What is a Magnetic material?

Every material has a unique composition that defines its magnetic behavior. Some materials get attracted towards a magnet and are called magnetic materials. Based on the magnetic properties held by each material, they are classified into different types.

The magnetic property of materials

Every material exhibits magnetic properties when placed in the vicinity of the magnetic field. The electrons located inside the atoms of the material are responsible for the magnetic behavior around a magnetic field. Each atom consists of a fixed number of atoms and each atom will act like a minute magnet that creates its magnetic field. Since the magnetic field is directly related to current, electrons can be considered as loops that conduct current. The revolving current will produce a magnetic moment.

The movement of electrons can be classified into two types. Electrons revolve around the nucleus in their designated orbitals. This is the orbital motion of electrons. The moment of spin arises when electrons rotate around their own axis. Both motions contribute to the behavior of the material in presence of a magnetic field.

Classification of materials based on magnetic properties

According to the behavior in the external electric field, materials are classified into the following five types.

• Diamagnetic materials
• Paramagnetic materials
• Ferromagnetic materials
• Antiferromagnetic materials
• Ferrimagnetic materials

Diamagnetic materials

When a material is weakly repelled on being exposed to an external magnetic field, then the material is classified as diamagnetic material. When placed near a magnet, weak repulsion force is experienced by the material. Electrons inside the atoms of diamagnetic materials are dependent on this behavior. The orbitals are filled, and all electrons are paired up. So, the spin number will be zero. Materials such as Copper, Bismuth, Gold, Silver, Water, Sodium chloride, Antimony, and Zinc exhibit diamagnetism.

Anton Brugmans first discovered the diamagnetic property in Bismuth. The diamagnetic property of materials is a quantum mechanical effect. The magnetic permeability of diamagnetic material is very small. It is less than one, which is the permeability of the vacuum. The diamagnetic substances have a high coercivity and low permeability.

All materials have diamagnetic properties, but it is a weak force. Sensory lab instruments are required to detect and measure the diamagnetic properties of materials.

Superconductors

Perfect diamagnets are considered as superconductors. These materials block any magnetic field that passes through them. This effect is known as the Meissner effect. Because of this effect, Electric current can pass through these materials without the loss of charge. Materials such as aluminum, mercury, and doped silicon carbide acts like superconductors.

Paramagnetic materials

The new material is weakly attracted when exposed to an external magnetic field, so the material can be classified as a paramagnetic material. When such materials are placed in front of a magnet, a weak attractive force can be experienced. The magnetic permeability of paramagnetic materials is greater than one and that is the cause of the attraction. Squid magnetometers are easily used to measure the paramagnetic property of materials in laboratories. Most chemical compounds and elements exhibit paramagnetic properties.

Paramagnetic materials exhibit such behavior due to the presence of electrons that are unpaired in the orbital of the atom. The unpaired electrons contributed to incompletely filled orbitals and this leads to paramagnetism. The external magnetic field will affect the dipole moment of the unpaired electrons. In the absence of the magnetic field, these unpaired electrons will be aligned in random directions, so, no contribution will be made to the magnetic force. However, when exposed to an external magnetic field, these magnetic dipoles will align in a parallel direction to the applied magnetic field. The parallel alignment of magnetic dipole moment will induce attractive forces, so paramagnetic materials can be considered to be temporarily magnetized. They cannot retain magnetic properties. When the external field is absent, materials will lose their magnetic properties and behave normally rather than like a magnet.

The induction of magnetism into paramagnetic materials depends upon the number of unpaired electrons in the orbitals of the atom. This process is a quantum mechanical property that involves the spin and angular momentum of the atom.

The magnetic susceptibility of better magnetic materials is very low in the order of 1e^-3 to 1e^-5

Ferromagnetic materials

Materials that are strongly attracted by an external magnetic field can be classified as ferromagnetic materials. These can be permanently magnetized and can be used to make magnets. The magnetic permeability of the materials, which are considered a ferromagnet, is very high and the range is from hundreds to 1000.

The presence of a large number of unpaired electrons in the orbitals of an atom is responsible for the ferromagnetic behavior of the material. In the unmagnetized state of the material, the unpaired atoms will act as individual dipoles that group together to form magnetic domains. Each of these magnetic domains will be arranged randomly and the total effect in the absence of an external magnetic field is canceled out. In the presence of an external magnetic field, these domains will rearrange in a single direction that is parallel to the external magnetic field. This will cause strong attractive forces and a high value of magnetic susceptibility. So, they will act as permanent magnets with magnetic poles.

