Light scattering in a magnetically polarizable nanoparticle suspension

We investigate magnetic-field-induced changes on transmitted light intensity in a magnetic disordered phase of iron oxide nanoparticle suspension. We observe a dramatic decrease in the transmitted light intensity at a critical magnetic field. The critical magnetic field follows power-law dependence with the volume fraction of the nanoparticles suggesting a disorder-order structural transition. The light intensity recovers fully when the magnetic field is switched off. We discuss the possible reasons for the reduction in the light intensity under the influence of magnetic field. Among the various mechanisms such as Kerker’s condition for zero forward scattering, Faraday effect, Christiansen effect, photoinduced refractive index mismatch between the two components of the dispersion, etc., the resonances within the magnetic scatterers appear to be the plausible cause for the extinction of light. The circular pattern observed on a screen placed perpendicular to the incident beam confirms the formation of rodlike structures along the direction of propagation of the light.

Figure 1

(Color online) Schematic of the experimental setup. The direction of the applied magnetic field is along the direction of propagation of the beam. P, polarizer; SC, sample cuvette; S, solenoid; M, mirror; PMT, photomultiplier tube; DAS, data acquisition system (for backscattering intensity measurement); and IF, interference filter.

Figure 2

(Color online) Normalized forward transmitted light intensity as a function of applied magnetic field for magnetite-based ferrofluid of different volume fractions. The average particle diameter was $6.7\mathrm{nm}$.

Figure 3

Critical magnetic fields (${\mathrm{H}}_{\mathrm{C}1}$ and ${\mathrm{H}}_{\mathrm{C}2}$) as a function of magnetite volume fraction. The critical fields ${\mathrm{H}}_{\mathrm{C}1}$ and ${\mathrm{H}}_{\mathrm{C}2}$ follow power-law decay with volume fraction $\left({\mathrm{H}}_{\mathrm{C}}\infty {\varphi }^{-\mathrm{x}}\right)$ where the exponents are 0.423 and 0.283, respectively. Reprinted with permission from [53]. Copyright 2008, American Institute of Physics.

Figure 4

Normalized forward and backward scattered light intensity as a function of applied magnetic field for ferrofluid of 0.0067 volume fraction.

Figure 5

(Color online) Normalized transmitted light intensity using vertical $\left({\mathrm{E}}_{\mathrm{V}}\right)$ horizontal $\left({\mathrm{E}}_{\mathrm{H}}\right)$, and circular polarization of light as a function of applied magnetic field $\left({\mathrm{H}}_{\text{applied}}\right)$.

Figure 6

(Color online) Forward scattered patterns from ferrofluids of 0.0067 volume fraction at various magnetic fields: (a) 0, (b) 50, (c) 115, (d) 140, and (e) $200\mathrm{G}$. The transmitted light intensity starts to decrease above $50\mathrm{G}$ and is fully extinct at $115\mathrm{G}$ cabove which the forward scattered light shows a diffused ringlike structure.

Figure 7

(Color online) Scattering anisotropy factor $⟨\cos \theta ⟩$ as a function of size parameter $ka$, for spheres of three different magnetic permeability values 1, 2.7, and 5.

Figure 8

(Color online) Extinction efficiencies $Q_{\mathit{ext}}^E$ as a function of size parameter $ka$ for different incident angles $\alpha$ for an infinite cylinder.

Figure 9

(Color online) (a)–(d) The scattered wave, on a screen placed perpendicular to the incident beam, from a cylindrical needle (a) parallel, (b) 20°, (c) 45°, and (d) 90°.

Figure 10

(Color online) (a) and (b) The scattered waves from the ferrofluid suspension, when the applied magnetic field is (a) parallel and (b) perpendicular to the direction of propagation of the incoming laser light.