Transformer cores

A major application of ferro- and ferrimagnetic materials is in transformer and motor cores,
Materials should be magnetically soft with large power-handling capacity and low losses. Magnetically soft
materials have high permeability, are magnetised easily at low applied fields, have a low coercive

field and

tend to have low hysteresis losses or BH products. These properties are favoured in materials that have a small magnetostriction coefficient, λs, and a low magnetocrystalline anisotropy coefficient, K1. Soft magnetic
materials are, mechanically, those in which the domain walls migrate easily.
In addition to hysteresis losses, eddy current losses are a serious problem at high frequencies, especially in
materials of low resistivity, because eddy currents are proportional to (frequency)2. Eddy currents in metals
such as Fe can be reduced by alloying Fe with, for example, Ni or Si: alloys generally have higher resistivity
than the component metals. One great advantage of ferrimagnetic oxides such as ferrites and YIG is their high
resistivity and, consequently, negligibly small eddy currents. Thus, in YIG containing trivalent cations only,
there is no easy mechanism for electronic conduction to occur and σ25 ◦C ≈ 10−12 S cm−1 it is necessary to ensure that all the Fe is in the +3 state, otherwise
 Fe2+ → Fe3+ + e− redox
transfer may give rise to high
conductivity. For instance, magnetite, Fe3O4 (i.e. Fe2+ Fe3+
2 O4), has a conductivity of 102–103 S cm−1 at
room temperature; this is ∼15 orders of magnitude higher than that of YIG and is associated with the mixed
valency of iron.