MAGNETIC MATERIALS

MAGNETIC MATERIALS – Introduction MAGNETIC MATERIALS

THE MATERIALS WHICH GET EASILYMAGNETIZED IN A MAGNETIC FIELD.

MAGNET: A DEVICE THAT ATTRACTS IRON AND PRODUCES A MAGNETIC FIELD MANY OF OUR MODERN TECHNOLOGICAL DEVICES RELAY ON MAGNETISM AND MAGNETIC MATERIALS POWER GENERATORS, TRANSFORMERS, ELECTRIC MOTORS, RADIO, TELEVISION, TELEPHONES, COMPUTERS & COMPONENTS OF SOUND & VIDEO REPRODUCTION SYSTEMS.

Magnetic dipole: The two equal and opposite magnetic poles are separated by a small distance. Magnetic dipole moment: Product of pole strength and length of the magnet. = ml Magnetic Induction (or) magnetic flux density (B): It represents the magnitude of the internal field strength within a substance that is subjected to an H field. Magnetic field Intensity (H): Ratio between the magnetic induction and the permeability of the medium

Magnetic Permeability (µ) : Ratio of the magnetic induction to the applied magnetic field intensity Magnetic Susceptibility (χ): Ratio between the intensity of magnetization to the applied magnetic field intensity Intensity of Magnetization(I or M) : The process of converting a non magnetic material into a magnetic material. Magnetic moment per unit volume.. Unit- A/m

Relation between µ & χ: The relative permeability or Substituting the B value, we get,

CLASSIFICATION OF MAGNETIC MATERIALS Classified into two categories, 1. Without permanent magnetic moments: i) Diamagnetic materials 2. With permanent magnetic moments: i) Paramagnetic materials ii) Ferromagnetic materials iii) Anti-Ferromagnetic materials iv) Ferri magnetic materials

An Introduction MAGNETIC MATERIALS – Ferro magnetism Generally Magnet  Ferro Magnetic material Ability to pickup the material like iron It is permanent magnet even in no field. It exhibits a magnetic moment in the absence of the field.

Explanation MAGNETIC MATERIALS – Ferro magnetism Net intrinsic magnetic dipole moment – due to spin of e -n The neighbor dipoles having strong interaction even in no field - SPIN EXCHANGE INTERACTION Exchange interaction aligns parallel and spreads over a small finite volume is called domain All moments with in the domain is in same direction More domains

Properties MAGNETIC MATERIALS – Ferro magnetism Having spontaneous magnetization and domain structure strongly magnetized in the direction of the external field If it is suspended freely, it set itself parallel to the external field. exhibit hysteresis and relative permeability is large. The value magnetic susceptibility is large &depends on T Above certain temperature, the ferromagnetic material becomes paramagnetic material. This temperature is called Curie temperature. Shows magnetostriction effect.

MAGNETIC MATERIALS – Ferro magnetism Permanent & large magnetizations due to parallel alignment of neighboring magnetic moments. Magnetic susceptibilities is high When T < ‘θ’ the material is in ferromagnetic state χ is very large due to spontaneous magnetization. Due to large internal field, the dipoles arrange in same direction Each domain is spontaneous magnetized even no applied Field Spin Direction

Domain Theory of Ferromagnetism MAGNETIC MATERIALS – Ferro magnetism To Explain HYSTERESIS effect, Weiss proposed the concept of domains in Size  m It consists of spontaneously magnetized small regions where all the magnetic moments are aligned in same direction. This small region are called as domains.

Domain Theory of Ferromagnetism MAGNETIC MATERIALS – Ferro magnetism Absence of field, domains oriented in diff. directions But Magnetic Moments in same Direction Results  magnetisation is zero DOMAIN STRUCTURE In the field, domains aligned in the direction of field If the field is removed, the domains restores its original. This cause the hysteresis.

MAGNETIZATION OF DOMAINS MAGNETIC MATERIALS – Ferro magnetism When External field is applied two possible alignment of domain By motion of domain walls By rotation of domains The movement of domain walls – in weak magnetic fields Magnetic moment increases & boundary of domains are displaced, volume of domains changes If applied field is strong, the domains can rotate into the field direction.

INTERNAL ENERGY IN DOMAINS MAGNETIC MATERIALS – Ferro magnetism Its made up from the following contributions Magneto static (or) the exchange energy Crystalline energy (or) the anisotropy energy Domain wall energy (or) Bloch wall energy Magnetostriction energy.

MAGNETIC MATERIALS – Ferro magnetism Neighboring atomic magnetic dipoles are interacting with Each other and align themselves. Magnetostatic Energy or The Exchange Energy The interaction energy between the neighboring atomic magnetic dipoles is called exchange energy or the magnetic field energy.

MAGNETIC MATERIALS – Ferro magnetism Most of the crystals are Anisotropic. (having a different value when measured in different directions) have easy and hard directions of magnetization. higher fields  magnetised in hard directions. The excess of energy required to magnetize a crystal in a particular direction, over that required to magnetize it along easy direction is called crystalline energy or anisotropic energy. Crystalline Energy or The Anisotropic Energy

MAGNETIC MATERIALS – Ferro magnetism Domain Wall Energy or Block Wall Energy The transition layer that separates adjacent domains, magnetized in different directions is called domain wall or block wall. The energy of domain wall is due to both exchange energy and anisotropic energy.

MAGNETIC MATERIALS – Ferro magnetism Magnetostriction Energy Under magnetic field, the dimensions will change. This phenomenon is called magnetostriction. If the domains are magnetised in different directions they will either expand or contract. This means that work must be done against the elastic restoring forces. The work done by the magnetic field against these elastic restoring forces is called an magnetostriction energy or magnetic elastic energy.