Today in some sensor systems magnetic materials are key components. In
order to fully optimize a sensor system where magnetic materials is used,
for instance different kinds of magnetic nanoparticles, soft or hard
magnetic materials it is of crucial importance to fully understand the
magnetic properties of the material that is being used in the sensor
system. In order to gain this knowledge the most straightforward is to
magnetically characterize the material and when you analyze the result
you must have a deep knowledge in the different magnetic properties of
the material.
Magnetic analysis
At Imego in the electromagnetic sensor group we have a well equipped
magnetic lab where we can perform both static and dynamic magnetic
characterization of all sorts of magnetic materials both for external
customers as well in our magnetic sensor systems. We have also several
years of experience in magnetic material, magnetic properties and
magnetic characterization. At Imego we can help external customers in
choosing the right magnetic particle system or any magnetic material in
their applications or help in magnetic characterization as described
above in the text. We can also give magnetic courses on general magnetism
and magnetic material or on magnetic particles.
Static magnetic characterization
Static magnetic measurement gives the equilibrium magnetization versus
magnetic field at constant temperature, versus temperature at constant
magnetic field or magnetization versus time. In these measurements we can
determine the magnetization processes in different magnetic particle
systems or any other magnetic material. For instance the total
magnetization curve versus field at constant temperature (for instance at
room temperature) gives a very good process control in order to
manufacture well defined magnetic particle systems.
If we just study the low field portion of the magnetization curve it is
possible to define a magnetic susceptibility, which is the ratio of the
induced magnetization in the material to the applied magnetic field. For
instance in a magnetic particle system the initial magnetic
susceptibility at low magnetic fields is sensitive to the size of the
magnetic particles, the number of magnetic domains per particle, the
configuration of the magnetic domains in the total particle, the material
in the magnetic particles and of course the concentration of the magnetic
particles. This makes the susceptibility as a very good parameter in
order to have a good control of the manufacturing process. But as pointed
out earlier in order to have a complete control the whole magnetization
should be measured and analyzed. For instance the magnetic saturation is
only dependent on the concentration of the magnetic material (in the case
of measuring the magnetization on magnetic nanoparticles the magnetic
saturation is somewhat decreased due to surface effects of the small
nanoparticles).
Hard magnetic materials (which are used in permanent magnets) can be
characterized by their hysteresis loops where the remanence (residual
magnetization after the material has been magnetized), coercivity (the
magnetic field that brings the magnetization to zero which is high for
hard magnetic materials), the shape of the hysteresis loops (distribution
of coercivity fields) and magnetic saturation can be determined. In the
case of soft magnetic material the same hysteresis loops can be used and
in this case the same parameters as for the hard magnetic materials are
measured (in the case of soft magnetic materials the coercivity values
are very low and should be ideally zero) included the magnetic
susceptibility (which is usually very large for this kind soft magnetic
materil.
Dynamic magnetic characterization
In order to fully understand the magnetic material we can also perform
measurements how fast the magnetization is building up in the material.
The type of magnetization processes, for instance magnetization reversal
in magnetic singledomains (Néel relaxation), randomly rotation of
particles in a carrier liquid containing thermally blocked singledomains
(Brownian relaxation) or domain wall motion as in polydomain materials.
All of these magnetization process creates a specific pattern in the
dynamic magnetic properties for instance in the frequency dependent
susceptibility. In this case we measure the dynamic magnetic properties
such as the frequency dependent susceptibilities for fast magnetic
relaxations or measure the magnetization change versus time for slow
magnetic relaxations.
Measuring the frequency dependent susceptibilities is almost the same as
a spectroscopic detection where the different relaxation processes is
visualized as peaks at their specific frequencies. In our lab we are able
to detect the frequency dependent susceptibility up to several MHz and we
have build a portable dynamic susceptibility system were we can measure
frequency dependent susceptibilities up to 200 kHz. In the case of
detecting Brownian relaxation of particles including thermally blocked
singledomains we are able from the experimental data determine the size
distribution of the particles using an algorithm included in the software
of the dynamic susceptibility system. With the same instrument we are
also able to study possible clustering of the particles.