Residual attractive force between superparamagnetic nanoparticles

TL;DR

This paper demonstrates that superparamagnetic nanoparticles experience a residual attractive force akin to van der Waals forces, despite lacking a permanent magnetic moment, which has implications for various nanotechnology applications.

Contribution

The study reveals a residual magnetic attraction between superparamagnetic nanoparticles, analogous to van der Waals forces, even above their blocking temperature.

Findings

Superparamagnetic nanoparticles exhibit a net attractive force despite no permanent magnetic moment.

The force is similar in nature to van der Waals interactions between molecules.

This residual force could influence nanoparticle behavior in ferrofluids and biomedical applications.

Abstract

A superparamagnetic nanoparticle (SPN) is a nanometre-sized piece of a material that would, in bulk, be a permanent magnet. In the SPN the individual atomic spins are aligned via Pauli effects into a single giant moment that has easy orientations set by shape or magnetocrystalline anisotropy. Above a size-dependent blocking temperature $T_{b}(V,\tau_{obs})$, thermal fluctuations destroy the average moment by flipping the giant spin between easy orientations at a rate that is rapid on the scale of the observation time $\tau_{obs}$. We show that, depite the vanising of the average moment, two SPNs experience a net attractive force of magnetic origin, analogous to the van der Waals force between molecules that lack a permanent electric dipole. This could be relevant for ferrofluids, for the clumping of SPNs used for drug delivery, and for ultra-dense magnetic recording media.