Ferromagnetic materials are those substances which are strongly magnetized in the direction of the magnetic field when subjected to one. Examples of ferromagnetic materials include iron, nickel, cobalt, and rare earth materials.
Did You Know?
Earlier, the term ‘ferromagnetism’ was used for any material that could show self-generated magnetization.
Ferromagnetism is the ability by which a material forms a permanent magnet or is pulled in by a magnet when the material is placed in a magnetic field. There are various kinds of magnetism, out of which ferromagnetism is the strongest type. The materials that possess ferromagnetism are called ferromagnetic materials. The magnetic forces created by these materials are very high. Interestingly, many such materials are used in our everyday life. A refrigerator magnet is a good example. It is used for posting reminders and shopping lists, or as a decorative piece.
There are wide applications of ferromagnetic materials in devices like electric motors and generators, transformers, telephones, loudspeakers, magnetic recording devices, such as cassette tapes, floppy discs for computers, and the magnetic stripe at the back of credit cards.
Properties of Ferromagnets
A ferromagnetic material can produce a very intense magnetic field and is greatly attracted to a magnet.
A ferromagnet retains its magnetic property even after the magnetic field is withdrawn.
In ferromagnetic materials, the atoms have permanent magnetic moments. A magnetic moment is a quantity that determines the force of a magnet in a magnetic field. It is a vector quantity and points from the south to north pole of a magnet. The magnetic moment is due to the electrons of the magnet. A force is needed to keep the magnetic moments of atoms parallel to each other. An external magnetic field creates an internal driving force that leads to the parallel alignment of the electron moments.
A ferromagnetic material has a large number of small regions called domains where the atomic or ionic magnetic moments are aligned parallel to each other. The magnetic field is strong in this domain. The moments in the different regions of the material may be aligned with a different axis or with the same axis.
All ferromagnetic materials turn into paramagnetic materials (materials which shows no magnetism when the magnetic field is removed) above a temperature known as the Curie temperature. It is the critical temperature at which magnetic moments of the electrons of atoms change direction. Above the Curie temperature, a ferromagnetic material changes into a paramagnetic material.
When a ferromagnetic rod is set freely in a uniform magnetic field, it aligns itself in the direction of the applied field.
Ferromagnetic materials have a permeability of more than 1. It is the measure of the ability of a material to maintain a magnetic field within itself.
A ferromagnetic substance is strongly magnetized in the direction of the applied magnetic field.
The magnetic susceptibility (degree of magnetization in response to an applied magnetic field) of ferromagnets is great. According to the Curie-Weiss law, magnetic susceptibility reduces if the temperature is increased. The law states that above the Curie temperature, ferromagnetic materials become paramagnetic. The reason for this is that thermal energy can change the alignment of the magnetic moments.
If a ferromagnetic material is magnetized by increasing the intensity of the magnetic field, then the variation of flux through the material is not rapid, but gradual.
Ferromagnetic materials move from a less intense region of the magnetic field applied to a more intense region.
Examples of Ferromagnetic Materials
Magnetite is a ferromagnetic material which is formed by the oxidation of iron into an oxide. It has a Curie temperature of 580°C. Earlier, it was recognized as a magnetic substance. Magnetite has the greatest magnetism among all the natural minerals on earth.
Neodymium magnet, also called the Neo magnet, is a type of rare earth magnet. It is a permanent magnet that constitutes an alloy of neodymium, iron, and boron. It has a Curie temperature in the range of 100°C to 200°C depending upon the grade of the magnet. It has wide applications in motors, cordless tools, loudspeakers, headphones, magnetic resonance imaging, hard disk drives, and magnetic fasteners.
Iron is an amorphous metal that has a Curie temperature of 1043 K. Magnetic iron has applications in radio engineering and telecommunication. It can be used to read the encoding on a magnetic stripe card by sprinkling iron fillings in the stripe.
Nickel is a silvery-white shiny metal. It is also an amorphous transition metal and has a Curie temperature of 627 K. It is magnetized by cooling the liquid alloy in cold water or another liquid.
Cobalt, a transition metal, possesses a Curie temperature of 1388 K. Cobalt has applications in making strong magnets for carbon nanotubes and electronics.
In addition to iron, Alnico is made of aluminum, nickel and cobalt. It also has copper and at times, titanium. Alnico alloys are ferromagnetic and their applications are in making permanent magnets. Sintered alnico has a Curie temperature of 810 – 860°C. Cast alnico has a Curie temperature of 740 – 860°C.
Gadolinium is a rare earth metal and has a Curie temperature of 292 K. It is widely used as a neutron absorber in nuclear reactors.
Dysprosium is also a rare earth metal and has a Curie temperature of 88 K. It has a silvery shine and is very soft. It is highly susceptible to magnetic fields.
Permalloys comprise various proportions of iron and nickel. They can be used in microwave devices and in single-chip electronics. By varying the proportions of iron and nickel, the properties of the permalloy can be changed. Molybdenum permalloy has a maximum magnetic permeability (μmax) of 150,000, a magnetization of 0.85 tesla, and a Curie temperature of 400°C.
Awaruite is a ferromagnetic material and an alloy of nickel and iron. It has a Curie temperature of 620°C. It is used to study the constitution of meteorites. It also has geological uses.
This ferromagnetic material is an alloy of cobalt and iron. It has a Curie temperature of 986°C.
A samarium cobalt magnet is a strong permanent rare earth magnet. It constitutes an alloy of samarium and cobalt. It has a Curie temperature from 700°C (973 K) to 800 °C (1,070 K). These magnets can solely be magnetized in the axis of their magnetic orientation.
Chromium dioxide is an organic compound. It is a black magnetic substance and is used in data tapes. It has a Curie temperature of approximately 386 K.
Ferrite, also called α-ferrite or alpha iron, is a ferromagnetic material. It has a Curie temperature of 771°C. It possesses a strength of 280 N/mm2 and a hardness of around 80 Brinell.
Metglas is an amorphous metal alloy made by applying a solidification process of around 1,000,000 °C/s. This process builds unique ferromagnetic properties that lets the alloy be magnetized and de-magnetized in a quick manner. It has a high permeability, strength, and hardness. Metglas has different Curie temperatures depending upon its magnetic alloy.
MKM steel is an alloy comprising nickel and aluminum. This ferromagnetic material is tough and durable and hence used in the electronics sector, communication, aviation, and automobile industries.
Gallium manganese arsenide
It is a magnetic semiconductor having ferromagnetic properties. It has a Curie temperature of 170 K.
Sendust is a magnetic metal powder. It is made of 85% iron, 9% silicon, and 6% aluminum. Sendust core has a Curie temperature of above 410°C. It possesses high magnetic permeability, low coercivity, good temperature stability and a flux density of around 1 T. It is used in magnetic recording heads.
Suessite is a rare iron silicide mineral. It is ferromagnetic and has a Curie temperature of 823.15 K and a magnetic moment of 4.6 μB.
Ferromagnetic materials have been used widely since long. They find uses in many vital industry segments, and hence are of great importance in our lives.