Today, research and the manufacturing of magnetic particles with sizes
from a few nanometers up to micrometers has been introduced into many
different applications including information carriers and in
biotechnology and medicine. Magnetic nanoparticles of, for example,
magnetite or maghemite (common iron oxide materials used in different
biomedical applications or in data storage systems) with diameters
smaller than about 50 nm are singledomains. The definition of a
singledomain is that every spin in the particle has the same direction
meaning that the total magnetic moment of the particle is the sum of all
the spins.
Magnetic nanoparticles can be divided into particles that are
superparamagnetic or thermally blocked. Superparamagnetic particles have
magnetic relaxation times that are shorter compared to the typical time
scale of the measurement. Thermally blocked particles have magnetic
relaxation times that are larger when compared with a typical time scale
of measurement being used to study the particle system. If the
nanoparticles are placed in a solid matrix, the thermally blocked
nanoparticles will exhibit both remanence and coercivity while the
superparamagnetic particles will not show any remanence and coercivity.
By this we mean that the coercivity and remanence of a nanoparticle
system depends on the magnetic relaxation compared to the typical time
scale of the measuremet. Coercivity is the field that brings the
magnetization to zero value while remanance is the residual magnetization
of the particle system after being magnetically saturated with an
external magnetic field. This is visualized in the figure above.
As can be seen in figure, maximum coercivity (and remanence) occurs when
the particles are as large as possible but still singledomains. This
corresponds with the fact that the particles are as highly thermally
blocked as possible (the magnetization is locked very hard in its easy
axis direction). After the transition into polydomains where the spin
structure in the nanoparticles are split up into several magnetic
domains, coercivity decreases with particle sizes. In magnetic polydomain
particles, the magnetization direction is no longer constant and the
domains are divided by domain walls where the spin directions gradually
change from one domain to another adjacent domain. The magnetization
process in polydomains is dominated by domain wall motion at low magnetic
fields.
Depending on the size and subsequent change in magnetic property, the
magnetic nanoparticles are used in different applications as described
below. Since the relaxation time of magnetic nanoparticles can be changed
by changing the size of the nanoparticles or using different kinds of
materials, magnetic nanoparticles have been (and will be in the future) a
very useful tool in different kind of applications, from biomedical to
data storage systems.
This diagram displays the different relaxtion mechanisms and the
transition between singledomains and polydomains with respect to the size
of a typical magnetic nanoparticle. A schematic figure of how the
coercivity varies with the particle size can be seen at the foot of the
diagram.
Magnetic separation
One of the major applications of magnetic particles in biomedicine is in
the area of magnetic separation. In this case, it is possible to separate
a specific substance from a mixture of different other substances. The
separation time is one of the important parameters in the magnetic
separation method. In order to optimize this parameter it is very
important to know the magnetic properties of the magnetic particle system
as well as the magnets that are being used in the separation system.
Imego has expert knowledge in the field of magnetic characterization and
analysis and the magnetic optimization of both magnetic particle systems
and magnet systems.
Magnetic identification and data storage systems
Magnetic nanoparticles with long relaxation times (thermally blocked
nanoparticles) with a stable remanent magnetization can be used as
information carriers in magnetic identification and data storage systems
where it is crucial to have small regions of magnetic material. The two
directions of the magnetic moments (the remament magnetization) of the
magnetic nanoparticles gives the zeros (0) and ones (1) that make it
possible to store information on a hard disk in a computer or in other
types of media. The directions of the magnetic moment of the
nanoparticles must be stable with time, otherwise information can be
lost. Today, research into using magnetic nanoparticles for information
storage is developing rapidly.
Magnetic biosensor systems
At Imego we use magnetic nanoparticles in biosensor applications to study
how the Brownian relaxation (random particle rotation) time changes when
biomolecules bind to the surface of particles. We use magnetic induction
techniques to study the changes in Brownian relaxation. This requires
that the orientation of the magnetic moment of the particle must change
with the same rate as the rotation time of the particle itself. The
orientation of the magnetic moments in the singledomains must then be
constant, meaning that the total magnetic particle (that can contain
several singledomains locked in a solid matrix) must contain thermally
blocked single domains. This puts a lower limit of the sizes of the
nanoparticles. For singledomains of maghemite this lies at a domain
diameter of approximately 15 nm at room temperature. There are other
biosensor systems that are related to magnetic detection of magnetic
particles. These biosensor systems use SQUIDs (Superconducting Quantum
Interference Devices) or sensitive GMR (Giant Magnetic Sensors) to detect
the presence of magnetic particles. The sizes of the single domains are
dependent on the technique used and it possible to find both
superparamagnetic as well as thermally blocked particles in these
applications.
Local heat sources
If you apply an AC magnetic field with a specific frequency and amplitude
it is possible for the magnetic nanoparticles to absorb energy, resulting
in an increase in the local temperature around the nanoparticle system.
This is used in in-vivo applications in medicine to destroy tumour cells.
In such cases, magnetic nanoparticles with materials with Curie
temperatures at approximately 42 ° C (the temperature where the tumour
cells are destroyed) are preferred. For these materials, overheating
problems can be avoided. The particle system will then work as a
thermostat. In other applications where local heating is required,
magnetic particles can also be used. In all of these cases, it is
important to really understand the magnetic properties of the particle
systems. At Imego we have the capability required to magnetically
optimize the magnetic particle system. Depending on whether the
nanoparticles are free to rotate or locked in a solid matrix, the sizes
of the nanoparticles are chosen to be thermally blocked or
superparamagnetic.
Contrast substances
Another area in medicine where magnetic nanoparticles have attracted
attention is Magnetic Resonance Imaging (MRI). In this case, magnetic
nanoparticle systems are used as contrast substances. Since the magnetic
singledomains have a magnetic moment they are surrounded by a magnetic
field produced from the magnetic moment. This surrounding magnetic field
interacts with the hydrogen nucleus in the water molecules in the body
and thereby affects the resonance properties. From the contrast
substances, cancer tumours can be visualized earlier.
Damping systems
When a magnetic field is applied across a magnetic liquid (containing
magnetic particles in a suitable carrier liquid) the viscosity of the
total liquid system (magnetic particles and carrier liquid) can be
changed. It is then possible to adjust the damping in different
applications such as in different types of vehicles or other systems
where adjustable damping is required. Generally, the particles used in
this application are rather large and the particle system goes under the
name of magnetorheological liquids.