Why learning about magnetic materials is so important?
Engineering materials are important in everyday life
because of their versatile structural properties. Prime physical properties of materials include electrical
properties; thermal properties; magnetic properties; and
optical properties. The magnetic properties of engineering materials are
diverse, and so are their uses in different applications.
Ex.: motors, telephones, medical applications, etc.
What is Magnetism?
Magnetism is a phenomenon by which a material exerts an either attractive or repulsive force on another. The basic source of magnetic force is the movement of electrically
charged particles. Thus magnetic behavior of a material can
be traced to the structure of atoms. Electrons in atoms have a planetary motion in that they go
around the nucleus. This orbital motion and its own spin
cause separate magnetic moments, which contribute to the
magnetic behavior of materials. Thus every material can
respond to a magnetic field. However, how a material responds depends
much on its atomic structure and determines whether a
material will be strongly or weakly magnetic.
Bohr Magneton:
Magnetic moment due to the spin of an electron is known as
Bohr magneton, MB.
where q is the charge on the electron, h – is Planck’s constant, and me – is the mass of the electron
Bohr magneton is the most fundamental magnetic moment.
Magnetic Dipoles:
Magnetic dipoles are found to exist in magnetic materials,
analogous to electric dipoles. A magnetic dipole is a small magnet composed of north and
south poles instead of positive and negative charges. Within a magnetic field, the force of the field exerts a torque
that tends to orient the dipoles with the field. Magnetic forces are generated by moving electrically
charged particles. These forces are in addition to any
electrostatic forces that may already exist. It is convenient to think of magnetic forces in terms of
distributed field, which is represented by imaginary lines.
These lines also indicate the direction of the force.
Types of Magnetism:
A material is magnetically characterized based on the way it can be
magnetized. This depends on the material’s magnetic susceptibility – its magnitude
and sign.
Dia-magnetism: very weak; exists ONLY in presence of an external
field.
Para-magnetism: slightly stronger; When an external field is applied
dipoles line up with the field, resulting in a positive magnetization.
However, the dipoles do not interact.
Ferro-magnetism: very strong; dipoles line up permanently upon
application of the external field. Has two sub-classes:-
Anti-ferro-magnetism: dipoles line up, but in opposite directions,
resulting in zero magnetization.
Ferri-magnetism: similar to anti-ferro-magnetism, but dipoles of
varying strength cannot cancel each other out.
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| Types of Magnetism and their examples |
Temperature Effect:
Temperature does have a definite effect on a material’s magnetic
behavior. With rising temperatures, the magnitude of the atom's thermal
vibrations increases. This may lead to more randomization of
atomic magnetic moments as they are free to rotate. Usually, atomic thermal vibrations counteract forces between the
adjacent atomic dipole moments, resulting in dipole misalignment
up to some extent both in the presence and absence of an external field. As a consequence of it, saturation magnetization initially
decreases gradually, then suddenly drops to zero at a temperature
called Curie temperature, Tc. The magnitude of the Curie temperature is dependent on the
material. For example: for cobalt – 1120 ˚C, for nickel – 335 ˚C,
for iron – 768 ˚C, and for Fe3O4 – 585 ˚C.
Magnetic Domains:
In addition to susceptibility differences, the different types of
magnetism can be distinguished by the structure of the magnetic
dipoles in regions called domains. Each domain consists of magnetic moments that are aligned,
giving rise to a permanent net magnetic moment per domain. Each of these domains is separated from the rest by domain
boundaries/domain walls. Boundaries, also called Bloch walls,
are narrow zones in which the direction of the magnetic moment
gradually and continuously changes from that of one domain to
that of the next. The domains are typically very small about 50 μm or less, while
the Bloch walls are about 100 nm thick. For a poly-crystalline
specimen, each grain may have more than one microscopic-sized
domain. Domains exist even in absence of an external field.
The average magnetic induction of a ferromagnetic material is
intimately related to the domain structure. When a magnetic field is imposed on the material, domains that
are nearly lined up with the field grow at the expense of
unaligned domains. This process continues until only the most
favorably oriented domains remain. For the domains to grow, the Bloch walls must move, the
external field provides the force required for this moment. When the domain growth is completed, a further increase in the
magnetic field causes the domains to rotate and align parallel to
the applied field. At this instant material reaches saturation
magnetization and no further increase will take place in
increasing the strength of the external field.
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| Magnetization vs Magnetic Field (M-H Plot) |
Magnetic Hysteresis:
Once magnetic saturation has been achieved, a decrease in the
applied field back to zero results in a macroscopically permanent
or residual magnetization, known as remanence, Mr
. The
corresponding induction, Br, is called retentivity or remanent
induction of the magnetic material. This effect of retardation by the material is called hysteresis. The magnetic field strength needed to bring the induced
magnetization to zero is termed coercivity, Hc. This must be
applied anti-parallel to the original field. A further increase in the field in the opposite direction results in a
maximum induction in the opposite direction. The field can once
again be reversed, and the field-magnetization loop can be closed, This loop is known as the hysteresis loop or B-H plot, or M- H plot.
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| Magnetic Hysteresis |
Semi-Hard Magnets:
Once the area within the hysteresis loop represents the energy
loss per unit volume of material for one cycle. The coercivity of the material is a micro-structure-sensitive
property. This dependence is known as magnetic shape
anisotropy. The coercivity of recording materials needs to be smaller than
that for others since data written onto a data storage medium
should be erasable. On the other hand, the coercivity values
should be higher since the data need to be retained. Thus such
materials are called magnetically semi-hard.
Ex.: Hard ferrites based on Ba, CrO2
, γ-Fe2O3
; alloys based on
Co-Pt-Ta-Cr, Fe-Pt and Fe-Pd, etc
Soft Magnets:
Soft magnets are characterized by low coercive forces and
high magnetic permeabilities and are easily magnetized and
de-magnetized. They generally exhibit small hysteresis losses. Application of soft magnets includes cores for electromagnets,
electric motors, transformers, generators, and other
electrical equipment.
Ex.: ingot iron, low-carbon steel, Silicon iron, superalloy
(80% Ni-5% Mo-Fe), 45 Permalloy (55%Fe-45%Ni), 2-79
Permalloy (79% Ni-4% Mo-Fe), MnZn ferrite / Ferroxcube
A (48% MnFe2O4
-52%ZnFe2O4
), NiZn ferrite / Ferroxcube
B (36% NiFe2O4
-64% ZnFe2O4
), etc.
Hard Magnets:
Hard magnets are characterized by high remanent
inductions and high coercivities. These are also called permanent magnets or hard magnets. These are found useful in many applications including
fractional horse-power motors, automobiles, audio and
video recorders, earphones, computer peripherals, and
clocks. They generally exhibit large hysteresis losses.
Ex.: Co-steel, Tungsten steel, SmCo5, Nd2Fe14B, ferrite
BaO6Fe2O3, Cunife (60% Cu 20% Ni-20% Fe), Alnico
(alloy of Al, Ni, Co, and Fe), etc.