Ground state microstructure of a ferrofluid thin layer
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AND LIQUIDS
Ground State Microstructure of a Ferrofluid Thin Layer1 T. A. Prokopievaa, V. A. Danilova, and S. S. Kantorovicha,b a
Ural State University, Leninskii prosp. 51, Yekaterinburg, 620000 Russia Universität Stuttgart, Institute for Computational Physics, Pfaffenwaldring 27, 70569 Stuttgart, Germany email: [email protected]stuttgart.de
b
Received February 10, 2011
Abstract—Using a fine weave of theoretical analysis and computer simulations, we found various aggregates of magnetic singledomain nanoparticles, which can form in a quasitwodimensional (q2D) ferrofluid layer at low temperatures. Our theoretical investigation allowed us to obtain exact expressions and their asymptotes for the energies of each configuration. Thus, for ferrofluid q2D layers it proved possible to identify the ground states as a function of the particle number, size, and other system parameters. Our suggested approach can be used for the investigation of ground state structures in systems with more complex interparticle interactions. DOI: 10.1134/S1063776111100062 1
1. INTRODUCTION
Magnetic fluids (also called ferrofluids), i.e., sys tems of magnetic particles dispersed in nonmagnetic carriers, were synthesized in the early 1960s. Because of the small size (typically on the order of 10 nm), magnetic particles are subjected to Brownian motion, which prevents the systems from irreversible aggrega tion and coagulation. Each particle has its magnetic moment, the size and the orientation of which are determined by the internal structure of the particle and its spatial orientation. An external magnetic field, if applied, tends to orient the particles along its direc tion, which usually leads to changes in the macroprop erties of the system. It is the unique combination of fluidity and the ability to significantly interact with an external magnetic field that provides the basis for fer rofluid applications: magnetic seals [1], ink for print ing, loudspeakers, [2], the hyperthermia cancer treat ment method [3, 4], targeted drug delivery [5], and many others. To ensure effective applications of mag netic fluids, a fundamental understanding of their internal structure is necessary. The goal of this work is to study the ground state of a thin layer of magnetic fluid, which is a logical contin uation of a series of studies on the influence of cluster formation and magnetic correlations on the micro and macroproperties of magnetic nanoparticle sys tems [6–12]. It should be noted that the theory of magnetic fluids has been actively developed for over 40 years. While in the first theoretical models it was assumed that the ferrofluid was an ideal superpara magnetic gas, the magnetization of which obeys the 1 The article was translated by the authors.
Langevin law [13, 14], due to the accumulated exper imental data on the magnetic properties of ferrocol loids [15], it soon proved that the strong interparticle interactions in the system must without fail be taken into account. The first attempts to directly introduce a Weiss molecu
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