Biomedical Engineering Reference
In-Depth Information
chemistry, structure and, most importantly, the ability to tailor the
microstructure. Therefore, materials selected for soft magnetic applications
must be optimized in terms of their intrinsic and extrinsic magnetic
properties, as well as their morphology. Intrinsic magnetic properties such
as saturation magnetic induction B
s
and Curie temperature T
C
are
determined by alloy composition and crystal structure. Permeability
,
which is an extrinsic property, is usually determined by chemistry, crystal
structure, microstructure and morphology (shape).
In particular, alloys with small magnetocrystalline anisotropies and
magnetostrictive coefficients give rise to excellent soft magnetic properties.
Alloys for soft magnetic applications may be single phase (Type I,
amorphous) or bi-phasic materials (Type II, nanocrystalline) with a
nanocrystalline ferromagnetic phase and a residual amorphous phase at
the grain boundaries. The Type II nanocrystalline alloys possess:
μ
.
high resistivity (50-80
μΩ
cm)
.
low magnetocrystalline anisotropy
.
increased mechanical strength.
With properties such as these, nanocrystalline alloys have great potential as
soft magnetic materials. The most common compositions for soft magnetic
applications either in the amorphous or in the nanocrystalline state are
metal-metalloid based (Fe, Co, Ni)-(Si, B) alloys with small additions of
Mn, Nb, C and, for the nanocrystalline case, Cu. This alloy system has a
good glass-forming ability and is easily accessible by rapid solidification as a
thin ribbon in large-scale production.
There has been extensive research in amorphous and nanocrystalline
materials in melt-spun ribbon form, which exhibit excellent magnetic
properties: large saturation magnetostriction, high saturation magnetiza-
tion, low anisotropy energies and low coercivity. These factors have made
soft magnetic ribbon materials excellent candidates for sensors and actuator
devices. Despite their excellent magnetic properties, the as-cast melt-spun
ribbons suffer from high randomly orientated stresses, which give rise to a
complicated domain structure. However, it is well established that in the
stress relieved or magnetically annealed state, they exhibit excellent soft
magnetic properties. The disadvantages of these ribbon materials are the
difficulty of incorporation into submillimeter dimensional devices and, most
importantly, there is no suitable means of bonding such materials onto
microfabricated structures.
Ribbon materials are currently bonded to larger devices using epoxy
resin. It has been found that the optimized domain structure obtained by
magnetic annealing is disturbed on curing the epoxy resin, which induces
stress in the ribbon. Since metallic glasses are widely used for sensor
applications, a thin-film form of this material would be of great interest for
