Ferromagnetism or the meaning of ferromagnetism is a mechanism through which certain materials form permanent magnets. With the aid of a strong electrostatic field, these materials can be permanently magnetized. Ferromagnetic metal ions are grouped into small regions called solid-state domains. So every domain is acting like a tiny magnet. The domains of a ferromagnetic unmagnetized piece are randomly oriented so that their magnetic moments are canceled out. When this material is put in a magnetic field, all domains are oriented in the direction of the magnetic field, creating a powerful magnetic effect. Also, when the magnetic field is withdrawn and the ferromagnetic material becomes a permanent magnet, this order of domains remains the same. There are many different forms of magnetism, but ferromagnetism is of the strongest form and is responsible for the widespread occurrence of magnetism in magnets experienced in everyday life.
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Examples of Ferromagnetic Materials
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Co (Cobalt)
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Fe (Iron)
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MnBi
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Ni (Nickel)
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[Nd_{2}Fe_{14}B]
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MnSb
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[CrO_{2}] (Chromium dioxide)
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MnAs
Properties of Ferromagnetic Materials
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When a rod of this material is placed in a magnetic field, it quickly aligns itself in the field track.
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These substances show the permanent magnetism even in the absence of magnetic field
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When the substances are heated at high temperatures, the ferromagnetic substances transform to paramagnetic
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Permeability of ferromagnetic material is greater than 1.
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The mechanism of ferromagnetism is absent in liquids and gases.
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The intensity of magnetization (M), relative permeability ([mu_{r}]), magnetic susceptibility ([chi_{m}]), and magnetic flux density (B) of this material will always be positive.
[X_{m} = frac{M}{H}]
[mu_r = 1 + X_m]
B = [mu_0](H + M)
[mu_0 rightarrow] Magnetic Permittivity of the free space.
H [rightarrow] Applied Magnetic Field Strength.
Hysteresis Loop
The hysteresis loop is formed by altering the magnetizing force while at the same time measuring the material’s magnetic flux. When a ferromagnetic material is magnetized in one direction, removal of the imposed magnetizing field will not relax back to zero magnetization. A field in the opposite direction needs to drive it back to zero. When an alternating magnetic field is applied to the object, a loop called a hysteresis loop can be traced for its magnetization.
The absence of magnetization curve re-traceability is the property called hysteresis, which is due to the presence of magnetic domains in the material. Upon reorientation of the magnetic domains, it takes some energy to turn them back.
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This property is useful as a magnetic “memory” of ferromagnetic materials. The magnetic memory aspects of iron make them useful for recording audiotape and for storing data magnetically on computer disks.
Curie Temperature
There is a temperature, over which the ferromagnetic material is paramagnetic. This specific temperature is called temperature Curie. This is, if we rise above Curie temperature, it will cause the ferromagnetic materials to lose their magnetic properties. Curie temperature is represented by [T_{c}]. Thermal energy interrupts the magnetic ordering of the dipoles in the ferromagnetic material.
[E_{thermal} = k_{B} T]
Curie’s law is given by X = [frac{C}{T}]
[k_{B} rightarrow] Boltzmann Constant
C [rightarrow] Curie Constant
Examples,
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Ni – 627 K
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Gd – 293 K
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Co – 1388 K
What is Antiferromagnetism?
Antiferromagnetic materials are weakly magnetized in the direction of the field, in the presence of a strong magnetic field. This property of the materials is called antiferromagnetism and antiferromagnetic materials are called the materials which exhibit this property. The magnetic moments are aligned in opposite directions in antiferromagnetic materials and are equal in magnitude. Thus, when antiferromagnetic material is unmagnetized the net magnetization is zero due to the exact cancellation of magnetic moments of the adjacent atoms when added in a line.
Application of Ferromagnetic Materials
Ferromagnetic materials have many applications for electrical, magnetic storage, and electromechanical equipment.
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Permanent Magnets: Ferromagnetic materials are used for making permanent magnets because its magnetization lasts longer.
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Transformer Core: A material used to make the transformer core and choke is subjected to very rapid cyclical changes and the material must also have strong magnetic induction. Ferromagnetic materials are highly used to serve the purpose.
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Magnetic Tapes and Memory Store: The magnetization of a magnet is not only dependent on the magnetization field but also on the magnetization cycle it has undergone. Thus, the specimen’s magnetization value is a record of the magnetization cycles that it has undergone. Thus, such a machine will serve as a memory storage unit.
Ferromagnetism vs Paramagnetism vs Diamagnetism
Ferromagnetism is a process through which a metal or substance is turned into a highly magnetic substance or a strong permanent magnet when it is subjected to an electromagnetic field. When these substances are brought out of the magnetic field, they remain as strong permanent magnets. This process can only be found in solids and is absent in liquids and gasses. But, when the ferromagnetic substances are exposed to extreme heat, they lose their magnetic property. Paramagnetism is a similar mechanism in which some objects or metals are weakly attracted to magnetic objects. When ferromagnets are subjected to extreme heat, it disrupts the perfectly aligned atoms and changes the object into a paramagnet. On the other hand, diamagnetism is a process in which a substance acts as a weak magnet when the orbital motion of the atoms of the object is changed by an external electromagnetic field. Both the paramagnets and diamagnets are weak magnets and to find the difference between both, the rule of thumb is, a
n object is said to be paramgent if all the electrons within the objects are unpaired and an object is said to be diamagnetic if all the electrons in the object are paired.
Fun Facts
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Magnetar is the most powerful magnet in the universe.
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Hammering a magnet can cause its magnetic properties to lose out. Heating up a magnet is another means of destroying its magnetic properties. This is because the molecules lose their alignment north-south and get arranged in random directions.