On the possibility of structural and magnetic ordering in magnetic colloids

Abstract

Data on the formation of periodic and labyrinth structures in thin layers of a kerosene-based magnetic fluid (MF) under the action of electric and magnetic fields is given in [1,2]. In this paper, we analyze the possible reasons and mechanisms of the observed phenomenon and present new experimental results concerning the structurization processes in the MF. The total set of the existing data enables us to state that the magnetic ordering of single-domain particles in aggregates is possible. EXPERIMENTAL METHOD AND OBJECT OF RESEARCHTo study the structures formed and their transformation under the effect of various factors, we used the diffraction scattering of a He-Ne laser beam perpendicular to a thin layer of the MF. The scheme of the experimental setup is thoroughly described in [2]. The intensity of the scattered light was measured by a photoelectric detector (type FES-25) with a diaphragm or by a miniature photoresistor, depending on the actual aims of the experiment. The measuring cell consisted of two rectangular glass plates with a transparent conducting coating. A fluoroplastic film with a circular hole in the center was placed between the glass plates; this hole was filled with the MF in question. The thickness of the MF layer amounted to 20-40 µ m and was determined by the thickness of the fluoroplastic films. To generate the electric field, voltage from a stabilized d.c. source was applied to the plates. As the magnetizing system, we used Helmholtz coils generating a uniform field in a volume several times larger than that of the cuvette. The same cuvette was also used for visual optical-microscopic observation of the processes in the MF. To study one of the structure-sensitive parameters of the MF, its magnetic susceptibility, the bridge method was used.The magnetic susceptibility was calculated according to the change in the inductance of the measuring solenoid caused by the sample placed into it. The inductance was measured by an LCR-817 immittance meter at a measuring field frequency of 70 Hz; the error did not exceed 0.1%. The object of study was a kerosenebased MF with magnetite particles stabilized by oleic acid. The volume concentration of the dispersed phase was 11%; the dynamic viscosity amounted to 5 × 10 -3 Pa s. EXPERIMENTAL RESULTS AND DISCUSSIONEarlier studies [2] showed that periodic structural lattices can be formed in thin MF layers in an electric field in the near-electrode space and can be transformed following a change in the external conditions. The formation of structural lattices was observed when a certain (critical) voltage at the electrodes was reached. Its value depended on the temperature and the intensity of the applied magnetic field. It was concluded that the structural transformations can be interpreted as phase transitions in disperse systems. Such an interpretation of structural transformations in MFs was earlier proposed in many studies (e.g., see [3][4][5]).Meanwhile, our experimental investigations showed that stabilized kerosene-based MFs are sufficiently stable ...

On magnetic ordering in colloids of single-domain particles

“…Note also that the high value of the magnetic suscepti bility of colloids with coarse disperse particles may be associated not only with their size [7] but also with the presence of magnetized aggregates in them. It turned out that magnetic fluids containing mag netized aggregates may serve as efficient magnetosen sitive media for visualization of magnetic records and magnetic head fields; in addition, they are promising for flaw detection [14]. Visualization of records on magnetic disks makes it possible to evaluate the record quality under a microscope or from a diffraction pat tern formed when a laser beam reflects from the sur face of the disk.…”

“…One can therefore assume that structural objects made up of colloid particles may keep magnetization resulting from particle-particle interactions and under the action of external fields. A well developed system of magnetized aggregates in a kerosene based magnetic fluid (MF) after long term storage was observed in [8], and the technique and conditions for preparing such aggregates were described in [10]. In this work, some results of investigation of these aggregates and MFs in which they arise are reported.…”

The Influence of Magnetic Fields on Electrophoretic Processes in Magnetic Colloids with Different Stabilization Mechanisms

“…The calculation of the potential curve of the interaction of colloidal particles, taking into account these expressions for a magnetic fluid based on kerosene, was previously carried out in ref. [69]. It has two minima and a potential barrier that are characteristic of the interaction of colloidal particles (Figure 17).…”

Experimental and Theoretical Study of an Autowave Process in a Magnetic Fluid

“…The structure of the layer is such that it "retains" the ions injected from the electrode, as a result of which the charge sign of the particles in the layer changes, and an autowave process is initiated. Due to the close arrangement of single-domain particles in the layer to each other, the possibility of long-range magnetic order arises, as shown in [40,41] and the references. The magnetic interaction of the particles contributes to the stability of the layer.…”