Today, research on, and the manufacturing of, magnetic particles with sizes from a few nanometers up to micrometers have been introduced into many different applications including information carriers in biotechnology and medicine. Magnetic nanoparticles of, for example, magnetite or maghemite (common iron oxides used in different biomedical applications and in data-storage systems) with diameters less than about 50 nm are single domains. The definition of a single domain is that every spin in the particle has the same direction, which means 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 than the typical time scale of the measurement. Thermally blocked particles have magnetic relaxation times that are longer than 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 or 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 measurement. Coercivity is the field that brings the magnetization to zero value while remanence 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 the figure, maximum coercivity (and remanence) occurs when the particles are as large as possible but still single domains. This corresponds with the particles being 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 properties, 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 very useful in many applications, from biomedical to data-storage systems.
Magnetic separation
One of the major applications of magnetic particles in biomedicine is in magnetic separation. It is possible to separate a specific substance from a mixture of 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 of the magnets that are being used in the separation system. We have developed expert knowledge in the fields of magnetic characterization and analysis as well as 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 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.Research into using magnetic nanoparticles for information storage is evolving 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 the particles. We use magnetic-induction techniques to study the changes in Brownian relaxation. The orientation of the magnetic moment of the particle must change at the same rate as the rotation time of the particle itself. The orientation of the magnetic moments in the single domains must then be constant, which means that the total magnetic particle, which can contain several single domains locked in a solid matrix, must contain thermally-blocked single domains. This puts a lower limit to the sizes of the nanoparticles. For single domains of maghemite this lie at a domain diameter of approximately 15 nm at room temperature.
There are other biosensor systems that use the 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 is possible to find both superparamagnetic as well as thermally blocked particles in these applications.
Local heat sources
If an AC magnetic field with a specific frequency and amplitude is applied, it is possible for the magnetic nanoparticles to absorb energy, which increases the local temperature around the nanoparticle system. This is used in in-vivo applications in medicine to destroy tumor cells. In such cases, magnetic nanoparticles with materials with Curie temperatures around 42 ° C (the temperature at which the tumor cells are destroyed) are preferred. Overheating problems can be avoided with these materials. The nanoparticle system then works 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 understand fully the magnetic properties of the particle systems. We have the capability required to optimize the magnetic-particle system. The sizes of the nanoparticles are chosen to be thermally blocked or superparamagnetic depending on whether the nanoparticles are free to rotate or are locked in a solid matrix.
Contrast substances
Another area in medicine where magnetic nanoparticles have attracted attention is Magnetic Resonance Imaging (MRI) in which magnetic nanoparticle systems are used as contrast substances. Since the magnetic single domains 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. Cancer tumors can be visualized earlier with contrast substances.
Damping systems
When a magnetic field is applied across a magnetic liquid (one 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 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.