Even after removing the external magnetic field, ferromagnetic materials can retain the magnetic properties and will act as permanent magnets. Ferromagnetic substances have low coercivity and high permeability. They have very low hysteresis loss and high electrical resistivity.

Materials such as cobalt iron, nickel, and alloys of other transition metals exhibit ferromagnetic properties. Rare-earth metals are ferromagnetic in nature.

Soft ferromagnetic materials

Ferromagnetic materials with high relative permeability and a small hysteresis are known as soft ferromagnetic materials. These materials can be easily demagnetized and magnetized and are mainly used in electromagnets and other electrical applications. Magnetic screening is also done using soft ferromagnetic materials. These are not used to create permanent magnets.

Hard ferromagnetic materials

Ferromagnetic materials with low relative permeability are classified as hard ferromagnetic materials. Magnetizing and demagnetizing these materials is a very hard process and is time-consuming. They retain all of their magnetic property even after removing the external magnetic field. These are used to create permanent magnets in instruments and speakers.

One of the strongest permanent magnets is alnico magnets. It is created from the alloy of aluminum-nickel-cobalt.

Anti ferromagnetic materials

Materials that have the net magnetic moments of zero are classified as antiferromagnetic materials.

The unpaired electrons in antiferromagnetic materials will act as individual dipoles. These dipoles will have dipole moments and they will align to form magnetic domains. In the absence of an external magnetic field, the domains are arranged randomly and the net magnetic moments will be zero.

In the presence of an external magnetic field, these domains are arranged in parallel and anti-parallel fashion. Half of the domains will be arranged parallel to the external magnetic field and the other half will be arranged in the antiparallel direction. So, the net magnetic moments will remain zero even after applying the external magnetic field.

Antiferromagnetic materials exist only under low-temperature conditions. Above a particular temperature, called Neel temperature, anti-ferromagnetism is lost and materials exhibit paramagnetic properties. At Neel temperature, the value of magnetic susceptibility of the material will be the maximum value and it exhibits a nonlinear relation between magnetization and magnetizing field.

Materials such as chromium and oxides of transition metals exhibit antiferromagnetic properties.

Ferrimagnetic materials

Ferrimagnetic materials can be compared to antiferromagnetic materials, but these materials will have an unequal number of domains that are arranged in parallel and antiparallel directions. Because of this uneven distribution, the net magnetic moments will not be equal to 0.

Materials such as magnetite exhibit ferrimagnetic properties.

Relationship between magnetic properties and temperature

Magnetization of a material is dependent on the temperature. This is expressed by Curie’s law.

The law states that the magnetization of a material is inversely proportional to the temperature and the constant of the proportionality used is called the curie’s constant.

Curie temperature can be defined as the limiting temperature below which the materials will have their permanent magnetic property. Above this temperature, loss of such magnetic properties will happen, domains will disintegrate, and ferromagnetic and ferrimagnetic substances will act as paramagnetic substances.

Context and Applications

The topic is studied under various courses like-

• Bachelors in Technology in Electronics
• Masters in Technology in Electronics
• Bachelors in Science in Physics
• Masters in Science in Physics

Practice Problems

1. In a bar magnet, magnet field line travels in which direction?

1. perpendicular to the outer surface
2. from north pole to south pole
3. from south pole to north pole
4. none of these

Answer:  b

Explanation: In a bar magnet , the magnetic field lines emerge from the north pole and they travel to the north pole.

2. Which of the following is not a permanent magnet?

1. alnico magnets
2. neodymium magnets 
3. electromagnets
4. Bar magnet

Answer: c

Explanation: Electromagnets are activated by passing electric current, so they are temporary magnets.

3) What is the flux density inside a superconductor?

1. high value
2. negative value
3. Zero
4. one

Answer: c

Explanation: Superconductors do not allow the magnetic field to pass inside the material, so the flux density inside the material will be zero.

4. Which of the following is a rare-earth magnet?

1. samarium-cobalt magnet
2. ceramic magnets
3. neodymium magnets
4. all of these

Answer: a

Explanation: SmCo magnet is a rare-earth magnet. It is a strong permanent magnet.

5.  What is the value of susceptibility in diamagnetic materials?

1. Positive and large
2. positive and small
3. zero
4. negative and small

Answer: d

Explanation: Coercivity in diamagnetic materials is a negative and small value.

Common Mistakes

Selecting materials depending on their magnetic property is a very difficult process. Regarding the use and application, the student should consider properties such as susceptibility, permeability, coercivity, and hysteresis loss before selecting the materials.

Mistakes are commonly done while taking the values of these properties from the hysteresis curve of the material. The graph must be studied properly to avoid mistakes.

Related Concepts

• Magnets
• Permanent magnets
• Applications of magnetic materials