Off-Axis Electron Holography of Self-Assembled Co Nanoparticle Rings
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1026-C18-03
Off-Axis Electron Holography of Self-Assembled Co Nanoparticle Rings Takeshi Kasama1,2, Rafal E. Dunin-Borkowski1,3, Michael R. Scheinfein4, Steven L. Tripp5, Jie Liu5, and Alexander Wei5 1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom 2 Frontier Research System, The Institute of Chemical and Physical Research, Hatoyama, Saitama, 350-0395, Japan 3 Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, DK-2800, Denmark 4 Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada 5 Department of Chemistry, Purdue University, West Lafayette, IN, 47907 ABSTRACT We use off-axis electron holography in the transmission electron microscope (TEM) to study magnetic flux closure (FC) states in self-assembled nanoparticle rings that each contain between five and eleven 25-nm-diameter Co crystals. Electron holograms are acquired at room temperature in zero-field conditions after applying chosen magnetic fields to the samples in situ in the TEM by partially exciting the conventional microscope objective lens. Mean inner potential contributions to the phase shift are determined by turning the samples over, and subsequently subtracted from each recorded phase image to obtain magnetic induction maps. Our results show that most nanoparticle rings form FC remanent magnetic states, and occasionally onion-like states. Although the chiralities (the directions of magnetization) of the FC states are determined by the shapes, sizes and positions of the constituent nanoparticles, reproducible magnetization reversal of each ring can be achieved by using an out-of-plane magnetic field of between 1600 and 2500 Oe.
INTRODUCTION A full understanding of the magnetic properties of small ferromagnetic elements is important for the development of high-density magnetic information storage and random access memory applications. Nanoscale ring-shaped elements are of particular interest because they can support flux-closed (FC) magnetic vortex states that would not be stable in disk-shaped elements of similar size [1]. Both FC and onion-like magnetic states have recently been observed at remanence in Co rings [2-4], with FC states favored over onion-like states at lower diameters and larger thicknesses for diameters of 300-800 nm and thicknesses of 10-50 nm [5]. The magnetic states that can be supported in nanoparticle assemblies [6] depend strongly on the sizes, shapes and positions of the particles [7]. Tripp et al. [8] used off-axis electron holography in the transmission electron microscope (TEM) to show that FC states can be supported in selfassembled rings of 25-nm-diameter Co nanoparticles.
Here, we use electron holography to study the magnetic states and magnetization reversal behaviors of rings of self-assembled Co nanoparticles that contain between five and eleven crystals, following the application of out-of-plane (OOP) magnetic fields.
EXPERIMENTAL DETAILS Cobalt nanoparticles with diameters of 25-30 nm were dispersed in a
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