Magnetic
Characterization
& Analysis

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.
 
 

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.