Magnetic Nanoparticles

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.
 

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 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.
 

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.
 

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.
 

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.
 

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.