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Transcript of Magnetic materials
04/15/2023 J.Subrahmanyam Confidential 1
Magnetic materials
1) Magnetic Induction or Magnetic Flux density (B): The magnetic
induction or magnetic flux density is the number of lines of magnetic force
passing through unit area perpendicularly. Where Φ is the magnetic flux
and A is the area of cross section. Units: Weber/m2 or Tesla.
2) Magnetic Field Intensity or Intensity of Magnetic Field (H):
Magnetic Field Intensity at any point in the magnetic field is the force
experienced by an unit north pole placed at that point. Units: A/m.
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3) Magnetic Permeability (µ): It describes the nature of the material i.e.
it is a material property. It is the ease with which the material allows
magnetic lines of force to pass through it or the degree to which magnetic
field can penetrate a given medium. Mathematically it is equal to the ratio
of magnetic induction B inside a material to the applied magnetic field
intensity H. Units: H/m.
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Defnition
4) Magnetization: Process of converting a non magnetic material into
magnetic sample.
5) Intensity of Magnetization (M): It is a material property. It is
defined as magnetic moment per unit volume in a material. Units: A/m.
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• Created by current through a coil:
• Relation for the applied magnetic field, H:
L
INH
applied magnetic fieldunits = (ampere-turns/m)
current
Magnetic Properties
magnetic field H
current I
N = total number of turnsL = length of the coil
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• Magnetic induction results in the material
Response to a Magnetic Field
current I
B = Magnetic Induction (tesla) inside the material
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Origin of magnetic dipoles
The spin of the electron produces a magnetic field with a
direction dependent on the quantum number ml.
The spin of the electron produces a magnetic field
with a direction dependent on the quantum number
ms.
Origin of magnetic dipoles
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Electrons orbiting around the nucleus create a magnetic
field around the atom.
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M V V K Srinivas Prasad
Confidential 14
ORIGIN OF MAGNETISM IN MATERIALS
Nuclear spin
Orbital motion of electrons
Origin of Magnetism Spin of electrons
A moving electric charge, macroscopically or “microscopically” is responsible for Magnetism
Weak effect
Unpaired electrons required for net Magnetic Moment
Magnetic Moment resultant from the spin of a single unpaired electron→ Bohr Magneton = 9.273 x 1024 A/m2
This effect is Strong.
Permanent Dipoles
Alignment of dipoles
Direction of dipoles
Magnitudes of dipoles
Dia magnetic materials
Para, Ferro, Anti ferro,Ferri magnetic materials
YesNo
Para
Random Uniform
Ferro, Anti ferro, Ferri
Same
Ferro Anti ferro, Ferri
Sam
eAnti ferro
Ferri
Opposite
Different
Classification of magnetic Materials
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Diamagnetic Materials
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Properties • No permanent dipoles are present so net magnetic moment is
zero.
• The number of orientations of electronic orbits is such that the
vector sum of the magnetic moments is zero.
• External field will cause a rotation action on the individual
electronic orbits.
• Dipoles are induced in the material in presence of external
magnetic field.
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No Applied
Magnetic Field (H = 0)Applied
Magnetic Field (H)
none
oppo
sing
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• The external magnetic field produces induced magnetic
moment which is due to orbital magnetic moment..
• Induced magnetic moment is always in opposite direction of
the applied magnetic field.
• So magnetic induction in the specimen decreases.
• Magnetic susceptibility is small and negative.
• Repels magnetic lines of force.
• Diamagnetic susceptibility is independent of temperature and
applied magnetic field strength.
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• Susceptibility is of the order of -10-5.
• Relative permeability is less than one.
• It is present in all materials, but since it is so weak it can be
observed only when other types of magnetism are totally
absent.
• Examples: Bi, Zn, gold, H2O, alkali earth elements (Be, Mg,
Ca, Sr), superconducting elements in superconducting
state.
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paramagnetic Materials
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Properties • If the orbital's are not completely filled or spins not balanced,
an overall small magnetic moment may exist.
• i.e. paramagnetism is because of orbital and spin magnetic
moments of the electron.
