[Paper Review] Giant spin Hall magnetoresistance in metallic bilayers

Junyeon Kim, Sheng Peng+3 more|arXiv (Cornell University)|Mar 31, 2015

Magnetic properties of thin films37 references3 citations

TL;DR

This study reports a giant spin Hall magnetoresistance (SMR) in tungsten/CoFeB metallic bilayers, achieving nearly tenfold enhancement over previous heavy metal/magnetic insulator systems. The effect arises from spin current generated via the spin Hall effect at the interface, which significantly modulates transverse resistance, demonstrating SMR as a powerful probe for spin current dynamics in metallic heterostructures.

ABSTRACT

The spin Hall effect in heavy metals generates spin current that enables control of magnetization of an nearby magnetic layer. Conversely, such flow of spin current can influence the transport properties of the system itself. For example, the resistance of a Pt layer attached to a magnetic insulator changes as the direction of the magnetization is changed. Such effect, now commonly referred to as the spin Hall magnetoresistance (SMR), has been reported to be, however, order of magnitude smaller than the conventional anisotropic magnetoresistance in magnetic materials. Here we report giant spin Hall magnetoresistance in a metallic bilayer system. We find nearly a ten-fold increase of SMR in W/CoFeB compared to the previously studied heavy metal/magnetic insulator systems. The large SMR accounts for the unconventionally large transverse magnetoresistance found in this system, manifesting the profound effect of spin current generated via the spin Hall effect on the electrical transport properties. We show that the SMR is defined at the heavy metal/magnetic layer interface even for metallic heterostructures in which current flows into the magnetic layer. These results demonstrate that SMR can be used as a powerful tool to evaluate the flow and accumulation of spin current in metallic heterostructures.

Motivation & Objective

• To investigate the magnitude and origin of spin Hall magnetoresistance (SMR) in metallic bilayer systems.
• To understand why SMR is significantly enhanced in W/CoFeB compared to conventional heavy metal/magnetic insulator heterostructures.
• To demonstrate that SMR can serve as a quantitative probe for spin current flow and accumulation in metallic heterostructures.
• To clarify the role of the heavy metal/magnetic layer interface in generating measurable SMR effects.

Proposed method

• Measurement of transverse magnetoresistance in W/CoFeB bilayer structures under varying magnetization directions.
• Use of tungsten (W) as the heavy metal to induce strong spin Hall effect and generate spin current.
• Comparison of SMR response in metallic CoFeB layer with previously studied heavy metal/magnetic insulator systems.
• Analysis of interface-driven SMR contributions despite current flow into the magnetic layer.
• Systematic variation of magnetization orientation to isolate the spin Hall magnetoresistance effect from other anisotropic transport contributions.
• Evaluation of spin current accumulation at the interface through resistance modulation.

Experimental results

Research questions

• RQ1 What is the magnitude of spin Hall magnetoresistance in W/CoFeB metallic bilayers compared to prior systems?
• RQ2 How does the spin Hall effect influence electrical transport in metallic heterostructures with ferromagnetic layers?
• RQ3 To what extent is the SMR effect localized at the heavy metal/magnetic layer interface in metallic systems?
• RQ4 Why is the transverse magnetoresistance in this system significantly larger than expected from conventional mechanisms?
• RQ5 Can SMR be used as a reliable probe for spin current flow and accumulation in metallic heterostructures?

Key findings

• The spin Hall magnetoresistance in W/CoFeB bilayers is nearly ten times larger than in previously studied heavy metal/magnetic insulator systems.
• The giant SMR arises from spin current generated via the spin Hall effect at the W/CoFeB interface.
• SMR persists even when current flows into the magnetic layer, indicating interfacial origin of the effect.
• The observed transverse magnetoresistance is dominantly attributed to the SMR effect, not conventional anisotropic magnetoresistance.
• The SMR is defined at the heavy metal/magnetic layer interface, confirming its role as a sensitive probe for spin current dynamics.
• The results establish SMR as a powerful tool for evaluating spin current flow and accumulation in metallic spintronic heterostructures.