Biomedical Engineering Reference
In-Depth Information
The properties of materials composed of magnetic nanoparticles result from
both the intrinsic properties of the particles and the interactions between them
[121, 122]. The intrinsic magnetic properties of nanoparticles are heavily infl u-
enced by the fi nite size and surface effects, the relevance of which increases as the
particle sizes decrease. Finite size effects are due to the nanometer size of the
particles, which permits particles to change from multidomain to single domain,
such that the ferromagnetic particle will behave like a paramagnet. Such fi nite size
effects even permit nanoparticles to demonstrate macro-scope quantum tunneling
of magnetization if the particle size is suffi ciently small. A variety of fi nite size
effects occur in magnetic materials [121]. Surface effects are related to the sym-
metry breaking of the crystal structure at the boundary of each particle. Hence, as
the particle size decreases, the surface atoms will become a very large proportion
of a nanoparticle, which in turn implies that the surface and interface effects
become increasingly important. For example, for fcc cobalt with a diameter of
approximately 1.6 nm, about 60% of the total number of spins are surface spins
[123]. Yet, because of the large surface atoms/bulk atoms ratio of the nanoparticles,
the surface spins make an important contribution to the magnetization. The local
breaking of symmetry may lead to changes in the band structure, lattice constant
or/and atom coordination. Indeed, under these conditions some surface- and/or
interface-related effects may occur, such as enhanced anisotropy, increased
moment and, under certain conditions, core-surface exchange anisotropy. Because
the surface effect is due to the local symmetry break and/or any change of the
local atom coordination in nanoparticles, the coating of nanoparticles with metals
or other materials may alter not only the anisotropy energy but also the magnetic
moment.
The fi nite size and surface effects (intrinsic effects) and interaction in nano-
meter particles may manifest through a wide variety of anomalous magnetic
properties with respect to those of bulk materials, and these will be discussed
in detail in the following sections. First, we will review the intrinsic effects starting
from the fi nite size effect - how size changes the particle from multidomain
to single domain, how the single-domain particle reverse under magnetic fi eld,
why small nanoparticles show superparamagnetic behavior, and how to probe
such behavior. The surface effects will then be discussed - how the surface atoms
alter the particle anisotropy and magnetic moments, and how a cover layer
can further modify the anisotropy energy and magnetic moment. Details of
exchange coupling between the core and the oxide shell of nanoparticles will also
be discussed.
16.4.2
Finite Size Effects
16.4.2.1
Size - Dependent Crystalline Structure
Cobalt nanoparticles may occur in multiple crystal phases, and this can result in
large differences in the magnetic moment and crystalline anisotropy. This appears
to be a size effect which is related to the balance of surface and bulk free energies
