Experimental investigation of CoFe$_2$O$_4$ particle size and acoustic properties of magnetic fluid

In this paper magnetic particle size and acoustic properties of the ferrofluid with CoFe$_2$O$_4$
particles are presented. The results acquired from atomic force microscope measurements
will let us set average value for the hydrodynamic diameter of magnetic particles appearing in
CoFe$_2$O$_4$ ferrofluids. The knowledge of these sizes is extremely useful because on the further
stage of examinations it allows us to determine the size of surfactant coating particles. It is
possible by making additional magnetic measurements enabling the size of average magnetic
core contained in the ferrofluid. Also, it allows us to determine in a simple way the thickness
of the surfactant layer. Such information on magnetic particles in a given ferrofluid is very
valuable in the context of the application of the liquids either in technology or in medicine.
Moreover, ultrasonic measurements carried out are providing the valuable information for us
on the subject of the ferrofluid stability exposed to the action of the external magnetic field.
The experimental estimation of the ultrasonic wave absorption coefficient in the function of a
field for a few temperatures allows us to forecast how magnetic particles in a given magnetic
liquid are capable of joining together and creating clusters, and how big aggregates can be
created in this way.