• Possess permanent dipoles.
• In the absence of external magnetic field all dipoles are
randomly oriented so net magnetic moment is zero.
• Spin alignment is random.
• The magnetic dipoles do not interact
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No Applied
Magnetic Field (H = 0)
Applied
Magnetic Field (H)
rand
om
alig
ned
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• In presence of magnetic field the material gets feebly
magnetized.
• i.e. the material allows few magnetic lines of force to pass
through it.
• Relative permeability µr >1
• The orientation of magnetic dipoles depends on temperature
and applied field.
• Susceptibility is independent of applied mag. field & depends
on temperature
• C is Curie constant
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• With increase in temperature susceptibility decreases.
• Susceptibility is small and positive.
• These materials are used in lasers.
• Paramagnetic property of oxygen is used in NMR technique
for medical diagnose.
• The susceptibility range from 10-5 to 10-2.
• Examples: alkali metals (Li, Na, K, Rb), transition metals, Al,
Pt, Mn, Cr etc.
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Ferromagnetic Materials
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Properties • Origin for magnetism in Ferro mag. Materials are due to Spin
magnetic moment.
• Permanent dipoles are present so possess net magnetic
moment
• Material shows magnetic properties even in the absence of
external magnetic field.
• Possess spontaneous magnetization.
• Spontaneous magnetization is because of interaction between
dipoles called EXCHANGE COUPLING.
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alig
ned
alig
ned
No Applied
Magnetic Field (H = 0)Applied
Magnetic Field (H)
• Magnetic susceptibility is as high as 106.
• So H << M. thus B = µoM
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Magnetic induction
B (tesla)
Strength of applied magnetic field (H) (ampere-turns/m)
Ferromagnetic
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• When placed in external mag. field it strongly attracts magnetic
lines of force.
• All spins are aligned parallel & in same direction.
• Susceptibility is large and positive, it is given by Curie Weiss Law
• C is Curie constant & θ is Curie temperature.
• When temp is greater than curie temp then the material gets
converted in to paramagnetic.
• They possess the property of HYSTERESIS.
• Material gets divided into small regions called domains.
• Examples: Fe, Co, Ni.
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Ferro magnetic Materials
Even when H = 0, the dipoles
tend to strongly align over
small patches.
When H is applied, the domains
align to produce a large net
magnetization.
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Thermal energy can randomize the spin
Ferromagnetic ParamagneticTcurie
Heat
Tc for different materials: Fe=1043 K, Ni=631 K,
Co=1400 K, Gd= 298 K
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Curie Temperature
The temperature above (Tc) which ferromagnetic material become
paramagnetic.
Below the Curie temperature, the ferromagnetic is ordered and
above it, disordered.
The saturation magnetization goes to zero at the Curie
temperature.
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Antiferro magnetic Material
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Properties • The spin alignment is in antiparallel manner.• So net magnetic moment is zero.• Susceptibility depends on temperature.• Susceptibility is small and positive.• Initially susceptibility increases with increase in
temperature and beyond Neel temperature the susceptibility decreases with temperature.
• At Neel temperature susceptibility is maximum.
• Examples: FeO, MnO, Cr2O3 and salts of transition elements.
Nm TT
C
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Ferrimagnetic Materials
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Classification of Ferrimagnetic Materials
Ferrimagnetic Materials
Cubic Ferrites
MeFe2O4
Hexagonal Ferrites
AB12O19
Garnets
M3Fe5O12
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Properties • Special type of ferro and antiferromagnetic material.• Generally oxides in nature.• Ionic in nature• Ceramic in nature so high resistivity (insulators)• The spin alignment is antiparallel but different
magnitude.• So they possess net magnetic moment.• Also called ferrites.
