Structure of ferrofluid nanofilms in homogeneous magnetic fields

We report molecular dynamics simulations results for model ferrofluid films subject to an external, homogeneous magnetic field directed parallel or perpendicular to the film surfaces. The interactions between the magnetic nanoparticles are modeled via the Stockmayer potential. In a previous study [J. Jordanovic and S. H. L. Klapp, Phys. Rev. Lett. 101, 038302 (2008)] we have shown that an external field can control the number and internal structure of the layers characterizing the fluid films, in qualitative agreement with experiments. Here we explore the dependence of the layering effects on thermodynamic conditions, and we analyze the results from an energetic (microscopic and macroscopic) perspective. As a special case we investigate a monolayer to bilayer transition induced via a perpendicular field.

Figure 1

Global order parameter as a function of the external field at ${\rho }^{*}=0.6$. Simulation and MF results for the confined $(L_z^{*}=5.0)$ and bulk system are denoted by symbols and lines, respectively. The inset shows the behavior of $P_1$ for small fields on an expanded scale.

Figure 2

Simulation data for the total dipolar energy $U_{\mathrm{DD}}∕N$ at ${\rho }^{*}=0.6$, $L_z^{*}=5.0$, and two directions of the external field. Included are results for the bulk system.

Figure 3

(a) Global order parameter as function of the field strength at different densities $(L_z^{*}=5.0)$. Parts (b) and (c) show the density and magnetization profiles at ${\rho }^{*}=0.4$. For comparison, (c) includes data at $H_z^{*}=7.4$ and ${\rho }^{*}=0.6$ (see bottom).

Figure 4

Density profiles at ${\rho }^{*}=0.6$, $H_z^{*}=10$, and different dipolar coupling parameters $\lambda$. The inset shows corresponding magnetization profiles.

Figure 5

Interlayer- and intralayer correlation functions at (a) ${\rho }^{*}=0.4$ and (b) ${\rho }^{*}=0.8$ in zero field and in a perpendicular field. Top, in-plane (contact layer 1); middle, interlayer 1 vs 2; bottom, interlayer 1 vs 3. Included are sketches of the particle arrangements as suggested by the correlations. The inset in the top part of (b) additionally shows the density profile for ${\rho }^{*}=0.8$ at zero and perpendicular field.

Figure 6

Density profiles (a) and correlation functions (b) and (c) at $L_z^{*}=2.0$ in zero field (monolayer) and in a perpendicular field (bilayer). The overall density is ${\rho }^{*}=0.4$.

Figure 7

“Snapshots” of a system at $L_z^{*}=2.0$ $({\rho }^{*}=0.4)$ in zero field (a) and at $H_z^{*}=74$ (b) and (c).

Figure 8

(a) Density profile and (b) local magnetization at ${\rho }^{*}=0.8$, $L_z^{*}=5.0$, and various strength of the parallel field.

Figure 9

(a) Intralayer correlation function in contact layer 1 and (b) interlayer correlations between layers 1 and 2 in zero field and in a strong parallel field (${\rho }^{*}=0.8$, $L_z^{*}=5.0$).