Behaviour at high frequencies

Many of the specific applications of ferrites depend on their behaviour at high frequencies. When subjected to an ac field, ferrite permeability shows several dispersions as the field frequency increases, the various magnetisation mechanisms become unable to follow the field. The dispersion frequency for each mechanism is different, since they have different time constants. Fig. 4.59. The low-frequency dispersions are associated with domain wall dynamics and the high-frequency dispersion, with spin resonance the latter, usually in the GHz range, is discussed in Section 4.6.2. 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

NMR being a window-dependent technique, the range of Larmor frequency plays an important role in the study of CPs. In CPs there are many interactions which are possible for SLR. Whenever the relaxation has contributions from additional mechanisms such as reorientation of symmetric groups present in the dopants such as PFg, AsF6, etc. and presence of moisture then the scenario becomes more complicated and one has to carefully analyse the data. Further, the lower frequency measurements show 3D behaviour while at high frequencies the system... 

The presentation of the result in the frequency domain offers an additional advantage. The reactor behaviour modelled (Comb. Chamber (,i) can be directly compared with the behaviour calculated (Comb. Chamberc t,). The calculated frequency response of the reactor is the ratio of Che measured output and input spectra. This comparison is not possible in the time domain due to the measurement noise at high frequencies. This noise is amplified by the compensation of the measurement dynamics and thus no useful presentation of data is possible in the time domain. 

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