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MAGNETIC MATERIALS

MAGNETIC MATERIALS. Magnetism & Magnetic Materials. Magnetism is generally defined as that property of a material which enables it to attract pieces of iron. A material possessing this property is known as a MAGNET.

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MAGNETIC MATERIALS

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  1. MAGNETIC MATERIALS

  2. Magnetism & Magnetic Materials • Magnetism is generally defined as that property of a material which enables it to attract pieces of iron. • A material possessing this property is known as a MAGNET. • Materials that are attracted by a magnet, such as iron, steel, nickel, and cobalt, have the ability to become magnetized. These are called magnetic materials. • Materials, such as paper, wood, glass, or tin, which are not attracted by magnets, are considered nonmagnetic. Nonmagnetic materials are not able to become magnetized.

  3. Why are we interested about magnetic materials? • Storage Devices: Hard Drive, USB drives, Bluetooth, Mobiles. • Tape recorders, and video reproduction equipment use magnetized tape. • Speakers use magnets to convert amplifier outputs into audible sound. • Electricalmotors use magnets to convert electrical energy into mechanical motion. • Generators use magnets to convert mechanical motion into electrical energy.

  4. FUNDAMENTAL RELATIONS 1. RELATION BETWEEN B, H and M A magnetic field can be expressed in terms of Magnetic field intensity (H) and Magnetic flux density. In free space, these quantities are related as (1.1) In a magnetic material, above relation is written as (1.2) Here 0 = absolute permeability of free space,  = absolute permeability of the medium and / 0 = r = relative permeability of the magnetic material.

  5. MAGNETIZATION (M) Magnetization is defined as magnetic moment per unit volume and expressed in ampere/ meter. It is proportional to the applied magnetic field intensity (H). (1.3) Here,  = r – 1 = Magnetic susceptibility (cm-3). Let us consider Relation between B, M and H (1.4)

  6. CLASSIFICATION OF MAGNETIC MATERIALS Diamagnetic: Materials with –ve magnetic susceptibility . Examples:  (Au) = - 3.6 cm-3,  (Hg) = - 3.2 cm-3  (H2O)) = - 0.2X10-8 cm-3) Paramagnetic:Magnetic materials with +ve and small magnetic susceptibility . Examples:  (Al) = 2.2X10-5 cm-3  (Mn)= 98 cm-3 Ferromagnetic:Magnetic materials with +ve and very large magnetic susceptibility . Examples: Normally of the order of 105 cm-3.

  7. 2. A MICROSCOPIC LOOK In an atom, magnetic effect may arise due to: 1. Effective current loop of electrons in atomic orbit (orbital Motion of electrons); 2. Electron spin; 3. Motion of the nuclei.

  8. MAGNETIC MOMENTS ‘μ’ AND ANGULAR MOMENTUM ‘L’ Consider a charged particle moving in a circular orbit (e.g. an electron around a nucleus), 1. ORBITAL MOTION The magnetic moment  may be given as Frequency=ω/2π velocity v=rω But (2.1.1) Therefore,

  9. For an electron orbiting around the nucleus, magnetic moment would be given as (2.1.2) Where, = Bohr Magneton Ex: For p electron, l=1, For d electron, l=2, In the equation (2.1.2) (2.1.3) orbital gyro-magnetic ratio, .

  10. 2. ELECTRON SPIN Electrons also have spin rotation about their own axis. As a result they have both an angular momentum and magnetic moment. But for reasons that are purely quantum mechanical, the ratio between  to S for electron spin is twice as large as it is for a orbital motion of the spinning electron: (2.2.1) 3. NUCLEAR MOTION Nuclear magnetic moment is expressed in terms of nuclear magneton mp is mass of a proton. (2.3.1)

  11. What happens in a real atom? Due to the mixture of the contribution from the orbits and spins the ratio of  to angular momentum is neither -e/m nor –e/2m. Where g is known as Lande’s g-factor. It is given as Ex1. If S=1, L=1, J=0,1,2 g= 1, 1.5, 1.5 Ex2. For Ni2+ ion, If S=1, L=3 and J=4, g= 1.25

  12. DIAMAGNETISM : Langevin’s theory Diamagnetism is inherent in all substances and arises out of the effect of a magnetic field on the motion of electrons in an atom. Suppose an electron is revolving around the nucleus in atom, the force, F, between electron and the nucleus is When this atom is subjected to a magnetic field, B, electron also experiences an additional force called Lorentz force Thus when field is switched on, electron revolves with the new frequency, ’, given by

  13. For small B, and Thus change in magnetic moment is

  14. Suppose atomic number be Z, then equation may be written as, Where, summation extends over all electrons. Since core electrons have different radii, therefore If the orbit lies in x-y plane then, If R represents average radius of Atom, then for spherical atom

  15. For spherical symmetry, Therefore, Therefore, equation (2.10) may be written as

  16. Thus, If there are N atoms per unit volume, the magnetization produced would be Susceptibility, , would be This is the Langevin’s formula for volume susceptibility of diamagnetism of core electrons.

  17. Conclusions: Diamagnetic susceptibility • Since   Z, bigger atoms would have larger susceptibility. 2. dia depends on internal structure of the atoms which is temperature independent. 3. All electrons contribute to the diamagnetism even ‘s’ electrons. 4. All materials have diamagnetism although it may be masked by other magnetic effects. Example: If Z=1, R = 0.1 nm, N = 5x1028/ m3,

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