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ENGINEERING PHYSICS II MAGNETIC MATERIALS UNIT III MAGNETIC MATERIALS 3.1 INTRODUCTION Magnetic materials are the materials which can be made to behave as magnets. When these materials are kept in an external magnetic field, they will create a permanent magnetic moment in it. Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic and Ferromagnetic materials are the magnetic materials type. Magnetism originates from the magnetic moment of the magnetic materials due to the rotational motion of the changed particles. When an electron revolves around the positive nucleus, orbital magnetic arises and due to the spinning of electrons, spin magnetic moment arises. Let us see some of the basic definitions in magnetism. 3.2 BASIC DEFINITIONS Magnetic dipole moment A system having two opposite magnetic poles separated by a distance‘d’ is called as a magnetic dipole. If ‘m’ is magnetic pole strength and ‘l’ is the length of the magnet, then its dipole moment is given by M = ml Magnetic moment can also be defined as M = ia, where i is the electric current that flows through a circular wire of an area of cross section ‘a’. Bohr Magneton The total magnetic moment and the spin magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called Bohr magneton. 1 Bohr magneton = eħ/2m = 9.27 x 10-24 Am2 Magnetic field The space around the magnet or the current carrying conductor where the magnetic effect is felt is called magnetic field. Magnetic lines of force Magnetic field is assumed to consist of lines of magnetic forces. These lines of forces travel externally from North Pole to South Pole as shown in figure. Hence a magnetic line of force is defined as the continuous curve in a magnetic field. The tangent drawn at any point on the curve gives the direction of the resultant magnetic intensity at that point. 41 ENGINEERING PHYSICS II MAGNETIC MATERIALS Magnetic induction (or) Magnetic flux density (B) It is defined as the number of magnetic lines of force passing normally through unit area of cross section at that point. Magnetic field intensity (H) It is defined as the force experienced by a unit North Pole placed at the given point in a magnetic field Magnetisation (or) Intensity of Magnetisation (I) The magnetization is the process of converting a non-magnetic material into a magnetic material. It measures the magnetization of the magnetized specimen. It also defined as the magnetic moment per unit volume Magnetic susceptibility (χm) It is defined as the ratio between intensity of magnetization (I) and the magnetic field intensity (H) I χm = H Magnetic permeability (µ) It is defined as the ratio between the magnetic flux density (B) and the magnetic field intensity (H) µ= B H 42