Iron Oxide / Silica Nanocomposites
- PDF / 623,285 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 95 Downloads / 209 Views
L4.3.1
Iron Oxide / Silica Nanocomposites Michael Kröll, Markus Pridöhl, Guido Zimmermann Degussa AG, Creavis Technologies & Innovation Advanced Nanomaterials Rodenbacher Chaussee 4 63457 Hanau Germany ABSTRACT Nanoscaled Iron Oxide / Silica Nanocomposite particles are produced in a pyrogenic process. The iron oxide crystallites are separated from each other and covered with a silica layer. Their size can easily be controlled by adjusting the process parameters. The amount of iron oxide can also be controlled within certain limits. Due to their size these isolated magnetic particles show a superparamagnetic behaviour. The saturation magnetisation measured depends almost linearly on the size of the iron oxide crystallites showing an increase in the magnetisation with increasing particle size. Due to the composite structure the iron oxide is chemically stable. Furthermore it shows a thermal stability which is unusual for the given oxides.
INTRODUCTION Superparamagnetic materials are typically composed of small particles of a material which is ferro- or ferrimagnetic in its bulk form. The nanometer sized particles are dispersed in a matrix which may be either a solid or a fluid. In order to preserve the magnetic properties of the individual particles, they need to be separated from one another, e.g. by means of a coating with a non-magnetic shell. In ferromagnets, the individual atomic magnetic moments are coupled and aligned parallel, thereby giving rise to a macroscopic magnetic moment of the material. This macroscopic moment is oriented along a certain direction with respect to the crystal structure or the shape of the material – the so-called “easy axis” of magnetisation. The energy barrier to overcome, in order to turn the macroscopic magnetic moment out of the easy axis orientation, is the magnetic anisotropy energy. This anisotropy energy increases with the volume of the ferromagnetic material. When the material is down-sized below a critical particle size, the thermal energy may suffice to overcome the anisotropy barrier. As a consequence, the macroscopic magnetic moment is no longer stable, but rather fluctuates in time, i.e. the material becomes superparamagnetic. This critical particle size at a given temperature is called the “superparamagnetic limit” [1]. For a given particle size, there is a critical temperature – the “blocking temperature” TB, below which the thermal energy is no longer able to overcome the anisotropy barrier, and the magnetisation is blocked. Below TB, the material behaves almost like a normal ferromagnet, whereas above TB, it is superparamgnetic. Superparamagnetic materials (TB < room temperature) do not have a remanent magnetisation, and their magnetisation curves (magnetisation as function of the magnetic field) are anhysteretic.
L4.3.2
However, the susceptibility of a superparamagnetic material is almost as high as that of a ferromagnetic material. Maghemite (γ-Fe2O3) and magnetite (Fe3O4) with particle sizes below about 20 nm (depending on the substance and crystal shape), are
Data Loading...
