Nature of Magnetic Materials

Nature of Magnetic Materials

AU ; Dec.-05, 09, 14, 17, May-11,18

• In this section, we shall study the classification of the magnetic materials. Before actually starting with the classification, let us take a review of quantum theory in brief.

1. Origin of Magnetic Dipole Moment in the Material

• Basically the magnetic materials are classified on the basis of presence of magnetic dipole moments in the materials. A charged partical with angular momentum always contributes to the permanent magnetic dipole moments. In general, there are three important contributions to the angular moment of an atom.

i) Orbital magnetic dipole moment

ii) Electron spin magnetic moment and

iii) Nuclear spin magnetic moment

• In any atom, several electrons revolve in the circular orbits around the nucleus. This is very much analogous to a small current loop. The orbital state of motion of an electron in an atom is described with the quantum numbers n, I and ml. The first quantum number n indicates principle quantum which determines the energy of an electron. The second quantum number I represents orbital quantum number. Obviously it determines angular momentum        of orbit called orbital angular momentum. The last quantum number mZ is magnetic quantum number which determines the component of magnetic moment along the direction of an external field. So a simple current loop, placed in a magnetic field external to it, experiences a torque. Now this torque tries to align the magnetic field produced by the orbital electrons with an external field. Thus in general the orbital electrons produce magnetic field in such a way to align with an external magnetic field with no other magnetic moments considered.

• The angular momentum of an electron is called spin of the electron. As electron is a charged particle, the spin of the electron produces magnetic dipole moment. In an atom with completely filled orbits the contribution in spin magnetic moment is zero. In other words, the spins of the electrons in incompletely filled shells contribute more in the resultant spin magnetic moment.

• Similar to the electro spin, the nuclear spin contributes to the magnetic moment called nuclear spin magnetic moment. The mass of the nucleus is much larger than an electron. Thus the dipole moments due to the nuclear spin are very small.

The contribution of nuclear magnetic moment to the magnetic properties of materials is negligible.

• The total magnetic dipole moment of an atom can be calculated by summing up all the above mentioned magnetic dipole moments in appropriate manner.

2. Classification of Magnetic Materials

• According to the previous discussion, it is clear that the characteristics of the magnetic materials are decided by the different components of moments and also their summations. On the basis of the magnetic behaviour, the magnentic materials are classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic and supermagnentic.

• The magnetic materials can also be classified on the basis of magnetic property namely relative permeability µ_r. In general, a material is said to be non-magnetic if the value of µ_r is less than 1 or approximately equal to 1. While a material is said to be magnetic if the value of µ_r  is greater than or equal to 1. Generally the magnetic materials are classified into three major heads namely diamagnetic, paramagnetic and ferromagnetic. If a value of µ_r is slightly less than unity then it is a diamagnetic material (µ_r < l). If the value of µ_r is slightly greater than unity, then it is paramagnetic material (µ_r > 1). If the value of µ_r is very large than unity (µ_r >> l), then it is ferromagnetic marterial. For most of the practical cases, the value of µ_r is assumed to be unity for a paramagnetic and diamagnetic materials. Hence generally these materials are considered to be linear and non-magnetic materials. On the other hand, the ferromagnetic materials are always non-linear and magnetic materials. The classification of a magnetic material on the basis of relative permeability of a material is as shown in Fig. 8.6.1.

• The magnetic materials in which the orbital magnetic moment and electron spin magnetic moment cancel each other making net permanent magnetic moment of each atom zero are called diamagnetic materials. Thus with no external field, in diamagnetic materials the net torque produced on atom is zero with no effective realignment of magnetic moment. But an applied field makes spin moment slightly greater than that of orbital moment. This results in small magnetic moment which opposes the applied field. Hence when a diamagnetic material like bismuth is kept near either pole of a strong magnet gets repelled. Other examples of diamagnetic materials are lead, copper, silicon, diamond, graphite, sulphur, sodium chloride and inert gases.

• The magnetic materials in which the orbital and spin magnetic moments do not cancel each other resulting in a net magnetic moment of an atom are called paramagnetic materials. In the paramagnetic materials, atoms are oriented randomly. In the absence of an external field, the paramagnetic materials do not show any magnetic effect. But when an external field is applied, each atomic dipole moment experiences a torque. Due to this, all the atomic dipole moments tend to align with the external field. Thus inside material, value of the field increases than the value of the external field if the perfect alignment of the dipole moments is achieved. When the paramagnetic material is kept near the pole of a strong magnets, it gets attracted. The common examples of paramagnetic materials are potassium, tungston, oxygen, rare earth metals.

