ABSTRACT
A model is developed to describe the oscillations of optical anisotropy induced in a viscoelastic ferrocolloid (nanodispersion of magnetic particles) by an AC magnetic field. The viscoelasticity of the matrix (carrier medium) is assumed to obey the Jeffreys rheological scheme, whose advantage is that with the aid of just two viscous parameters and a single one for elasticity it enables one to vary the retarded mechanical response of the carrier from a weakly Maxwellian fluid to a medium with the rheology of a Kelvin gel. As the orientational motion of the particles driven by the AC field is always strongly affected by thermal motion, the occurring process is described with the aid of a kinetic (Fokker-Planck type) equation that combines diffusional and drift terms. On this basis, an exact evolution equation for the macroscopic optical anisotropy of a ferrocolloid is derived that is, however, just one link in an infinite chain of equations for statistical moments. The solution is obtained by applying effective field approximation: reducing the number of moment equations to their minimum and closing the chosen set. This solution is substituted to the scheme of a standard polarimetric set-up, and it is demonstrated how the peculiarities imparted by viscoelasticity should manifest themselves on the intensity of the light transmitted through the set up containing a ferrocolloid sample. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.
ABSTRACT
This work describes the use of mesoporous SBA-15 silicas as hard templates for the size-controlled synthesis of oxide nanoparticles, with the pores acting as nanoscale reactors. This fundamental work is mainly aimed at understanding unresolved issues concerning the occurrence and size dependence of phase transitions in oxide nanocrystals. Aqueous solutions of Fe(NO3)3*9H2O are deposited inside the pores of SBA-15 silicas with mesopore diameters of 4.3, 6.6, and 9.5 nm. By calcination, the nitrate salt transforms into FeOx oxides. The XRD peaks of nanocrystals are broad and overlapping, resulting in ambiguities attributed to a given allotropic variety of Fe2O3 (alpha, epsilon, or gamma) or Fe3O4. The association of XRD, SAED, and Raman information is necessary to solve these ambiguities. The metastable gamma-Fe2O3 variety is selectively formed at low Fe/Si atomic ratio (ca. 0.20) and when a low calcination temperature is used (773 or 873 K followed by quenching to room temperature once the targeted temperature is reached). The small size dispersion of the patterned nanoparticles, suggested on a local scale by TEM, is confirmed statistically by magnetic measurements. The nanoparticles have a superparamagnetic behavior around room temperature. Their magnetic moments (from 220 to 370 mB), their sizes (from 4.0 to 4.8 nm), and their blocking temperatures (from 36 to 58 K) increase with the silica template mesopore diameter. Their magnetic properties are compared to those of standard gamma-Fe2O3 nanoparticles of similar size, obtained by coprecipitation in water and stabilized by a citrate coating.
ABSTRACT
Theoretical model to describe magnetodynamics of a ferrogel, i.e., an assembly of ferromagnetic nanoparticles embedded in a gel, is proposed. The reorientations of the particles are determined by the influence of the elastic matrix and the rotational Brownian motion. Due to the interplay between these two factors, the main parameter characterizing the static magnetic susceptibility of the system is the ratio of the elastic modulus of the matrix times particle volume to the thermal energy. It is shown that the main components of the dynamic magnetic-susceptibility tensor are determined by the combinations of the reference rates of several processes inherent to the system, namely, the elastic restoration of the particle orientation, Brownian rotary diffusion, and viscous relaxation of the particle angular momentum. In the framework of the model, absorption of the energy of an alternating external field by a ferrogel is studied. With allowance for the ever present interaction of elastic and Brownian forces, the effective relaxation times for the longitudinal and transverse components of the ferrogel magnetization are evaluated.
ABSTRACT
As an important example of noisy rotary oscillations, the dynamic magnetization of an assembly of superparamagnetic particles is considered. In the presence of a bias field, there exists a mechanism that causes selective suppression of higher harmonics in the response spectrum of the system. Manifestation of this effect at temperature variation is known as the noise-induced resonance. Its manifestation at the change of excitation intensity, formerly unknown, we term the force-induced resonance.
