Dipolar Magnetism in Ordered and Disordered Low-Dimensional Nanoparticle Assemblies

TLDR

Magnetostatic dipolar interactions between nanoparticles can enable new nanocrystalline magnetic materials, and are strong enough to sustain ambient‑temperature magnetic order in closely‑spaced assemblies with moments ≥100 μB. The study aims to directly quantify how particle topology correlates with magnetic order to enable technological exploitation of quasi‑2D nanoparticle assemblies. Electron holography at sub‑particle resolution was employed to map the correlation between particle arrangement and magnetic order in self‑assembled 1D and quasi‑2D 15‑nm cobalt nanoparticle arrays. Initial states exhibited dipolar ferromagnetism, antiferromagnetism, or local flux closure depending on arrangement, yet after saturation measurements and simulations revealed that overall ferromagnetic order persists even in completely disordered 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.