Superparamagnetic Fe3O4 nanocrystals were prepared by a chemical coprecipitation method with a thin thickness-adjustable silica layer coated on the surface by hydrolysis of tetraethyl orthosilicate. The silica-coated Fe3O4 nanocrystals were well dispersed and consisted of a 6–7 nm diameter magnetic core and a silica shell about 2 nm thick, according to transmission electron microscopy observations. Fourier transform infrared spectra revealed that amino (–NH2) groups were successfully covalently bonded to the silica-coated Fe3O4 and then carboxyl (–COOH) groups were functionalized to the surface through the reaction of –NH2 and glutaric anhydride. The synthesized nanocrystals have a cubic spinel structure as characterized by x-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Their magnetic properties were carefully investigated by a SQUID magnetometer. The results showed that the nanocrystals were superparamagnetic and the blocking temperature TB shifted from 131 K down to 92 K after they were coated with a thin nonmagnetic layer, since this layer can effectively suppress the magnetic dipolar interaction between particles; the chemically inert silica layer can limit the outside environment effect on the Fe3O4 cores quite well due to the excellent magnetic reproducibility of the coated nanocrystals after ageing for 7 months at room temperature. In addition, the dependence of their high-field specific magnetization on temperature has a T2 relationship. These functionalized silica-coated Fe3O4 superparamagnetic nanocrystals have great potential in biomagnetic applications.

The properties of cylindrical and spherical dust–ion acoustic solitary waves (DIASW) in an unmagnetized dust plasma comprised of relativistic ions, Boltzmann electrons and stationary dust particles are investigated for the first time in this paper. By using the standard reductive perturbation method, a cylindrical/spherical KdV equation is obtained. The change in the DIASW structure due to the effects of geometry, relativistic streaming factor, ion density, and electron temperature is studied by numerical calculation of the cylindrical/spherical KdV equation. It is observed that the properties of DIASW in cylindrical and spherical geometry differ from those in planar one-dimensional geometry, and the amplitude of the solitary wave decreases with increasing electron temperature and increases with increasing relativistic streaming factor, ion density, and vice versa. It is shown that the soliton height is closely related to the nonlinearity coefficient: the height increases as the nonlinearity coefficient decreases.