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Magnetic materials

This document discusses various magnetic properties of materials. It defines magnetic induction, magnetic field intensity, and magnetic permeability. It describes diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic materials. Diamagnetic materials have no unpaired electrons and are repelled by magnetic fields. Paramagnetic materials have some unpaired electrons and are weakly attracted to magnetic fields in a temperature-dependent manner. Ferromagnetic materials have strongly interacting unpaired electrons that align to produce spontaneous magnetization and strong attraction to magnetic fields.

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Overview of magnetic induction, field intensity, permeability, and magnetization.

Magnetic properties created by current through coils and response of materials to magnetic fields.

Discussion on magnetic dipoles and their origins related to electron spin and motion.

Overview of magnetic materials including permanent dipoles and classifications like diamagnetic.

Properties of diamagnetic materials, magnetic susceptibility, and examples.

Characteristics of paramagnetic materials, their behavior in magnetic fields, and examples.

Introduction to ferromagnetic materials, their properties, and the concept of hysteresis.

Hysteresis loop, thermal effects on domains, and properties of antiferromagnetic materials.

Details of ferrimagnetic materials, their classifications, and properties.

Understanding of domain structures and their behavior under magnetic fields, including hysteresis.

Applications of magnetic materials in hard drives, including media types for storage.

Functionality and applications of relays in various systems and devices.

Comparison of types of magnetic materials: ceramic, alnico, and rare earth types.

Definition and applications of sensors, including those that utilize magnetic properties.

