Samarium-Cobalt magnets (SmCo)

Magnets have an essential role in many sectors of the economy. This is due to electromagnetism and to the permanent magnetism of the magnets. The magnetic alloy of samarium and cobalt is very important for the production of permanent magnets. The metal alloy SmCo5, developed in 1966 does not contain any iron, in contrast to the alloy Sm2Co17, which contained a percentage of iron between 20-25%. Until the end of the ‘70s, this alloy was the one with the highest density of magnetic flux.

Fundamentals of magnetism

In general, the magnetic fields are displacements of electrically charged particles. This happens each time the current flows through an electrical conductor (for example, metals such as copper, aluminum, etc.). Even electrically charged particles (particles with spin), like electrons, have a fraction in which are magnetic and therefore represent a small magnets. Since all the electrons are particles with spin we can deduce that all matter has the magnetic properties. In most cases these magnetic moments exist no longer after a short period of time so that the matter seems non-magnetic outside. After exposure to a magnetic field the material undergoes a change. Depending on the behavior under the influence of an external magnetic field the materials are distinguished in these categories: diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials. In fact, all the materials are initially diamagnetic because they contain all the electrons invariably associated with opposite spin. By the action of an external magnetic field within the subject is formed a weaker magnetic field. This phenomenon is so minimal that usually cannot be seen. Are there are unpaired electrons, they are magnetized by an external magnetic field, which usually results in a magnetization unstable (paramagnetism), which magnetizes the matter in his favor. After removing the magnetic field the effect is stopped. The paramagnetism is usually so weak that it can not be observed without technical aids. However, if the magnetism keeps the same orientation of the spin of unpaired electrons stably, it is called ferromagnetism. Materials with these characteristics are therefore suitable for the production of permanent magnets.

Permanent magnets

The materials with ferromagnetic properties are the main base for the production of permanent magnets. These are substances, which due to the quantum mechanics have unpaired electrons with parallel spin in their atoms and may therefore form its own magnetic field. Thanks to the action of an external magnetic field, the spin of the electrons will align and create a permanent magnetic field. Such materials include iron, cobalt, nickel, and some lanthanides as samarium-cobalt, alnico, neodymium or ferrite.

The properties of ferromagnetic materials

Ferromagnetic materials contain within them the so-called molecular magnets, generated by unpaired electrons with parallel spin. An external magnetic field does change only the direction of these elementary magnets and magnetizes the material. The field strength is independent of the external magnetic field. The magnetism of ferromagnetic materials is a phenomenon due to complicated quantum-physical processes, which include an exchange of electrons that in this way stabilize the magnetization and the direction of the magnetization. During this process energy is added to the material, which mass is defined energy product. As soon as the external magnetic field is lacking this phenomenon ceases to exist. However, it remains a residue, which is called remanence. The coercive force instead determines the stability of the magnetic field. The magnetic field can be destroyed by using elevated temperatures (Curie temperature) or by using some mechanical treatments, as well as through the use of a strong external magnetic field. The challenge in the development of permanent magnets is finding materials that can create a magnetic field that is strong and resists high temperatures and high mechanical stress. The magnetic alloys that have good features are AlNiCo, SmCo, NdFeB and ferrite.

The properties of the alloys of samarium and cobalt

There are two types of SmCo alloys, which can reach a very high density of magnetic energy: 130-200 kJ/m3 for the alloy SmCo5 and 160-260 kJ/m3 for Sm2Co17. The energetic product is therefore very high. In addition to this, their magnetic field is stable and resistant to external influences. The alloys of cobalt-samarium are difficult to demagnetize. The Curie temperature is 450 degrees Celsius. Above this temperature, the magnetization disappears. The magnet can be easily used at a temperature of about 350 degrees without loss of the magnetic field. The temperature coefficient of residual flux density is very low and is between 0.003 and 0.004% per degree. This means that the magnetic field, with an increase in temperature, decreases only slightly. In addition, the coercive force is extremely high and exceeds some other permanent magnets. Samarium-cobalt alloys have the advantage of being highly resistant to corrosion. However, you should be pay attention using inorganic acids and alkali.

Production of samarium cobalt magnets

Samarium and cobalt are melted in an inert argon gas atmosphere, because they would react with oxygen present in the air. After that, they are poured into ingot forms. Various additions of other materials help to improve the thermal properties. During solidification of the melt the material forms crystals that prevent the stabilization of the magnetic field. The alloy is easily demagnetized and is not suitable as a magnetic material. The ingots obtained are pulverized under an inert gas atmosphere and subjected to temperatures of 1150-1250°C for the sintering process. The individual powder particles are "baked" together. During this process the real magnetization happen. After this process is no longer possible to modify the magnets. The material may splinter and possibly catch fire.

The use of permanent magnets

Permanent magnets, including those in samarium and cobalt, have many uses in daily life, for example, in closure systems, for notes and much more. However, special uses are found in the conversion and production of energy.
In generators for example the rotor is composed of many magnets that generate electricity. Furthermore, the permanent magnets may also be used in electric motors for the conversion of electrical energy into mechanical energy.

The main fields of application of SmCo

These magnets are used when a very strong magnetic field in extreme conditions (temperatures from -40 up to 350 ° C) is necessary.

The main application areas include:

- Generators

- Engines

- Sensors

- Several indicators

Nowadays the most used alloy is Sm2Co17 because it is less expensive because of lower percentage of samarium. Specific applications make use of SmCo5 especially when you need very strong magnetic field strengths.

Comparative analysis of other types of magnets with samarium cobalt magnets

Samarium cobalt magnets have as other permanent magnets, a high energy density, resistance to high temperatures and high coercive forces. However, there are differences that may be advantageous or disadvantageous for you, depending on the expected use.

Comparison with neodymium magnets

The components of these magnets are neodymium, iron and boron. This combination of metals has been used since 1970. The energy density is significantly higher than that of samarium-cobalt magnets. However, neodymium magnets can only be used at temperatures up to 80 ° C. Moreover, this alloy is less resistant to other magnets that has got better with some additions to the mixture that managed to increase the resistance to temperature and corrosion, but the values do not get even close to the magnets SmCo.

Comparison with ferrite magnets

Ferrite magnets are composed of an alloy of iron oxide and barium or strontium, and has a consistency similar to ceramic. Ferrite magnets are in use all over the world and are very popular because they are cheap and at the same time highly resistant to corrosion. In addition, they may be exposed to temperatures between -40 degrees up to 250 ° C. These magnets do not meet the criteria if you are searching for magnets with a stronger magnetic field. In this case, we suggest neodymium magnets or samarium cobalt.

Comparison with Alnico magnets

The AlNiCo magnets are made of an alloy of aluminum, cobalt and nickel. They can be used up to 550° C, they are highly resistant to corrosion and have high retention. However, they have a low coercivity and can be easily demagnetized by external magnetic fields. In many applications it has been already replaced by ferrite magnets or for uses at higher temperatures by SmCo magnets.

Conclusion

SmCo magnets have a high density of magnetic energy and can be used at temperatures up to 350 °C. They are resistant to external magnetic fields and are also highly resistant to corrosion. Their production is more expensive than other magnets because samarium is considered a rare metal. The uses of these magnets are in areas where you need a high magnetic field intensity at high temperatures such as construction of generators, motors, sensors and measuring devices.