In order to fully understand magnetic materials dynamic susceptometry is a valuable magnetic tool. Dynamic magnetic measurements gives information on 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), domain wall motion as in polydomain materials or magnetic resonances of the magnetization in the material.
All of these magnetization process creates a specific pattern in the dynamic magnetic properties for instance in the frequency dependent susceptibility. 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. The dynamic magnetic properties can be revealed by measuring the frequency dependent susceptibility for fast magnetic relaxations or measure the magnetization change versus time for slow magnetic relaxations. For fast magnetic relaxation measurements we are also working with pulsed magnetic fields and measurements of the magnetization decay which gives a much faster measurements of the dynamic magnetic processes in the material compared to measuring the response in the frequency domain.
In our lab we are able to detect the frequency dependent susceptibility up to several MHz both with a commercially purchased sensitive instrument and also that we develop a portable dynamic susceptibility systems were we can measure frequency dependent susceptibilities up to 200 kHz (or even higher up in the MHz region). In the case of detecting Brownian relaxation of particles including thermally blocked singledomains, we are able from the experimental data to determine the size distribution of the particles.