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4/30/2015 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. 4/30/2015 J.Subrahmanyam Confidential 2
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. 4/30/2015 J.Subrahmanyam Confidential 3
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. 4/30/2015 J.Subrahmanyam Confidential 4
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• Created by current through a coil: • Relation for the applied magnetic field, H: L IN H  applied magnetic field units = (ampere-turns/m) current Magnetic Properties magnetic field H current I N = total number of turns L = length of the coil 4/30/2015 J.Subrahmanyam Confidential 8
• Magnetic induction results in the material Response to a Magnetic Field current I B = Magnetic Induction (tesla) inside the material 4/30/2015 J.Subrahmanyam Confidential 9
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4/30/2015 J.Subrahmanyam Confidential 11 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 4/30/2015 J.Subrahmanyam Confidential 12
4/30/2015 J.Subrahmanyam Confidential 13  Electrons orbiting around the nucleus create a magnetic field around the atom.
4/30/2015 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 1024 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 Para Uniform Ferro, Anti ferro, Ferri Ferro Anti ferro, Ferri Anti ferro Ferri Classification of magnetic Materials 4/30/2015 Confidential 15 J.Subrahmanyam
4/30/2015 J.Subrahmanyam Confidential 16 Diamagnetic Materials
4/30/2015 J.Subrahmanyam Confidential 17 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.
4/30/2015 Confidential 18 No Applied Magnetic Field (H = 0) Applied Magnetic Field (H) none opposing
4/30/2015 J.Subrahmanyam Confidential 19
4/30/2015 M V V K Srinivas Prasad Confidential 20 • 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.
4/30/2015 J.Subrahmanyam Confidential 21 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 22 paramagnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 23 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
4/30/2015 M V V K Srinivas Prasad Confidential 24 No Applied Magnetic Field (H = 0) Applied Magnetic Field (H) random aligned
4/30/2015 M V V K Srinivas Prasad Confidential 25 • 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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4/30/2015 M V V K Srinivas Prasad Confidential 27 • 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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4/30/2015 M V V K Srinivas Prasad Confidential 30 Ferromagnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 31 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.
4/30/2015 M V V K Srinivas Prasad Confidential 32 aligned aligned No Applied Magnetic Field (H = 0) Applied Magnetic Field (H)
• Magnetic susceptibility is as high as 106. • So H << M. thus B = µoM 4/30/2015 M V V K Srinivas Prasad Confidential 33 MagneticinductionB(tesla) Strength of applied magnetic field (H) (ampere-turns/m) Ferromagnetic
4/30/2015 M V V K Srinivas Prasad Confidential 34 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 35 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.
4/30/2015 M V V K Srinivas Prasad Confidential 36 Thermal energy can randomize the spin Ferromagnetic Paramagnetic Tcurie Heat Tc for different materials: Fe=1043 K, Ni=631 K, Co=1400 K, Gd= 298 K
4/30/2015 M V V K Srinivas Prasad Confidential 37 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.
4/30/2015 M V V K Srinivas Prasad Confidential 38 Antiferro magnetic Material
4/30/2015 M V V K Srinivas Prasad Confidential 39 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. N m TT C  
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4/30/2015 M V V K Srinivas Prasad Confidential 42 Ferrimagnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 43 Classification of Ferrimagnetic Materials Ferrimagnetic Materials Cubic Ferrites MeFe2O4 Hexagonal Ferrites AB12O19 Garnets M3Fe5O12
4/30/2015 M V V K Srinivas Prasad Confidential 44 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
4/30/2015 M V V K Srinivas Prasad Confidential 45 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
4/30/2015 M V V K Srinivas Prasad Confidential 46 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
4/30/2015 M V V K Srinivas Prasad Confidential 47
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. 4/30/2015 M V V K Srinivas Prasad Confidential 48
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4/30/2015 M V V K Srinivas Prasad Confidential 51 Domain Theory of Ferromagnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 52
4/30/2015 M V V K Srinivas Prasad Confidential 53 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?
4/30/2015 M V V K Srinivas Prasad Confidential 54 Ferromagnetism • Materials that retain a magnetization in zero field • Quantum mechanical exchange interactions favour parallel alignment of moments • Examples: iron, cobalt
4/30/2015 M V V K Srinivas Prasad Confidential 55 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 56 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
4/30/2015 M V V K Srinivas Prasad Confidential 57 Magnetic domains • Applying a field changes domain structure • Domains with magnetization in direction of field grow • Other domains shrink
Domain Structure and the Hysteresis Loop 1. 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 4/30/2015 M V V K Srinivas Prasad Confidential 58
4/30/2015 M V V K Srinivas Prasad Confidential 59  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.
4/30/2015 M V V K Srinivas Prasad Confidential 60 Magnetic domains • Applying very strong fields can saturate magnetization by creating single domain
4/30/2015 M V V K Srinivas Prasad Confidential 61 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).
4/30/2015 M V V K Srinivas Prasad Confidential 62
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) Magnetic induction(B) 0 Bsat H H H H H ferromagnetic or ferrimagnetic material initially unmagnetized 4/30/2015 M V V K Srinivas Prasad Confidential 63
• 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 4/30/2015 M V V K Srinivas Prasad Confidential 64
4/30/2015 M V V K Srinivas Prasad Confidential 65  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 4/30/2015 M V V K Srinivas Prasad 66
4/30/2015 M V V K Srinivas Prasad Confidential 67 remanent magnetization = M0 coercivity = Hc
4/30/2015 M V V K Srinivas Prasad Confidential 68 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).
4/30/2015 M V V K Srinivas Prasad Confidential 69 Hysteresis, Remanence, & Coercivity of Ferromagnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 70 “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: 4/30/2015 M V V K Srinivas Prasad Confidential 71
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. 4/30/2015 M V V K Srinivas Prasad Confidential 72
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 Material Have 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.
4/30/2015 M V V K Srinivas Prasad Confidential 77 Applications of Magnetic Materials
4/30/2015 M V V K Srinivas Prasad Confidential 78 Simulation of hard drive courtesy Martin Chen. Reprinted with permission from 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.
4/30/2015 M V V K Srinivas Prasad Confidential 79 --Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy)
4/30/2015 M V V K Srinivas Prasad Confidential 80 --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10- 30nm! (hard drive)
4/30/2015 M V V K Srinivas Prasad Confidential 81
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Magnetic hard drives 4/30/2015 M V V K Srinivas Prasad Confidential 83
4/30/2015 M V V K Srinivas Prasad Confidential 84 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
4/30/2015 M V V K Srinivas Prasad Confidential 85
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4/30/2015 M V V K Srinivas Prasad Confidential 87
4/30/2015 M V V K Srinivas Prasad Confidential 88 RELAYS
4/30/2015 M V V K Srinivas Prasad Confidential 89 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 90 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 91 Applications • Telecommunication system • Computer interfaces • Domestic appliances • Air conditioning • Traffic control • Control of motors • Business machines • Electric power control
4/30/2015 M V V K Srinivas Prasad Confidential 92 • 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.
4/30/2015 M V V K Srinivas Prasad Confidential 93 • 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
4/30/2015 M V V K Srinivas Prasad Confidential 94 • MAGNETIC MATERIALS • The three primary types of magnetic materials used are; • A) Ceramic Types • B) Alnico Types • C) Rare Earth Types
4/30/2015 M V V K Srinivas Prasad Confidential 95 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.
4/30/2015 M V V K Srinivas Prasad Confidential 96 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 low 5) The low energy product will drive up the volume of magnet 6) magnetic flux can be lost rapidly with the introduction of small demagnatising forces.
4/30/2015 M V V K Srinivas Prasad Confidential 97 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.
4/30/2015 M V V K Srinivas Prasad Confidential 98 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
4/30/2015 M V V K Srinivas Prasad Confidential 99 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.
4/30/2015 M V V K Srinivas Prasad Confidential 100 Samarium – Cobalt Family • This family of magnets was developed in the 1970’s. • Applications requiring high magnetic energy with little volume were 1) 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
4/30/2015 M V V K Srinivas Prasad Confidential 101 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.
4/30/2015 M V V K Srinivas Prasad Confidential 102 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.
4/30/2015 M V V K Srinivas Prasad Confidential 103 SENSORS
4/30/2015 M V V K Srinivas Prasad Confidential 104 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
4/30/2015 M V V K Srinivas Prasad Confidential 105 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.
4/30/2015 M V V K Srinivas Prasad Confidential 106 Magnets can be used to sense – Position – Force – Torque – Speed – Rotation – Acceleration – current and magnetic field

