Ferrite Magnets

Ferrite magnets – this is why they attract us!

Ferrite magnets - Who does not know these little blacks magnets? Impossible not to know them! Even though most of you think immediately of magnets for blackboards. Ferrite magnets have a much larger potential! They are in fact used in the construction of generators, loudspeakers or microphones! They can also be found in some of computer components and in cars. The magnets used for these purposes are almost always made of hard ferrite.
Ferrite magnets are becoming increasingly popular - not only because of their relatively low price and their high resistance to corrosion, but also for their versatility.

Magnets in hard or soft ferrite

Ferrite magnets have a constant magnetic field and they are called permanent magnets, which consist of a magnetizable piece of ferrite material. The ferrite magnet in accordance with the magnetic order is considered ferrimagnetic: a hybrid between the ferromagnetic and antiferromagnetic category since the ferrite does not lose its static magnetism.
There is no doubt: ferrite magnets are the most produced in the world. The production methods of these magnets are very similar to those of other ceramic materials.

The ferrite material is divided into two types, hard ferrite magnets and soft ferrite magnets.
The soft ferrite is characterized by a low coercivity, whereas hard ferrite has a very strong magnetic field. As magnetic coercivity means the force of the magnetic field, which is necessary to completely demagnetize a ferromagnetic material. From this it follows that the higher this value, the higher the resistance of this material to an exposure to another magnetic field.
How can I determine if my magnet is made of hard or soft ferrite? The instrument used for this purpose is the hysteresis curve - also called the hysteresis loop. This curve indicates the magnetization, which is represented by two factors: the flux density and the field intensity in their context.

The production of these magnets is mainly located in Asia. Chinese producers developed an easier mass production method which allows a huge production in reduced times.

Isotropic and anisotropic magnets

There are two categories of ferrite magnets depending on their production method. Isotropic ferrite, rather weak, and anisotropic ferrite, much stronger.
The production of isotropic magnets is made with the use of high pressure. The later magnetization is due to an external magnetic field. These magnets are produced in different ways.
The pressure production system is also called “dry” the other one is called “moist” depending on the state of the materials while forming the magnets. The dry preparation has a great advantage: it is possible to magnetize the magnets in different directions - depending on the desired magnetization.

In contrast, anisotropic magnets are produced using the moist preparation. After forming the magnets in a second step the magnetic field takes the predetermined direction. The advantage is in this case the high energy density.

Manufacturing process and production

Ferrite magnets are generally produced in a sintering and calcination process. This is a chemical reaction of the starting materials of iron oxide and barium or strontium. This process is called, using an outdated term, "pre-sintering". Subsequently, the product is ground into fine powder and then placed in molds for the drying and sintering process. The magnetization occurs also here using an external magnetic field. The particles are carried in a preferred orientation (anisotropy).
The so-called dry pressing method can also be used for the construction of small and simple geometric shapes. The cause of the lower values ​​compared to the wet production is mainly due to the tendency of smaller particles to re-agglomerate.

Soft ferrite is composed of materials which are ground and then pressed at the beginning of the process, after that there is a sintering process, however this happens under specific conditions - such as lack of oxygen.

The peculiarity of the wet press process is the emulsion produced from powders of alloys and liquid. Filling channels ensure that the emulsion is inserted in the forms. The subsequent compaction causes the alignment of the particles, that is, the adjustment of anisotropy, with the action of an external magnetic field. During this process, some of the liquid is leaking out of the forms. The sintering is carried out after a drying process which is carried out at room temperature or in special furnaces. There is no doubt that the wet pressing process is more complex than the dry pressing process, this however guarantees best properties magnets and higher resistance.

Advantages and disadvantages

A big advantage of these magnets is the high coercivity and high resistance. Another pro for the ferrite magnets is their low price and a higher corrosion resistance than other magnets. A special surface treatment is not necessary.
Their great characteristic is their particular resistance to high temperatures: can be used at temperatures between -40 °C and +250 °C, this is due to the sintering process which is made at high temperatures. They have also a particular resistance to chemical agents.
The only disadvantage of these magnets is the strength of the magnetic field, which is not comparable to neodymium magnets or samarium-cobalt magnets.

Ferrite magnets are produced using two metal alloys iron with either strontium or barium. A great advantage of SrFe - compared to BaFe - is that this is the only approved alloy for the use in contact with drinking water.

Purposes

Horseshoe or disc, blocks or rings or even customized shapes! Ferrite magnets are known and famous. In electrical engineering they are in demand because their electrical conductivity is almost not existent. These characteristics are ideal for the basic material for the coils and transformers - at higher frequencies.

The soft ferrite magnets instead are used as magnetic heads for tape recorders, computer hard drives or disk drives. The hard ferrites are applied for example in the construction of loudspeakers, electric motors etc.
Improving the quality of these magnets can make an important contribution to the solution of many issues that affect our society and economy today: this includes the question of energy saving, as well as environmental issues or the intelligent use of resources in general. These magnets are not only advantageous for the price but also because they are sustainable.

Economy, ecology a union between humans and nature. What could be better?

That's why the ferrite magnets attract us so much. 

Photo Credits:

• Generator: I, Dergenaue [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons
• Lautsprecher: By Gary Luck (Own work) [Public domain], via Wikimedia Commons
• Entstördrossel: By Nightflyer at de.wikipedia(Original text : --Nightflyer 21:52, 2. Jan. 2007 (CET)) [GFDL or CC-BY-SA-3.0], from Wikimedia Commons
• Ferrit-Zirkulator: By Ulfbastel.Ulfbastel at de.wikipedia [GFDL or CC-BY-SA-3.0], from Wikimedia Commons
• Hysteresekurve: http://commons.wikimedia.org/wiki/File:Hysteresekurve.gif, public domain