Effects of pulse magnetic field on the optical transmission properties of thin ferrofluid (FF) layers were experimentally investigated. It was observed that, under an influence of an external uniform magnetic field, pulses applied to the samples surfaces in normal direction decrease the optical transmission with further returning it to its original state, even before the end of the field pulse. The dependencies of the observed effects on the magnetic pulse magnitude and the samples thickness were investigated. The experimental results are explained using FF columnar aggregates growth and lateral coalescence under influence of a magnetic field, leading to a light scattering type Rayleigh-to-Mie transition. Further evolution of this process comes to a geometrical optics scale and respective macroscopic observable opaque FF columnar aggregates emergence. These changes of optical transmission are non-monotonic during the magnetic field pulse duration with minimal value in the case of Mie scattering, which is known as a magneto-optical extinction trend inversion. The residual inversion was detected after the external magnetic field pulse falling edge. Using molecular dynamics simulation, we showed that a homogeneous external magnetic field is enough for the formation of columnar aggregates and their fusion. The results clarify the known Li theory (Li et al., J. Phys. D: Appl. Phys. 37 (2004) 3357, and Sci. Technol. Adv. Mate. 8 (2007) 448), implying an inhomogeneous field as a required prerequisite for the magneto-optical extinction trend inversion phenomenon
The phase transition between a massive dense phase and a diluted superparamagnetic phase has been studied by means of a direct molecular dynamics simulation. The equilibrium structures of the ferrofluid aggregate nucleus are obtained for different values of a temperature and an external magnetic field magnitude. An approximate match of experiment and simulation has been shown for the ferrofluid phase diagram coordinates "field-temperature". The provided phase coexistence curve has an opposite trend comparing to some of known theoretical results. This contradiction has been discussed. For given experimental parameters, it has been concluded that the present results describe more precisely the transition from linear chains to a dense globes phase. The theoretical concepts which provide the opposite binodal curve dependency trend match other experimental conditions: a diluted ferrofluid, a high particle coating rate, a high temperature, and/or a less particles coupling constant value.
The optical transmission of a thin ferrofluid layer was investigated at various optical radiation wavelengths. The turning on of the durable external magnetic field pulse leads to nonmonotonic changes of the optical transmission value with minimal value during the field pulse. This phenomenon is related to the formation of columnar nanoparticle aggregates and transformation in the ferrofluid bulk. It was shown that time interval corresponding to the optical transmission minimum is proportional to the laser wavelength, which can be explained with Mie-like optical extinction on the ferrofluid aggregates and its dependence on the diameters of columnar aggregates. Hence, a simple experimental approach was proposed to measure and control the ferrofluid aggregates diameters in submicron spatial dimension ranges. Particularly, this approach could be used for the formation of composite nanomaterials consisting of polymers and magnetic nanoparticles with controlled structural parameters. These materials could be reused after parameters changes (e.g., lattice constant, aggregate size, and magnetic permeability tensor) with a heating/cooling cycle without the need for preparation of a new material from scratch.
Досліджено особливості оптичного пропускання тонким шаром магнетної рідини (МР) у зовнішньому магнетному полі. Спостережений ефект інвер-сії напрямку оптичної екстинкції (ІНОЕ) виникає через деякий час після вмикання магнетного поля та його вимикання. Час настання ІНОЕ зале-жить від величини амплітуди поля та від довжини хвилі зондувального оптичного випромінення. Існування ІНОЕ пов'язується з динамікою тран-сформування (утворення або руйнування) кластерів магнетних наночас-тинок у МР під дією магнетного поля: в момент часу, коли розмір D клас-тера, утвореного з магнетних наночастинок МР, стає співмірним із дов-жиною хвилі (D 1 , де -деякий безрозмірний коефіцієнт), сумарна дія розсіяння та поглинання оптичного випромінення стає максимальною (момент виникнення ІНОЕ), а потім починає зменшуватися. Обговорення одержаних експериментальних результатів здійснюється на основі за-пропонованого моделю формування та трансформування кластерів магне-тних наночастинок у МР під дією зовнішнього магнетного поля, важли-вим елементом якої є латеральна аґреґація ланцюжкових кластерів маг-нетних наночастинок. Проведено аналізу практичної значущости одер-
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