Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO 2 and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.

Polarons in crystalline and non-crystalline materials

Fine particles of cobalt ferrite were synthesized by the sol?gel method. Subsequent heat treatment at different temperatures yielded cobalt ferrites having different grain sizes. X-ray diffraction studies were carried out to elucidate the structure of all the samples. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. The variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to Maxwell?Wagner type interfacial polarization in general, with a noted variation from the expected behaviour for the cold synthesized samples. High permittivity and conductivity for small grains were explained on the basis of the correlated barrier-hopping model.

https://iopscience.iop.org/article/10.1088/0022-3727/40/6/001/pdf

Finite size effects on the electrical properties of sol–gel synthesized CoFe2O4 powders: deviation from Maxwell–Wagner theory and evidence of surface polarization effects

DC conductivity and dielectric studies of CoxZn1−xFe2O4 (x = 0.32, 0.49, 0.79), BaFe12O19, and Mg0.25Zn0.75Fe2O4 are reported as a function of temperature and frequency. The lattice constant and the existence of a single phase are established from X-ray studies. The dielectric behaviour is attributed to the Maxwell-Wagner polarisation. A dispersion region is observed in one of the cobalt-zinc ferrites and the barium ferrite. Activation energies are calculated for these two ferrites from dielectric measurements. The conductivity of these ferrites is explained by the hopping mechanism. Activation energies and the transition temperatures are presented.



Ergebnisse von Gleichstromleitfahigkeits- und dielektrischen Untersuchungen an CoxZn1−xFe2O4 (x = 0,32; 0,49; 0,79) BaFe12O19 und Mg0,25Zn0,75Fe2O4 in Abhangigkeit von der Temperatur und Frequenz werden mitgeteilt. Die Gitterkonstante und die Existenz einer einzelnen Phase werden aus den Rontgenuntersuchungen festgestellt. Das dielektrische Verhalten wird der Maxwell-Wagner-Polarisation zugeschrieben. Ein Dispersionsbereich in einem der Kobalt-Zink-Ferrite und der Barium-Ferrite wird beobachtet. Fur diese beiden Ferrite werden aus den dielektrischen Messungen Aktivierungsenergien berechnet. Die Leitfahigkeit dieser Ferrite wird durch einen Hopping-Mechanismus erklart. Aktivierungsenergien und Ubergangstemperatur werden angegeben.

DC conductivity and dielectric behaviour of cobalt-zinc ferrites

Thermolectric-power measurements were carried out in the temperature range 80-300 K on five synthetic single crystals of magnetite, ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, differing in oxygen stoichiometry. The anomalous behavior of the Seebeck coefficient below the Verwey temperature was interpreted with a model of mixed conduction.

Thermoelectric properties of magnetite at the Verwey transition

Measurements are reported of the resistivity and the thermoelectric power as a function of temperature for nickel-iron ferrites. The thermoelectric power shows a transition from a temperature independent to a temperature activated region. The results are broadly in agreement with the Jonker hopping model but some disagreement with the accepted simple theory is found. A 2-energy-level model is proposed for the Fe2+ ions; this removes most, though not all, discrepancies between predicted and experimental results.

The thermoelectric power of the system NiFe204-Fe304

The electrical properties of nickel ferrite are discussed in terms of a model proposed by Jonker. It is shown that some parameters governing the intrinsic conduction may be conveniently estimated from measurements on slightly doped materials. The magnitude of the energy gap between Fe2+ and Ni2+ levels and the mobilities of electrons and of positive holes in nickel ferrite are evaluated. The agreement of experimental results with the Jonker model is satisfactory.

https://iopscience.iop.org/article/10.1088/0508-3443/17/5/302/pdf

Electrical conduction in nickel ferrite

Measurements are reported of the electrical resistivity of nickel ferrites having cobalt substitutions of up to 5%. For certain compositions large resistivities are found due to the buffering action of the cobalt ions, which behave in this respect similarly to managanese ions in nickel ferrite. Some differences are observed between cobalt- and manganese-doped materials and these are explained on the assumption that, although Co2+ ions may be readily substituted for Ni2+ ions, there exists a limiting concentration to which cobalt can replace iron in the ferrite lattice. Magnetization measurements are found to be in good agreement with this hypothesis.

The effect of cobalt substitution on electrical conduction in nickel ferrite

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The thermoelectric power of the system NiFe204-Fe304

Electrical conduction in nickel ferrite

The effect of cobalt substitution on electrical conduction in nickel ferrite