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Monday, September 3, 2018

Ferrite, Ferrite structure and Ferrite properties


What is Ferrite?

  • Ferrite is ceramic, homogeneous material and has ferrimagnetic properties. In simple words, Ferrite is a ceramic material formed by reacting metal oxides into a magnetic material.

Ferrite Properties

  • Ferrite exhibits ferrimagnetism due to the exchange communication between metal electrons and oxygen ions.
  • Ferrites are composed of many oxides, where iron oxide acts as the main component.
  • Ferrite has opposite spins which lowers magnetization, while ferromagnetic metals have parallel spins.
  • Ferrite has higher resistivity compared to ferromagnetic metals because of the basic atomic level communication between oxygen and metal ions.
  • Ferrites can have several different crystal structures. Here we are only focussing on the soft magnetic ferrites, which have a cubic crystal structure. The crystal lattice of ferrite is spinel.
  • Based upon the chemical composition, soft ferrites can be divided into two main categories; manganese-zinc ferrite and nickel-zinc ferrite.
  • In each of these categories by changing the chemical composition or manufacturing technology, many different MnZn and NiZn material varieties can be manufactured.
  • The chemical formula of ferrite is commonly expressed as MeFe2O4. Where Me represents a divalent metal ion. e.g. Fe²⁺, Ni²⁺, Mn²⁺, Mg²⁺, Co²⁺, Zn²⁺, Cu²⁺ etc.
  • In general most common of these ferrites are Zn²⁺, when changed with Ni and Mn ferrite, represented as NiZnFe2O4 and MnZnFe2O4.

What is Ferrimagnetism?

  • Ferrites show a type of magnetism known as ferrimagnetism.
  • This means there is net magnetic moment in molecular level as a result of electronic communication between Metal and oxygen ions called super exchange.
  • The ferrimagnetic material does not lose its magnetism even in the absence of external magnetic field.
  • In ferrimagnetic materials atoms have opposing magnetic field strengths/moments, but these are unequal. Therefore, there exists some net magnetic moment.
  • In a bulk ferrite, there are domains (fields) called Wiess* Domains in which all these molecular magnets are aligned in one direction.
  • Domain wall separates different domains associated in random directions and in the presence of an external magnetic field these moments can be forced to align in one direction.
  • Figure 1 at the below shows spins in Ferrimagnetism.
Spins in Ferrimagnetism
Spins are aligned antiparallel and do not cancels each other
*Wiess Domains: In ferrimagnetism, it refers to microscopically small magnetized domains in the crystals of the magnetic materials.

Ferrite Structure

How's the structure of Ferrite Crystal?
  • The magnetic property of ferrite is the presence of nature of ions and their relative lattice position.
  • Ferrite exists in spinel lattice structure.
  • Metal ions are normally located at octahedral and tetrahedral places. Fe2+, Ni2+, Mn2+ etc ions usually occupies the octahedral sites and Fe3+ and Zn2+ usually occupies tetrahedral sites.
  • NiZn ferrite and MnZn ferrites has inverse spinel structure where the part of the B atoms occupies in the tetrahedral site and A atom occupy the Octahedral site.
  • Depends on composition and process situations such as sintering temperature and atmosphere, the lattice site occupancy changes, mainly to the change in magnetic and electrical properties.
  • This show that in ferrite manufacturing both composition and process conditions is very critical to get the essential quality.
  • Below Figure 2 shows the spinal structure of ferrite with the indication of tetrahedral and octahedral sites.
Spinal structure of ferrite
The spinel structure of ferrites with the indication of tetrahedral and octahedral sites

Conclusion

Ferrites have a benefit over other types of magnetic materials because of their high electrical resistivity, low eddy current losses and other magnetic properties over different frequency range. This is because of presence of chemical composition of oxygen and metal ions and their spinel structure.

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