Question

A major application of ferro- and ferrimagnetic materials is in transformer and motor cores. Materials are required that are magnetically soft (high permeabilities – they are magnetized easily at low applied fields and a low coercive field) with large power-handling capacity and low losses. Why cannot pure iron metal be used in transformer cores?

Account for the magnetic behavior of the following spinels:
(a) ZnFe2O4 is antiferromagnetic
(b) MgFe2O4 is ferrimagnetic and its magnetic moment increases with temperature

c) MnFe2O4 is ferrimagnetic and its magnetic moment is independent of temperature

Answer 1

1. Zinc Ferrite (ZnFe₂O₄) has magnetic properties different to most spin ferrites. It has low ordering temperature and an antiferromagnetic ground state. The compound has a normal spinal structure with the iron atom located at octahedral B site and the Zn atoms occupying tetrahedral A site. Though most iron-rich ferrites are ferrimagnetically ordered but the stoichiometric well crystalized zinc ferrite a antiferromagnet with low transition Neel temperature of Tn=10K.The difference is because of the weaker super exchange interactions between the B site rather than the A-B interactions. There for a small migration of Fe atoms from B to A can produce ferrimagnetic regions with strong A-B super exchange interactions.
2. In case of MgFe₂O₄, Mg2⁺ has 0 unpaired electrons and if it is a completely inversed spinel system antiferromagnetic system is expected. But MgFe₂O₄ is mixed spinel and there is a slightly higher concentration of Fe3+ ions in the octahedral sites than in the tetrahedral sites. MgFe₂O₄ exhibits small net magnetic moment and is ferrimagnetic material. But as the temperature increases it gradually transforms into normal spinel causing the magnetic moment to increase.
3. In case of MnFe₂O₄, which is about 20% inverse degree of inversion does not affect the overall magnetic moment because both Mn2⁺ and Fe3⁺ have d5 electron configuration and are independent to the raise in temperature.