• General form MFe2O4
• Susceptibility is very large and positive.• Examples: ferrous ferrite, nickle ferrite
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Ion
Mn2+ 3d5
E.C Spin Orientation Net Spin S Magnetic Moment
5/2 5µB
Fe2+ 3d6 2 4µB
Co2+ 3d7 3/2 3µB
Ni2+ 3d8 1 2µB
Cu2+ 3d9 1/2 1µB
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Unpaired electrons give rise to ferromagnetism in alkali metals
Net magnetic moment
Na 3s1 1 B
Fe 3d64s2 4 B
atom crystal
2.2 B
Co 3d74s2 3 B 1.7 B
Ni 3d84s2 2 B 0.6 B
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Ferrimagnetism
• All Fe2+ have a spin magnetic moment.
• Half of Fe3+ have a spin moment in on direction, the other half in the other (decreasing the overall moment to just that contributed by the Fe2+ ions).
Simpler picture showing a net magnetic moment.
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Domain Theory of Ferromagnetic Materials
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Lots and lots of domains in Ferro- (or Ferri-) Magnets
Domains form for a reason in ferro- and
ferrimagnetic materials. They are not random
structures.
What happens when magnetic field is applied to the ferromagnetic crystal?
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Ferromagnetism
• Materials that retain a magnetization in zero field
• Quantum mechanical exchange interactions favour parallel alignment of moments
• Examples: iron, cobalt
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• According to Becker, there are two independent processes which take place and lead to magnetization when magnetic field is applied.
1. Domain growth: Volume of domains oriented favourably w. r. t to the field at the expense of less favourably oriented domains.
2. Domain rotation: Rotation of the directions of magnetization towards the direction of the field.
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Magnetic domains• Ferromagnetic
materials tend to form magnetic domains
• Each domain is magnetized in a different direction
• Domain structure minimizes energy due to stray fields
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Magnetic domains
• Applying a field changes domain structure
• Domains with magnetization in direction of field grow
• Other domains shrink
Domain Structure and the Hysteresis Loop1.Domain growth:
1. Each domain is magnetized in a different direction
2. Applying a field changes domain structure. Domains with magnetization in direction of field grow.
3. Other domains shrink
2.Domain rotation: Finally by applying very strong fields can saturate
magnetization by creating single domain
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Bloch walls - The boundaries between magnetic domains.
Domain Structure and the Hysteresis Loop
The entire change in spin direction between domains does not occur in one sudden jump across a single atomic plane rather takes place in a gradual way extending over many atomic planes.
Bloch Wall
The magnetic moments in adjoining atoms change direction continuously across the boundary between domains.
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Magnetic domains
• Applying very strong fields can saturate magnetization by creating single domain
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Hysteresis Curve
• Means lagging or retarding of an effect behind the cause
of the effect.
• Here effect is B & cause of the effect is H.
• Also called B H curve.
• Hysteresis in magnetic materials means lagging of
magnetic induction (B) or magnetization (M) behind the
magnetizing field (H).
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Domain Structure and the Hysteresis Loop
• As the applied field (H) increases...
---the magnetic moment aligns with H.
• “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!
H = 0
Applied Magnetic Field (H)
Mag
netic
in
duct
ion
(B)
0
Bsat
H
H
H
H
H
ferromagnetic or ferrimagnetic material initially unmagnetized
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• Notice the permeability values depend upon the magnitude of H.
When a magnetic field is first applied to a magnetic material, magnetization initially increases slowly, then more rapidly as the domains begin to grow.
Later, magnetization slows, as domains must eventually rotate to reach saturation.
Domain Structure and the Hysteresis Loop
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Hysteresis loop - The loop traced out by magnetization in a ferromagnetic or ferrimagnetic material as the magnetic field is cycled. OR
Hysteresis Loop
• Removing the field does not necessarily return domain structure to original state. Hence results in magnetic hysteresis.
Applied Magnetic Field (H)
1. initial (unmagnetized state)
B 2. apply H, cause alignment
4
Negative H needed to demagnitize!
. Coercivity, HC
3. remove H, alignment stays! => permanent magnet!
Ferromagnetism: Magnetic hysteresis
Ms – Saturation magnetization
Hc – Coercive force (the field needed to bring the magnetization back to zero)
Mrs – Saturation remanent magnetization
M
H
Mrs
Hc
Ms
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remanent magnetization = M0
coercivity = Hc
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Domain growth reversible boundary displacements.
