please refer to our recent work on Nickel ferrite to understnd the dielectric measurements, and the variation in the dielectric constant, tan delta, and imaginary part (e'') of dielectric constant measured on Nickel ferrite.

1) It may appear as if it is zero, but it not really zero. (see our paper), and can show a constant with frequency in the low frequency range. It all depends how low a frequency are you measuring.  Pl see the attached paper of our on a ferrite.

2) If you are seeing the imaginary part of the dielectric constant )e"") decreasing with frequency, maybe there is a relaxation.  You are probably seeing a diffused peak at a certain frequency, and further decrease in the frequency shows the lowering of e"" with frequency.

this can be due to a low frequency relaxation phenomena, and why it occurs, needs careful analysis. 

3) You must take dielectric measurements over a wide frequency range.

(preferably from 1 mHz to 10 MHz).

I have attached my paper hopefully it will useful to you understand further.

K. Sreenivas

low frequency relaxation behaviour are unavoidable in polycrystalline samples, since the role of grain boundary is greatly influenced due to the trapped charges and irregular charge distribution  i.e space charges, the porosity also influence the dielectric behavior ,but their presence generally decreases the dielectric constant and dielectric relaxation.

On sintering grain becomes dominant were the density of the sample is increased and relaxation of grain boundary appear to be suppressed or merges with grain, thereby the grain boundary contributions are reduced, were  the relaxation behavior in the low frequency is also suppressed, thus you observe almost the constant plot in e" vs frequency.

as you can clearly seen in prof. sreenivas paper. the 1200 oC is optimum sintering for nickel ferrite. also note that in high frequency at >10 KHz  of 1200 oC the dielectric constant e' is relatively increased than for 1100 oC, this is were the net effect of porosity role appears, see also tan d.  as porosity is reduced in 1200 oC the dielectric constant is high in region >10 KHz were as in lower frequency for 1100 oC , you find a strong relaxation in e" and tan d, also high dielectric in e' in low frequency indicate the  trapped charges and irregular charge distribution i.e space charges in grain boundary.

In case of 1300 oC oxygen vaccany may become more prevalent,and interdiffusion of charge cations in the lattice may also alter the microstructure typically thereby leading to increase in the space charge in the sample.

Dear Sir,

for soft ferrites we have a Ferromagnetic resonance in a middle frequency region. This is a increasing of µ ´´ . For dielectrics we have no polarisation losses in the low frequency range.

All the best

F. Gräbner

Since the mechanisms of loss (dissipation) of energy are associated with dynamic changes in the system state, then the losses increase with increasing velocity (and amplitude) of these changes. Accordingly, the losses in the low frequency region described by the imaginary part of permeability, below\ the losses (which are described by the imaginary part of permeability) become lower at low frequencies.

In addition, some of these materials (for example, MnZn ferrites having appreciable electrical conductivity) have contribution to losses make eddy currents Foucault, which decrease with increasing frequency electromagnetic field. However, in most experiments it is impossible to clearly distinguish the value of e" deposits purely dielectric and electric (Joule effect) losses.

However, in most experiments it is impossible to clearly distinguish the contributions of purely dielectric and ‘electric” (Joule effect) losses in the value of e".

Best regards,

Alexander Morchenko

I have been studying the optical properties of materials by using DFT..There I have seen that sometimes the imaginary part of the dielectric constant at the zero frequency is not zero, but theoretically it must be zero..Is that due to artefact of DFT softwares???

it is due to ionic conduction loss