Effects of array shape and disk ellipticity in dipolar-coupled magnetic metamaterials

Sam D Slöetjes,Ambjørn D Bang,Erik Folven,Alpha T N’Diaye,Rajesh V Chopdekar,Einar S Digernes,Jostein K Grepstad,Anders Strømberg,Fredrik K Olsen

doi:10.1103/physrevb.104.134421

Abstract

Two-dimensional lattices of dipolar-coupled thin film ferromagnetic nanodisks give rise to emergent superferromagnetic (SFM) order when the spacing between dots becomes sufficiently small. In this paper, we define micron-sized arrays of permalloy nanodisks arranged on a hexagonal lattice. The arrays were shaped as hexagons, squares, and rectangles to investigate finite-size effects in the SFM domain structure for such arrays. The resulting domain patterns were examined using x-ray magnetic circular dichroism photoemission electron microscopy. At room temperature, we find these SFM metamaterials to be below their blocking temperature. Distinct differences were found in the magnetic switching characteristics of horizontally and vertically oriented rectangular arrays. The results are corroborated by micromagnetic simulations.

Highlights

Dipolar-coupled magnetic metamaterials have attracted much attention in the past decade, the most prominent example being artificial spin ice [1]
We have previously shown that it is possible to realize such systems in arrays of monodomain nanoscale disks acting as magnetic dipoles [8]
We investigate the magnetic order in finitesized arrays of dipolar-coupled permalloy (Py) nanodisks using x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM)

Summary

INTRODUCTION

Dipolar-coupled magnetic metamaterials have attracted much attention in the past decade, the most prominent example being artificial spin ice [1]. The possibility to tailor the shape of the individual nanomagnets provides an additional handle for control of the magnetic properties on a microscopic scale. Hexagonal arrays of such magnetic dipoles have been extensively studied theoretically. Varón et al [11,12] have investigated the magnetic order in self-assembled aggregates of 15 nm Co nanoparticles They found that formation of longitudinal domain walls (DWs) is energetically favored over transverse DWs for rectangular-shaped arrays. We find that the arrays are below their blocking temperature and prevented from reaching their magnetic ground state. We use micromagnetic simulations that allow for nanomagnet ellipticity, lattice symmetry, and geometric shape of the arrays to reproduce their magnetic switching characteristics

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS