6.11.4.1 Soft Ferrites

Soft ferrites, as we explained earlier, are materials which are easy to magnetize or demagnetize i.e. materials with low coercive field strengths and thus so that they can reverse the direction in alternating fields without dissipating much energy since the area of B-H (or M-H) loop is small.

Typical soft ferrites used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese based ferrites. These materials also have higher resistivities than typical ferromagnetic metals, of the order of  10^-1 to 10⁶Ω.m, which leads to low eddy currents in the core, another source of energy loss.

Because of their comparatively low losses at high frequencies, they are also extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies (SMPS).
The most common soft ferrites are Mn_xZn_(1-x)Fe₂O₄, Ni_xZn_(1-x)Fe₂O₄. Ferrites of Ni-Zn show higher resistivity than those containing Mn-Zn, and are, therefore, more suitable for frequencies above 1 MHz.  Mn-Zn  ferrites, in comparison, have higher permeability and saturation induction.

The properties of soft ferrites can be tailored by compositional modifications. For instance, in Mn_1-xZn_xFe₂O₄ and Ni_1-xZn_xFe₂O₄, increasing the Zn  content leads to an increase in the magnetic permeability just before the magnetic transition but at the same the magnetic transition temperature also decreases. The increase in magnetic permeability near magnetic transition has been attributed to reduced magnetic anisotropy. The increase in relative permeability is about an order of magnitude in Mn_1-xZn_xFe₂O₄ for doping levels up to 50 at.% and about 2-3 orders of magnitude in Ni_1-xZn_xFe₂O₄  for doping levels up to 70 at.%.

Change in grain size also has a profound effect on the relative permeability with permeability increasing with increasing grain size. This is related to the decrease in the grain boundary concentration resulting in less pinning of domain walls by grain boundaries and hence facilitating easy magnetic switching.
Electrical resistivity of ferrites is again composition dependent. As we have discussed earlier in module 3, electrical conduction in ceramics takes place by hopping of electrons between say two valence states of an ion. In ferrites, d-group elements are susceptible to valence fluctuations. For example Mn-Zn  ferrites are more susceptible to valence fluctuations of Mn and Fe as compared to Ni-Zn  ferrites. This is also controlled very strongly by processing conditions such as firing temperatures, atmosphere and rate of cooling after sintering.

For example in Ni_1-xZn_xFe_2+αO_4-β , if all of the iron is present in 3+  valence state, then α=0. Any increase in the iron content i.e. α > 0 is compensated by the formation of Fe²⁺  which creates favourable conditions for electron hopping between Fe³⁺ and Fe²⁺ promoting n-type conduction.  On the other hand, deficiency of iron i.e. α < 0 is usually compensated by oxygen vacancies, resulting in a large increase in the resistivity, about 8 orders of magnitude, as shown below.

In addition, the resistivity of nickel ferrites is also increased by addition of small amounts of Cobalt. Reduction of cobalt from Co³⁺ to Co²⁺ state minimizes the reduction of iron to Fe²⁺ state. Since Co ions are sparsely located in the lattice, hopping of electrons between Co²⁺ and Co³⁺ states is minimal.
Another method of increasing the resistivity in polycrystalline ferrites can be via grain boundary modification either by preferential oxidation of grain boundaries or by addition of additives like CaO or SiO₂ so that Fe and Ca ions are incorporated into ferrite regions closer to the grain boundaries. Both of these approaches make grain boundaries more resistive than the grains.

Another interest in soft ferrites is the frequency dependence if their properties such as permeability and loss which we have not discussed in great detail except for a bit of discussion in Section 6.10. These properties are very useful for microwave applications, switch mode power supplies, inductors and other high frequency broadband applications. Interested readers could go through Chapter 9 (Magnetic Ceramics) of Electroceramics; Materials, Properties and Applications by A.J. Moulson and J.M. Herbert.