Domain growth irreversible boundary displacements.
Magnetization by domain rotation
Hysteresis Loop
• Means lagging or retarding of an effect behind the cause of the effect.
• Here effect is B & cause of the effect is H.
• Also called B H curve.• Hysteresis in magnetic
materials means lagging of magnetic induction (B) or magnetization (M) behind the magnetizing field (H).
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Hysteresis, Remanence, & Coercivity of Ferromagnetic Materials
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“hard” ferromagnetic material has a large M0 and large Hc.
“soft” ferromagnetic material has both a small M0 and Hc.
Hard versus Soft Magnets
High initial permeability.
Low coercivity.
Reaches to saturation magnetization with a
relatively low applied magnetic field.
It can be easily magnetized and demagnetized.
Low Hysteresis loss.
Applications involve, generators, motors, dynamos,
Cores of transformers and switching circuits.
Characteristics of soft magnetic materials:
Soft Magnets:
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Importance of Soft Magnetic Materials:
Saturation magnetization can be changed by altering composition of the materials.
Ex:- substitution of Ni2+ in place of Fe2+ changes saturation magnetization of ferrous-Ferrite.
Susceptibility and coercivity which also influence the shape of the Hysteresis curve are sensitive to the structural variables rather than composition.
Low value of coercivity corresponds to the easy movement of domain walls as magnetic field changes magnitude and/ or direction.
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Hard versus Soft Magnets
Characteristics of Hard magnetic materials:
Low initial permeability.
High coercivity and High remanence.
High saturation flux density.
Reaches to saturation magnetization with a
high applied magnetic field.
It can not be easily magnetized and
demagnetized.
High Hysteresis loss.
Used as permanent magnets.
Hard Magnets:
Importance of Hard magnetic material: Two important characteristics related to applications of these materials are
(i) Coercivity and (ii) energy product expressed as (BH)max with units in kJ/m3.
This corresponds to the area of largest B-H rectangle that can be constructed within the second quadrant of the Hysteresis curve.
Larger the value of energy product harder is the material in terms of its magnetic characteristics.
Schematic magnetization curve that displays hysteresis. Within the second quadrant are drawn two B–H energy product rectangles; the area of that rectangle labeled (BH)max is the largest possible, which is greater than the area defined by Bd–Hd
Who to get larger area of (BH)max i.e., who to produce Hard magnets?
Energy product represents the amount of energy required to demagnetize a permanent magnet.
Hysteresis behaviour depends upon the movement of domain walls.
The movement of domain walls depends on the final microstructure.Ex: the size, shape and orientation of crystal domains and impurities.
Microstructure will depend upon how the material is processed.
In a hard magnetic material, impurities are purposely introduced, to make it hard. Due to these impurities domain walls cannot move easily.
Finally the coercivity can increase and susceptibility can be decrease.
So large external field is required to demagnetization i.e., difficult to move the domain walls.
Baskar, Naren & G.Srinivas
Hard Magnetic Material Soft Magnetic MaterialHave large hysteresis loss. Have low hysteresis loss.
Domain wall moment is difficult Domain wall moment is relatively easier.
Coercivity & Retentivity are large. Coercivity & Retentivity are small.
Cannot be easily magnetized & demagnetized
Can be easily magnetized & demagnetized.
Magneto static energy is large. Magneto static energy is small.
Have small values of permeability and susceptibility
Have large values of susceptibility and permeability.
Used to make permanent magnets. Used to make electromagnets.
Iron-nickel-aluminum alloys, copper-nickle-iron alloys, copper–nickel– cobalt alloys
Iron- silicon alloys, ferrous- nickel alloys, ferrites, garnets.
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Applications of
Magnetic Materials
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Simulation of hard drive courtesy Martin Chen.Reprinted with permissionfrom International Business Machines Corporation.
• Head can... --apply magnetic field H & align domains (i.e., magnetize the medium). --detect a change in the magnetization of the medium.• Two media types:
MAGNETIC STORAGE• Information is stored by magnetizing material.
