What is a 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 (O2-), having A cations occupying one-eighth of the tetrahedral holes and B cations taking place half of the octahedral holes, which means A2+B23+O42-.
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.
FAQs on Ferrite
1. What exactly are ferrites and what is their general chemical formula?
Ferrites are a class of ceramic compounds composed of iron(III) oxide (Fe₂O₃) combined with one or more other metallic elements. They are chemically inert, hard, and are notable for being electrical insulators. The general chemical formula for many common ferrites is MFe₂O₄, where 'M' represents a divalent metal ion such as manganese (Mn²⁺), nickel (Ni²⁺), zinc (Zn²⁺), or magnesium (Mg²⁺).
2. What is the difference between 'soft' and 'hard' ferrites?
The main difference lies in their magnetic coercivity, which is their ability to resist demagnetisation.
- Soft Ferrites: These have low coercivity, meaning they can be easily magnetised and demagnetised. This property makes them ideal for applications like high-frequency transformer cores and inductors. Examples include Manganese-zinc ferrite (MnZn) and Nickel-zinc ferrite (NiZn).
- Hard Ferrites: These have high coercivity and retain their magnetism permanently. They are used to make permanent magnets for applications like refrigerator magnets, loudspeakers, and small electric motors. Examples include Barium ferrite (BaFe₁₂O₁₉) and Strontium ferrite (SrFe₁₂O₁₉).
3. What are some important uses of ferrites in modern technology?
Ferrites are crucial components in many electronic and magnetic applications due to their unique properties. Some important uses include:
- High-Frequency Applications: Used as cores for inductors and transformers in power supplies and telecommunications because their high electrical resistivity minimises energy loss from eddy currents.
- Permanent Magnets: Hard ferrites are used to make cost-effective permanent magnets for loudspeakers, electric motors, and magnetic stripe cards.
- Data Storage: They form the magnetic medium on recording tapes and hard disks.
- Stealth Technology: Ferrite particles are used in radar-absorbing materials and paints for stealth aircraft to reduce their radar signature.
4. How is the ferrimagnetism in ferrites different from the ferromagnetism found in iron?
The key difference is in the alignment of atomic magnetic dipoles. In ferromagnetic materials like iron, all magnetic moments align in the same direction, resulting in a very strong net magnetic moment. In ferrimagnetic materials like ferrites, the magnetic moments of atoms on different sub-lattices align in opposite (anti-parallel) directions. However, since the opposing moments are unequal in magnitude, they do not completely cancel each other out, leading to a net magnetic moment that is strong but weaker than in ferromagnetic materials.
5. Why do ferrites have high electrical resistivity, and what is the importance of this property?
Ferrites are ceramic materials with an ionic crystal structure. Unlike metals where electrons can move freely, the electrons in ferrites are localised to specific ions. This lack of free electrons makes them excellent electrical insulators with high resistivity. The importance of this property is most evident in high-frequency alternating current (AC) applications. The high resistivity prevents the formation of eddy currents—wasteful electrical currents induced within the core material—which would otherwise cause significant energy loss and heating in metallic cores.
6. In metallurgy, what does 'ferrite' refer to, and how does it differ from the ceramic ferrites used in magnets?
This is a common point of confusion as the term 'ferrite' has two distinct meanings:
- Ceramic Ferrite: This refers to the iron-oxide-based ceramic compound (e.g., MFe₂O₄) with ferrimagnetic properties, used for magnets and electronic cores as discussed.
- Metallurgical Ferrite (or alpha-iron, α-Fe): This refers to a specific solid solution of iron with a body-centered cubic (BCC) crystal structure. It is essentially a micro-constituent of steel and cast iron. It is soft, ductile, and ferromagnetic, not ferrimagnetic. These two are completely different materials in terms of chemical composition, crystal structure, and properties.
7. What is the significance of the spinel crystal structure for the properties of many ferrites?
Many common ferrites (MFe₂O₄) adopt the spinel crystal structure. In this structure, the oxide ions form a face-centered cubic lattice, and the metal cations (M²⁺ and Fe³⁺) occupy specific interstitial sites, known as tetrahedral and octahedral sites. The magnetic properties of the ferrite are critically dependent on how these different metal ions are distributed among these two types of sites. The anti-parallel alignment of magnetic moments between ions on the tetrahedral sites and ions on the octahedral sites is what gives rise to ferrimagnetism.
