Magnetic vortices in nanocaps induced by curvature

Ahmed M Abdelgawad,Mukund Bapna,Hao Chen,Sara A Majetich,Nikhil Nambiar

doi:10.1063/1.5007213

Ahmed M Abdelgawad, Mukund Bapna + Show 3 more

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https://doi.org/10.1063/1.5007213

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Abstract

Magnetic nanoparticles with room temperature remanent magnetic vortices stabilized by their curvature are very intriguing due to their potential use in biomedicine. In the present study, we investigate room temperature magnetic chirality in 100 nm diameter permalloy spherical caps with 10 nm and 30 nm thicknesses. Micromagnetic OOMMF simulations predict the equilibrium spin structure for these caps to form a vortex state. We fabricate the permalloy caps by sputtering permalloy on both close-packed and sparse arrays of polystyrene nanoparticles. Magnetic force microscopy scans show a clear signature of a vortex state in close-packed caps of both 10 nm and 30 nm thicknesses. Alternating gradient magnetometry measurements of the caps are consistent with a remnant vortex state in 30 nm thick caps and a transition to an onion state followed by a vortex state in 10 nm thick caps. Out-of-plane measurements supported by micromagnetic simulations shows that an out-of-plane field can stabilize a vortex state down to a diameter of 15 nm.

Highlights

There has been a growing interest in magnetic chirality at the nanoscale and its potential applications in spintronics and biomedical applications
A magnetic vortex state has been shown to be stable at room temperature in permalloy disks 60 nm in diameter (ø) down to 35 nm in height.[4]
We experimentally demonstrate the existence of a remanent vortex state at room temperature in ø100 nm permalloy caps with 30 nm and 10 nm thicknesses

Summary

Introduction

There has been a growing interest in magnetic chirality at the nanoscale and its potential applications in spintronics and biomedical applications Stabilizing this chirality typically requires symmetry breaking inducing Dzyaloshinskii–Moriya interaction (DMI)[1] or curvature in surfaces or nanoparticles (NPs)[2] that lead to a flux closure and chiral spin patterns. Streubel et al.[6] studied chirality in ø330 nm close-packed permalloy caps and did micromagnetic simulations that predicted the lack of a vortex state in close-packed caps of smaller sizes and that an onion state is preferred

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