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--Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy)
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~60nm --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10-30nm! (hard drive)
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Magnetic hard drives
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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RELAYS
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• Relays are electromagnetically operated switch.
• A relay is a control device consisting of a small electromagnet which, when energized by a current in its winding, attracts a piece of magnetic material, thus operating a switch in another circuit.
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• A relay is a remote controlled switch capable of switching multiple circuits, either individually, simultaneously or in sequence.
• Relays are used where it is necessary to control a circuit by a low power signal or where several circuits are to be controlled by one signal.
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Applications• Telecommunication system
• Computer interfaces
• Domestic appliances
• Air conditioning
• Traffic control
• Control of motors
• Business machines
• Electric power control
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• Consists of a coil of wire surrounding a soft iron core and a movable iron armature and one or more set of contacts.
• When electric current is passed through the coil, it generates a magnetic field that attracts armature and a contact is made.
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• Modern relays to use a permanent magnet for assisting both the energized and the deenergized conditions.
• These magnets must maintain their strength under all temperature and vibration extremes.
• Loss of magnetic field strength could cause the relay to change key operating
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• MAGNETIC MATERIALS
• The three primary types of magnetic materials used are;
• A) Ceramic Types
• B) Alnico Types
• C) Rare Earth Types
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Ceramic Type
• Ceramic magnets are composed of Strontium or Barium Ferrite.
• Ceramic magnets are hard and brittle and are extensively used in consumer products.
Advantages
1) They are the least expensive magnets.
2) They are very resistant to corrosion.
3) They are stable up to approximately 300°C.
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Disadvantages
1) They are difficult to machine.2) They have a low energy product (3MGOe)3) They have a low/moderate coercively
(2KOe).4) magnets is cost is very low5) The low energy product will drive up the
volume of magnet6) magnetic flux can be lost rapidly with the
introduction of small demagnatising forces.
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Alnico Type• Alnico magnets are made of alloys of
Aluminum, Nickel and Cobalt.
Advantages • 1) They are relatively inexpensive.• 2) They are stable up to very high
temperatures (550°C).• 3) They are very resistant to corrosion.
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Disadvantages • 1) They are very difficult to machine.• 2) They have a low coercively (1KOe).• 3) They have a moderate energy product
(5MGOe).• Alnico does hold its magnetic properties at
very high temperatures• It can lose it’s magnetic strength under
conditions of shock
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Rare Earth Type
• Alloys of the Rare Earths are the most advanced commercialized permanent magnet materials.
• These materials represent a significant improvement in permanent magnet properties.
• The two primary materials are the Samarium-Cobalt family and the Neodymium-Iron-Boron family.
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Samarium – Cobalt Family• This family of magnets was developed in the
1970’s.• Applications requiring high magnetic energy
with little volume were1) Very high energy product (30MGOe).
2) Very high coercivity (10KOe).
3) Stable at high temperatures (350°C).
4) They are very resistant to corrosion.
5) They are the most expensive.
6) They are difficult to machine
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Neodymium – Iron – Boron
• The discovery of Neodymium-Iron-Boron magnets discovered late in 1983 by Sumitono Special Metals and General Motors.
• These magnets are the highest energy permanent magnets.
• Less expensive than SmCo.
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Advantages 1)Exceptionally high energy product (40MGOe).
2) Exceptionally high coercivity (15KOe).
3) Relatively easy to machine.
4) They are relatively inexpensive
Disadvantages
1) They do not resist corrosion.
2) They are not stable above 150°C.
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SENSORS
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SENSORS ?• American National Standards Institute• A device which provides a usable output in response to a specified
measure
• A sensor acquires a physical quantity and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical)
• Nowadays common sensors convert measurement of physical phenomena into an electrical signal
• Active element of a sensor is called a transducer
Sensor
Input Signal Output Signal
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Definition of a sensor• Def.
– A sensor is a device that receives a signal or stimulus and response with an electrical signal.
– Sensor is a device that measures a physical quantity and converts it into a signal which can be read by an absorber or by an instrument.
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Magnets can be used to sense
– Position– Force– Torque– Speed– Rotation– Acceleration– current and magnetic field