Electron Holography of Ferromagnetic Nanoparticles Encapsulated in Three-Dimensional Arrays of Aligned Carbon Nanotubes
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Electron Holography of Ferromagnetic Nanoparticles Encapsulated in Three-Dimensional Arrays of Aligned Carbon Nanotubes Krzysztof K. Koziol1, Takeshi Kasama1,2, Rafal E. Dunin-Borkowski1, Prabeer Barpanda1, and Alan H. Windle1 1 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom 2 The Institute of Physical and Chemical Research, Frontier Research System, Hatoyama, Saitama, 350-0395, Japan
ABSTRACT Closely-spaced ferromagnetic nanoparticles are of interest for applications that include data storage, magnetic imaging and drug delivery. Here, we use off-axis electron holography and micromagnetic simulations to study the magnetic properties of iron nanoparticles encapsulated in three-dimensional arrays of carbon nanotubes. The nanotubes constrain the shapes, sizes and separations of the nanoparticles, as well protecting them from oxidation. We record magnetic induction maps from individual particles that each contain a single magnetic domain. We also discuss the use of electron holography to assess magnetostatic interactions between adjacent particles. INTRODUCTION Since their discovery [1, 2], carbon nanotubes have attracted considerable interest as a result of their outstanding mechanical, electronic and thermal properties. Here, we use off-axis electron holography [3, 4] in the transmission electron microscope (TEM) to characterise the magnetic properties of carbon nanotubes that have been synthesised in highly-aligned arrays using chemical vapour deposition (CVD) and contain closely-spaced ferromagnetic iron catalyst particles. We compare our measurements with magnetic induction maps determined from micromagnetic simulations. Our results provide high spatial resolution quantitative magnetic information that cannot be obtained using techniques such as magnetic force microscopy (MFM) [5] and magnetometry. The latter techniques have previously been applied to the characterization of the magnetic properties of carbon-related materials [6] and metallic particles formed within [7, 8] and on the surfaces of [9, 10] carbon nanotubes. EXPERIMENTAL DETAILS Carbon nanotubes were synthesised by CVD on quartz substrates from a toluene/ferrocene feedstock solution that decomposes into hydrocarbon species and metal particles. Highly-aligned nanotubes grow from the particles perpendicular to the substrate (Fig. 1), with excess iron encapsulated in the nanotubes. The encapsulated particles are typically spaced few hundred nanometres apart and approximately ellipsoidal in shape. Both the diameter
of the nanotubes and the spacing of the particles can be controlled by tuning the CVD process [11]. Off-axis electron holograms of individual carbon nanotubes were acquired at 300 kV using a Philips CM300 field emission gun TEM. The technique involves using an electron biprism to overlap an electron wave that has passed through the region of interest on the specimen with a reference wave that has passed only through vacuum. The resulting holographic int
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