[Paper Review] External-Field-Free Spin Hall Switching of Perpendicular Magnetic Nanopillar with a Dipole-Coupled Composite Structure

Zhengyang Zhao, Angeline Klemm Smith+2 more|arXiv (Cornell University)|Feb 8, 2016

Magnetic properties of thin films25 references14 citations

TL;DR

This paper proposes a dipole-coupled composite structure that enables external-field-free spin Hall switching of perpendicular magnetic nanopillars by leveraging a local in-plane magnetic layer to generate a compensating dipolar field. The method achieves robust, field-free switching with a large in-plane compensation field of ~135 Oe and an out-of-plane loop shift of ~45 Oe per 10⁷ A cm⁻², validated by experiments and micromagnetic simulations in both rectangular and 10×10 nm² circular pillars.

ABSTRACT

Spin Hall effect (SHE) induced reversal of perpendicular magnetization has attracted significant interest, due to its potential to lead to low power memory and logic devices. However, the switching requires an assisted in-plane magnetic field, which hampers its practical applications. Here, we introduce a novel approach for external-field-free spin Hall switching of a perpendicular nanomagnet by utilizing a local dipolar field arising from an adjacent in-plane magnetic layer. Robust switching of perpendicular CoFeB nanopillars in a dipole-coupled composite stack is experimentally demonstrated in the absence of any external magnetic field, in consistent with the results of micromagnetic simulation. Large in-plane compensation field of about 135 Oe and out-of-plane loop shift of about 45 Oe / 10 7 A cm-2 are obtained in the nanopillar devices with composite structure. By performing micromagnetic simulations, we confirm the composite external-field-free switching strategy can also work for a 10 x 10 nm2 circular pillar. Compared with other proposed methods for external-field-free spin Hall switching of perpendicular magnetization, the dipole-coupled composite structure is compatible with a wide range of spin Hall systems and perpendicular magnetic tunnel junctions, paving the way towards practical SHE-based MRAM and logic applications.

Motivation & Objective

• To overcome the need for an external in-plane magnetic field in spin Hall effect (SHE)-induced switching of perpendicular magnetic nanostructures.
• To develop a practical, field-free switching mechanism compatible with existing spin Hall and magnetic tunnel junction technologies.
• To demonstrate robust, reproducible switching in perpendicular CoFeB nanopillars without any applied external magnetic field.
• To validate the approach through both experimental measurements and micromagnetic simulations across different geometries.

Proposed method

• Integration of a perpendicular CoFeB magnetic nanopillar with an adjacent in-plane magnetic layer to form a dipole-coupled composite structure.
• Utilization of the dipolar field from the in-plane layer to compensate for the net magnetic anisotropy field, enabling field-free switching.
• Application of spin Hall currents via a heavy metal layer (e.g., W or Pt) to induce spin-orbit torque in the perpendicular magnetic layer.
• Measurement of magnetic hysteresis loops under varying current densities to quantify switching behavior and loop shifts.
• Conducting micromagnetic simulations to model and confirm the switching dynamics in both rectangular and circular pillar geometries.
• Use of a 10×10 nm² circular pillar model to demonstrate scalability and robustness of the field-free switching mechanism.

Experimental results

Research questions

• RQ1 Can external-field-free spin Hall switching be achieved in perpendicular magnetic nanopillars using a dipole-coupled composite structure?
• RQ2 What is the magnitude of the in-plane compensation field and out-of-plane loop shift achievable in such a system?
• RQ3 How does the dipole-coupled structure enable robust switching without any external magnetic field?
• RQ4 Is the proposed mechanism scalable to smaller device dimensions, such as 10×10 nm² pillars?
• RQ5 How well do micromagnetic simulations reproduce the experimental switching behavior?

Key findings

• Robust external-field-free switching of perpendicular CoFeB nanopillars is experimentally demonstrated using the dipole-coupled composite structure.
• A large in-plane compensation field of approximately 135 Oe is achieved, effectively counteracting the net anisotropy field.
• An out-of-plane loop shift of about 45 Oe per 10⁷ A cm⁻² is measured, indicating strong spin Hall torque efficiency.
• Micromagnetic simulations confirm the feasibility of the method for a 10×10 nm² circular magnetic pillar, indicating scalability.
• The composite structure is compatible with a wide range of spin Hall materials and perpendicular magnetic tunnel junctions, enhancing practical integration potential.
• The results show consistent agreement between experimental data and simulation, validating the proposed switching mechanism.