title: Dipolar magnetism in ordered and disordered low-dimensional nanoparticle assemblies.
abstract: Magnetostatic (dipolar) interactions between nanoparticles promise to open new ways to design nanocrystalline magnetic materials and devices if the collective magnetic properties can be controlled at the nanoparticle level. Magnetic dipolar interactions are sufficiently strong to sustain magnetic order at ambient temperature in assemblies of closely-spaced nanoparticles with magnetic moments of ≥ 100 μ(B). Here we use electron holography with sub-particle resolution to reveal the correlation between particle arrangement and magnetic order in self-assembled 1D and quasi-2D arrangements of 15 nm cobalt nanoparticles. In the initial states, we observe dipolar ferromagnetism, antiferromagnetism and local flux closure, depending on the particle arrangement. Surprisingly, after magnetic saturation, measurements and numerical simulations show that overall ferromagnetic order exists in the present nanoparticle assemblies even when their arrangement is completely disordered. Such direct quantification of the correlation between topological and magnetic order is essential for the technological exploitation of magnetic quasi-2D nanoparticle assemblies.
authors: Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
authors: M
authors: M
authors: Varón
authors: M
authors: M
authors: Beleggia
authors: T
authors: T
authors: Kasama
authors: R J
authors: RJ
authors: Harrison
authors: R E
authors: RE
authors: Dunin-Borkowski
authors: V F
authors: VF
authors: Puntes
authors: C
authors: C
authors: Frandsen
mesh_terms: D003035:Cobalt / Q000737:chemistry; D060526:Magnetic Fields; D008280:Magnetics; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure
chemical_list: D003035:Cobalt
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