What Is a Ferrite Magnet?
Ferrite magnets are made by mixing iron oxide with cobalt, nickel, and manganese. The molecular formula is represented as MFe2O4 (M = Mn, Fe, Co, Ni, Cu, Mg, Zn, Cd, etc.).
The composite metal M is dominated by divalent cations, and Fe3O4, where M = Fe, is black and is a well-known raw material called magnetite. Other compounds exist where the compound metal M is 1, 3, or even tetravalent cations, called ferrite.
The manufacturing process involves pressing powdered ferrite and baking it at high temperatures. It is also a type of ceramic. It can be synthesized from iron oxide Fe2O4 and is therefore inexpensive.
The characteristics of ferrite magnets are that they can be easily formed into any shape and are easy to process and that they are chemically stable and resistant to corrosion by rust and chemicals.
Applications of Ferrite Magnets
Applications of ferrite magnets can be divided into two types: hard ferrite and soft ferrite.
1. Hard Ferrite
Hard ferrite magnets are ferrite magnets that become permanent magnets once they are attached to a strong magnet (by applying a strong magnetic field). U-shaped magnets are the most common type of magnet we see in our daily lives.
U-shaped magnets are typical examples. They are also used in small motors, speakers, headphones, and cassette tapes.
2. Soft Ferrite
Soft ferrite is a ferrite magnet that becomes a magnet when in contact with a magnetic field and ceases to be a magnet when removed from the field. It is often used as a magnetic core and is suitable for transformers and coils.
Examples of easy-to-understand applications are used in radios, televisions, game consoles, automobiles, personal computers, microwave ovens, vacuum cleaners, and refrigerators.
Principle of Ferrite Magnets
The magnetic properties of ferrite magnets differ between hard ferrites and soft ferrites. First, let us explain the magnetic properties. Figure 1 shows the spin state of each magnetic property.
1. Magnetic Properties
- Ferromagnetic: A material whose magnetic moment (a vector quantity that expresses the strength and direction of a magnet) is aligned without the application of a magnetic field is called a ferromagnetic material.
- Ferrimagnet: A ferrimagnetic material is one in which the magnetic moments of neighboring atoms are opposite in direction but different in magnitude so that the material as a whole is magnetized. All ferrite magnets are ferrimagnetic.
- Paramagnetic: When there is no magnetic field, the magnetic moments are oriented in various directions, but when a magnetic field is applied, the magnetic moments become aligned.
The types and characteristics of each type of ferrite magnet are shown in Figure 2. Saturation magnetization is the maximum magnetization at which the magnetization of a material does not increase even if the magnetic field is increased. Curie temperature is the temperature at which the magnet changes from ferromagnetic to paramagnetic.
2. Hard Ferrite
Hard ferrite is a ferromagnetic material and a permanent magnet. Hard ferrites can be further classified into isotropic and anisotropic magnets according to the orientation of the magnetic poles of their molecules.
- Isotropic Magnets: The magnetic moment is oriented in various directions. Since the orientation of the magnetism is not uniform, it can be magnetized from any direction, but the magnetic force is weak.
- Anisotropic Magnets: The orientation of the magnetic moment of the molecules is aligned, providing directionality but a strong magnetic force. It is produced by aligning the magnetic poles of each ferrite molecule by applying a magnetic field during the hardening process.
3. Soft Ferrite
Soft ferrite is magnetic only while an external magnetic field is applied. Although the magnetic field is smaller than that of hard ferrite, it has excellent magnetic properties over a wide frequency range.
For example, a spinel-type crystal structure has a high magnetic permeability (the degree to which a material is magnetized) over a wide range of frequencies. The garnet type has the property that single crystals are not easily broken in the microwave frequency range.
Comparison With Alnico Magnets
Alnico magnets are manufactured by adding aluminum (Al), nickel (Ni), cobalt (Co), and other additive elements to iron and using casting methods or powder sintering.
Alnico magnets are characterized by their extremely high Curie temperature (the temperature at which they cease to be permanent magnets) of 860°C, allowing them to be used in high-temperature environments. Alnico magnets can recover their original magnetic force when returned to room temperature at temperatures ranging from room temperature to 400℃. Those manufactured by casting also have excellent mechanical strength.
Applications for Alnico magnets include electric motors, sensors, speaker units, and magnetic pickups for electric guitars.
Differences From Ferrite Magnets
Ferrite magnets are primarily composed of iron oxide, while Alnico magnets are composed primarily of iron with aluminum, nickel, and cobalt added. The retention of magnetic force in an Alnico magnet is small and easily demagnetized.
There is a limitation that they must have a long shape due to the need for a long distance between the poles. Also, the supply of the raw material, cobalt, is unstable and expensive, so ferrite magnets are less expensive.
Comparison With Samarium Cobalt Magnets
Samarium cobalt magnets are rare earth magnets composed of samarium (Sm) and cobalt (Co). They are divided into SmCo5 (1-5 series) and Sm2Co17 (2-17 series) according to their composition ratio, with the 1-5 series, which contains less samarium, being widely used today.
Samarium cobalt magnets are characterized by a high Curie temperature of approximately 800°C at maximum. Because of its excellent corrosion resistance, it can be used as is without surface treatment, and its shape is highly selectable. Magnetic properties are higher than those of ferrite magnets and second only to those of neodymium magnets.
Differences From Ferrite Magnets
Because they can be used in environments up to about 350°C, they are used in space-saving, high-temperature environments where higher magnetic force than ferrite magnets is required. On the other hand, it has the disadvantage of low strength, which makes it prone to cracking and chipping. The raw materials samarium and cobalt are both rare, making them very expensive compared to ferrite magnets.
Comparison With Neodymium Magnets
A neodymium magnet is a magnet composed mainly of neodymium (Nd), iron (Fe), and boron (B). Neodymium magnets are characterized by their easy oxidation and high thermal dependence.
Because they oxidize easily, their surfaces are nickel-plated before use. They are usually used at temperatures below 80°C. Because of its relatively high strength, it is also resistant to cracking and chipping.
Differences From Ferrite Magnets
Compared to ferrite magnets, magnetic properties are extremely high, with about four times the magnetic holding power and 10 times the maximum energy product. It is more expensive than ferrite magnets, but less expensive than samarium cobalt magnets.