Self-assembly of Dipolar Crystals from Magnetic Colloids

TL;DR

This study uses molecular dynamics simulations to explore how magnetic colloids self-assemble into ordered crystals, revealing the influence of dipole strength and cooling protocols on crystal structure formation.

Contribution

It demonstrates a novel spontaneous transition mechanism for crystallization in magnetic colloids and details how dipole strength affects resulting crystal structures.

Findings

Weak dipoles form FCC crystals with chain alignment.

Higher dipoles lead to BCO structures due to chain compression.

Crystallization occurs via a spontaneous transition, not nucleation.

Abstract

We study the self-assembly of magnetic colloids using the Stockmayer (SM) model characterized by short-range Lennard-Jones interactions and long-range dipole-dipole interactions. Using molecular dynamics simulations, we design cooling protocols that yield perfectly assembled single-domain magnetic crystals. We identify cooling rates at which the system transforms from an amorphous glass to a crystal, where magnetic ordering promotes crystalline order. Remarkably, we observe that the latter develops via a spontaneous transition rather than through the traditional nucleation and growth mechanism. For a weakly dipolar fluid ($\mu=1$), this self-assembly results in a face-centered cubic (FCC) colloidal crystal with dipole moments chained along the (111) direction. For fluids with higher dipole moment ($\mu = 2.5$), the crystal structure shifts towards a body-centered orthorhombic (BCO) arrangement due to the compression of chains from strong dipolar attractions. These results provide valuable insights into the mechanisms driving crystallization in magnetic fluids, opening new avenues for understanding the formation of magnetically responsive colloidal magnetic crystals with promising applications.