Biomedical Engineering Reference
In-Depth Information
If one considers Ni as an example with an anisotropy constant
K=4.5
103 J m
3
, then for a particle size of 20mm, the particle will
show a blocking temperature (T
B
)at
6
55K using equation 2.1. Below T
B
,
the magnetization will be relatively stable and will show ferromagnetic
behavior. Above T
B
, the thermal energy will be sufficient to suppress
ferromagnetic behavior and the particles thus become superparamagnetic.
2.4
Colossal magnetoresistance (CMR)
Colossal magnetoresistance [103-109] is a property of some materials,
mostly manganese-based perovskite oxides, that enables them to dramati-
cally change their electrical resistance in the presence of a magnetic field.
The magnetoresistance of conventional materials enables changes in
resistance of up to 5%, but materials featuring CMR may demonstrate
resistance changes by very high orders of magnitude.
CMR was initially discovered in mixed-valence perovskite manganites in
the 1950s by G. H. Jonker and J. H. van Santen [133]. In the initial double-
exchange model to explain the phenomenon, the spin orientation of adjacent
Mn-moments is associated with kinetic exchange of e
g
-electrons.
Consequently, alignment of the Mn-spins by an external magnetic field
causes higher conductivity. However, the double-exchange model did not
adequately explain the high insulating resistivity above the transition
temperature. One later model is the so-called half-metallic ferromagnetic
model, which is based on spin-polarized band structure calculations using
the local spin-density approximation (LSDA) of the density functional
theory (DFT) where separate calculations are carried out for spin-up and
spin-down electrons. The half-metallic state is concurrent with the existence
of a metallic majority spin band and a non-metallic minority spin band in
the ferromagnetic phase.
This model can be contrasted with the Stoner model of itinerant
ferromagnetism [134-138]. In the Stoner model, a high density of states at
the Fermi level makes the non-magnetic state unstable. With spin-polarized
calculations on covalent ferromagnets, the exchange-correlation integral in
the LSDA-DFT takes the place of the Stoner parameter. The density of
states at the Fermi level does not play a special role. A significant advantage
of the half-metallic model is that it does not rely on the presence of mixed
valency as does the double-exchange mechanism and it can therefore explain
the observation of CMR in stoichiometric phases like pyrochlore Tl
2
Mn
2
O
7
.
Microstructural effects have also been investigated for polycrystalline
samples and it has been found that the magnetoresistance is often
dominated by the tunneling of spin-polarized electrons between grains,
giving rise to an intrinsic grain-size dependence to magnetoresistance.
