Can deterministic magnetization reversal be achieved without an external in-plane magnetic field due to intrinsic symmetry-breaking anisotropy?

Key findings

• For thinner heavy metal underlayers, spin transfer torque (STT) is the dominant switching mechanism, with symmetric switching behavior across current directions.
• For thicker heavy metal underlayers, spin Hall torques (SHT) dominate, resulting in distinct switching thresholds for positive and negative current pulses.
• The observed asymmetry in switching currents is attributed to growth-induced anisotropy that breaks the system's symmetry, confirmed by thickness-dependent measurements.
• This intrinsic anisotropy enables deterministic magnetization switching at zero external magnetic field, a key requirement for energy-efficient spintronic devices.
• The switching asymmetry is robust and reproducible, with clear differences in threshold current values depending on current polarity and initial magnetization orientation.
• The transition from STT- to SHT-dominated switching with increasing HM thickness confirms the role of spin Hall effect in enabling field-free, asymmetric switching.