Properties of Magnetic Materials

Properties of Magnetic Materials

AU : May-15,16

 • The magnetic materials are classified based on the presence of magnetic dipole moments in the materials.

 • The magnetic materials are classified as ferromagnetic and ferrimagnetic materials

. • The materials in which the atoms have large magnetic dipole moments which are lined up in parallel fashion are called ferromagnetic materials. Most of the electrical components and devices use ferromagnetic materials. Iron, nickel, cobalt and various alloys of iron are the examples of ferromagnetic materials. The chrome steel, alnicos certain copper-nickel alloys are hard ferromagnetic materials. The nonlinear B-H relationship, high permeability, saturation, hysteresis are the important properties of ferromagnetic materials. The materials in which the magnetic dipole moments are lined up in antiparallel fashion are called ferrimagnetic materials. Ferrites are the special ferrimagnetic materials having very low electrical conductivity and used as a.c. inductors and core of the transformers. The nickel ferriate, nickel-zinc ferrite and various mixed oxides of iron are the examples of ferrites.

1. Magnetization Characteristics and Hysteresis Loop

AU : May-15, 16

• The variation of magnetic flux density (B) against the magnetic field strength (H) is called the magnetization characteristics of the magnetic material. It is nonlinear in nature.

 • When a magnetic material is subjected to a cycle of magnetization and demagnetization then the graph of magnetic flux density (B) against magnetic field intensity (H) is called hysteresis loop of that magnetic material. Such a hysteresis loop indicates important properties of that magnetic material.

• The Fig. 1.17.1 shows a general hysteresis loop and the its characteristics.

• The point 'D' shows saturation indicating that there cannot be increase in B further though H is increased. The H value corresponding to D is denoted as H., and is the maximum H of the cyclic variation. The corresponding flux density is denoted as B_m

When H is reduced and becomes zero at point 'E', the material retains a considerable degree of magnetization. This is called residual magnetism. The property of retaining the magnetism though applied magnetic force is zero is called retentivity of the magnetic material.

 The driving magnetic force must be reversed and increased to large value to make the magnetization zero again. This is denoted by point 'F'. This force is called coereive force and property is called coercivity of the material.

 • When H is increased further in reversed direction, then saturation in opposite direction is achieved. The curve traces a loop if magnetic force is increased again which is called hysteresis loop.

• The classification of magnetic materials as soft and hard is based on the value of coercive force.

2. Soft Magnetic Materials

 • The soft magnetic materials are easy to magnetise and easy to demagnetise. Hence to obtain quick response, such materials are used in the devices like transformers where they are subjected to alternating electric fields.  

• To minimize the hysteresis loss in a cycle, the area of hysteresis loop should be as small as possible.

• The materials with very small hysteresis loop are classified as soft magnetic materials.

Key Point : The hysteresis loss can be reduced by selecting good quality magnetic material.

• The area of hysteresis loop should be narrow Silicon steel is employed for the core material so that hysteresis loss can be minimised.

Key Point : The eddy current loss can be reduced by using thin laminations for the core.