Ferrite

Ferrite is defined as a ceramic-like material having magnetic properties, which are useful in several types of electronic devices. Ferrites are brittle, hard, iron-containing, and generally black or gray and are polycrystalline - it means made up of a large count of small crystals. These are composed of iron oxide and either one or more other metals in the chemical combination.

Ferrimagnetism is a type of permanent magnetism that occurs in the solids.

Composition

Usually, the ferrites are ferrimagnetic ceramic compounds, which are derived from iron oxides. A famous example is given as magnetite (Fe₃O₄) is. Similar to most of the other ceramics, ferrites are brittle, poor, and hard conductors of electricity.

Several ferrites adopt the spinel structure having the formula AB₂O₄, where A and B indicate different metal cations, usually including iron (Fe). Usually, spinel ferrites adopt a crystal motif, which is consisting of cubic close-packed (fcc) oxides (O^2-), having A cations occupying one-eighth of the tetrahedral holes and B cations taking place half of the octahedral holes, which means A²⁺B₂³⁺O₄^2-.

Structure

Ferrite crystals are the ones that do not adopt the ordinary spinel structure; however, rather, the inverse spinel structure: One-eighth tetrahedral holes are occupied by the B cations, one-fourth of the octahedral sites can be occupied by the A cations. and the other one-fourth by the B cation. Also, it is possible to have mixed structure spinel ferrites with having the formula [M²⁺1-δ Fe³⁺_δ][M²⁺_δ Fe³⁺_2-δ  ]O₄.

Where δ is given as the degree of inversion.

The magnetic material, which is called "ZnFe," contains the formula as ZnFe₂O₄, with Zn²⁺ occupying the tetrahedral sites and Fe3+ occupying the octahedral sites; it is an example of a common structure of spinel ferrite.

A few ferrites adopt the hexagonal crystal structure, like strontium and barium ferrites SrFe₁₂O₁₉ (SrO:6Fe₂O₃) and BaFe₁₂O₁₉ (BaO:6Fe₂O₃).

Concerning their magnetic properties, often, the various ferrites are classified as either "soft", "semi-hard," or "hard," which refers to their low or high magnetic coercivity.

Soft Ferrites

Ferrites, which are used in electromagnetic or transformer cores, contain zinc, nickel, and/or manganese compounds. They contain a low coercivity and are known as soft ferrites. The low coercivity means the magnetization of the material can easily reverse direction without dissipating more energy (hysteresis losses), while the high resistivity of the material prevents eddy currents in the core, the other energy source loss. Due to their comparatively low losses at the higher frequencies, they can be extensively used in the RF transformers and inductor cores in applications like loopstick and switched-mode power supplies antennas used in the AM radios.

Semi-hard Ferrites

Cobalt ferrite with the chemical formula CoFe₂O₄ (CoO·Fe₂O₃) is in between both soft and hard magnetic material and can be usually classified as a semi-hard material. It is majorly used for its magnetostrictive applications such as actuators and sensors, a good thing to its high saturation magnetostriction (at ~200 ppm). Also, the cobalt ferrite has the benefits of being rare-earth-free, which makes it a good substitute for the Terfenol-D.

Furthermore, its magnetostrictive properties may be tuned by inducing a magnetic uniaxial anisotropy. This may be done by the magnetic field-assisted compaction, magnetic annealing, or the reaction under uniaxial pressure. This last solution contains the advantage of being ultra-fast (just within 20 min), with the use of spark plasma sintering. Also, the induced magnetic anisotropy in the cobalt ferrite can be beneficial to enhance the magnetoelectric effect in the composite.

Hard Ferrites

In contrast, the permanent ferrite magnets are produced of hard ferrites that contain a high remanence and high coercivity after magnetization. Barium and iron oxide or strontium carbonate can be used in the manufacturing of hard ferrite magnets. High coercivity is defined as the materials are more resistant to becoming demagnetized, an important characteristic for a permanent magnet. Also, they hold a high magnetic permeability.

These particular so-called ceramic magnets are very cheap and widely used in the household products like refrigerator magnets. The maximum magnetic field strength H is up to 30 to 160 kiloampere turns per meter (400 to 2000 oersteds), and the magnetic field B is up to 0.35 tesla. At the same time, the density of ferrite magnets is up to 5 g/cm3.

Production

Ferrites can be produced by heating a oxides' mixture of the constituent metals at higher temperatures, given in this idealized equation:

Fe₂O₃ + ZnO → ZnFe₂O₄

In a few cases, the finely-powdered precursor mixture is pressed into a mould. Typically, for strontium and barium ferrites, these particular metals are supplied as their carbonates, SrCO₃ or BaCO₃. These carbonates undergo calcination during the heating process:

MCO₃ → MO + CO₂

After this, the two oxides combine to produce the ferrite. The oxide's resulting mixture undergoes sintering.

Uses

Ferrite cores can be used in transformers, electromagnets, and electronic inductors, where the ferrite's high electrical resistance leads to very low eddy current losses. Commonly, they are seen as a lump in a computer cable, which is called a ferrite bead that helps to prevent high-frequency electrical noise (which is called the radio frequency interference) either from exiting or entering the equipment.

The early computer memories stored the data in the residual magnetic fields of the hard ferrite cores that were assembled into core memory arrays. Ferrite powders can be used in magnetic recording tape coatings.