• The materials in which the atoms have large dipole moment due to electron spin magnetic moments are called ferromagnetic materials. In the ferromagnetic materials, the adjacent atoms line up their magnetic dipole moments in parallel fashion in the lattice. The regions in which large number of magnetic moments lined in parallel are called domains. When an external field is applied, the domains increase their size increasing internal field to a high value. When the external field is removed, the original random alignment of dipole moments is not achieved. Some of the moments remain in a small region which results in residual field or remanant field. This effect is called hysteresis. Iron, nickel and cobalt are the examples of ferromagnetic materials.

• By comparing ferromagnetic materials with diamagnetic and paramagnetic materials, the ferromagnetic materials show following unique properties.

1) These materials can be strongly magnetized with a magnetic field.

2) Evenafter removing the magnetic field, these materials are capable of retaining a considerable amount of magnetization.

3) When these materials are heated above certain temperature, their ferromagnetic properties are lost and these materials turn to be linear paramagnetic in nature. Such a temperature is commonly called Curie temperature (For iron Curie temperature is 770 °C)

4) As these are non-linear materials, the relation  fails in such materials because the relative permeability µ_r depends on .

• The behaviour of a ferromagnetic material can be represented interms of magnetic domains. A magnetic domain is a small region in which all the magnetic dipoles are perfectly aligned as shown in the Fig. 8.6.2. But the direction of alignment of the dipoles vary from domain to domain. Hence this virgin material is said to be in non-magnetized state.

• When a current carrying wire is wound around the magnetic material, a magnetic field is produced. So when the magnetic material is placed in an external magnetic field, all the magnetic dipoles will try to align in the direction of magnetic field. The domains in the material, which are already in the direction of magnetic field, grow in size at the cost of neighbouring domains. The remaining domains rotate their dipoles in the direction of magnetic field. Thus magnetic flux density within the magnetic material increases.

• Due to the current in wire,   is produced in material. The applied field  produces   field within the medium. The movement of the domain walls is reversible till the  field within medium is weak.

• When the current in wire is increased, the   field increases and  field becomes stronger and stronger. This is due to more number of magnetic dipoles align with the  field. Now if we measure  field, it is observed that initially  field increases slowly, then it increases rapidly. Then again slowly  field increases and then it flattens off finally as shown in the Fig. 8.6.4 (a).

• The changes in  are due to the changes in  is magnetization. The flattened region indicate that almost all the magnetic dipoles are aligned themselves in the direction of  field.

• Now if we lower thefield by decreasing current in wire, the  field does not follow the same path but it slowly decreases as shown by the dashed path in the Fig. 8.6.4 (a). Thus it is clear that even though the  field becomes zero, there exists certain magnetic field density in material. So it is called residual flux density or remanant flux density. It is denoted by . The magnetic material which retains high residual flux density is called hard magnetic material.

• When the direction of the current through wire is reversed, the flux density  becomes zero at a certain value of  in opposite direction. The value of  to make  zero is called coercive force denoted by. By increasing and decreasing  field in both the directions, we get a loop which is called hysteresis loop. The area of this hysteresis loop determines the loss in the energy per cycle which is known as hysteresis loop. Thus to minimise the hysteresis loss, the area of the hysteresis loop should be small. That means the residual flux density of the material should be as small as possible. The material with small values of the residual flux density is called soft material.

• The materials in which the dipole moments of adjacent atoms line up in antiparallel fashion are called antiferromagnetic materials. The net magnetic moment in such materials is zero. Thus when a specimen of antiferromagnetic material is kept near a strong magnet gets neither attracted nor repelled. This property is observed in materials like many of the oxides, chlorides and sulphides at low temperatures.

• The materials in which the magnetic dipole moments are lined up in antiparallel fashion, but the net magnetic moment is non-zero are called ferrimagnetic materials. The specimen of ferrimagnetic material gets affected in the external strong fields much lower than ferromagnetic materials. Ferrites are special ferrimagnetic materials having very low electrical conductivity. So these materials are used as ac inductors and core of transformers as the eddy currents are reduced and ohmic losses are reduced. The common examples of ferrites are nickel ferrite, nickel-zinc-ferrite and iron-oxide-magnetite.

• In supermagnetic materials, the ferromagnetic materials are suspended in the dielectric matrix. The important property of the supermagnetic material is that eventhough each particle of it contain large magnetic domains but can not penetrate adjacent particles. The common examples of supermagnetic materials are magnetic tapes used for audio, video and data recordings.

Review Questions

1. Classify different magnetic materials with suitable examples.

2. Give a brief note on the magnetic materials.

AU : Dec.-09, 17, Marks 6

3. Discuss the phenomenon of hysteresis associated with ferromagnetic materials.

AU : Dec.-05, Marks 8

 5. Describe the classification of magnetic materials and draw a typical magnetization (B-H) curve.

AU : Dec.-14, Marks 8