Self - assembly of magnetic

We use Monte Carlo and quaternion molecular dynamics simulations to study the self-assembly of intriguing structures which form in colloidal sus-pensions of small magnetite particles. We show that the only stable isomers with few particles, a ring and a chain, can be eeciently interconverted using a magnetizable tip. We propose to use the oscillating dipole eld of the tip to locally anneal the aggregates to either a ring in zero eld or a chain in nonzero applied eld. 1 Given the present advanced stage of miniaturization, the most promising way to sig-niicantly reduce the dimension of devices involves a transition from micro-manufacturing to self-assembly of nanostructures 1]. Inspired by the richness of structures observed in aggregates of magnetic nanoparticles 2,3] and the possibility of their structural transformation 4,5], we propose a hybrid thermodynamic self-assembly technique capable of producing magnetic patterns of unprecedented packing density 6]. The key ingredients are a system of magnetic nanoparticles in a colloidal suspension, resonant magnetic heating on the nanome-ter scale that we postulate, and the possibility to manipulate individual nanostructures using a local magnetic eld. In the following, we prove our technique to work using realistic Monte Carlo and Molecular Dynamics simulations addressing the self-assembly, the eld-assisted interconversion, and the long-term stability of the magnetic nanostructures. In the following, we will describe microscopically the structural and magnetic transitions in microcanonical and canonical ensembles of few magnetic particles. Commercially available spherical nanoparticles of magnetite are covered by a thin surfactant layer to inhibit irreversible coalescence in a viscous liquid at room temperature 2]. Such colloidal suspensions, called ferroouids, have recently become a focus of experimental and theoretical attention due to their interesting behavior in applied magnetic elds 2,3,7{12]. We will discuss the eeect of eld and temperature on the stability of the individual isomers, which { for few particles { are known to be either a chain or a ring 4]. More important, we will show how to locally modify their equilibrium structure by changing the eld and temperature (assembly of nanostructures) and how to distinguish magnetically between the diierent isomers (detection of nanostructures). Our model system consists of six 13] spherical magnetite particles with a diameter = 200 A, mass m = 1:3110 7 amu, inertia I = 5:2510 7 amu A 2 , and a large permanent magnetic moment 0 = 1:6810 5 B. The potential energy of this system in an external magnetic …