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Magnetic materials

  • 1.
  • 2.
    Magnetic materials 1) MagneticInduction 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. 4/30/2015 J.Subrahmanyam Confidential 2
  • 3.
    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. 4/30/2015 J.Subrahmanyam Confidential 3
  • 4.
    Defnition 4) Magnetization: Processof 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. 4/30/2015 J.Subrahmanyam Confidential 4
  • 5.
    4/30/2015 M VV K Srinivas Prasad Confidential 5
  • 6.
  • 7.
  • 8.
    • Created bycurrent through a coil: • Relation for the applied magnetic field, H: L IN H  applied magnetic field units = (ampere-turns/m) current Magnetic Properties magnetic field H current I N = total number of turns L = length of the coil 4/30/2015 J.Subrahmanyam Confidential 8
  • 9.
    • Magnetic inductionresults in the material Response to a Magnetic Field current I B = Magnetic Induction (tesla) inside the material 4/30/2015 J.Subrahmanyam Confidential 9
  • 10.
  • 11.
    4/30/2015 J.Subrahmanyam Confidential11 Origin of magnetic dipoles  The spin of the electron produces a magnetic field with a direction dependent on the quantum number ml.
  • 12.
     The spinof the electron produces a magnetic field with a direction dependent on the quantum number ms. Origin of magnetic dipoles 4/30/2015 J.Subrahmanyam Confidential 12
  • 13.
    4/30/2015 J.Subrahmanyam Confidential13  Electrons orbiting around the nucleus create a magnetic field around the atom.
  • 14.
    4/30/2015 M V VK 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 1024 A/m2 This effect is Strong.
  • 15.
    Permanent Dipoles Alignment of dipoles Directionof dipoles Magnitudes of dipoles Dia magnetic materials Para, Ferro, Anti ferro, Ferri magnetic materials Para Uniform Ferro, Anti ferro, Ferri Ferro Anti ferro, Ferri Anti ferro Ferri Classification of magnetic Materials 4/30/2015 Confidential 15 J.Subrahmanyam
  • 16.
    4/30/2015 J.Subrahmanyam Confidential16 Diamagnetic Materials
  • 17.
    4/30/2015 J.Subrahmanyam Confidential17 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.
  • 18.
    4/30/2015 Confidential 18 NoApplied Magnetic Field (H = 0) Applied Magnetic Field (H) none opposing
  • 19.
  • 20.
    4/30/2015 M VV K Srinivas Prasad Confidential 20 • 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.
  • 21.
    4/30/2015 J.Subrahmanyam Confidential21 • 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.
  • 22.
    4/30/2015 M VV K Srinivas Prasad Confidential 22 paramagnetic Materials
  • 23.
    4/30/2015 M VV K Srinivas Prasad Confidential 23 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
  • 24.
    4/30/2015 M VV K Srinivas Prasad Confidential 24 No Applied Magnetic Field (H = 0) Applied Magnetic Field (H) random aligned
  • 25.
    4/30/2015 M VV K Srinivas Prasad Confidential 25 • 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
  • 26.
    4/30/2015 M VV K Srinivas Prasad Confidential 26
  • 27.
    4/30/2015 M VV K Srinivas Prasad Confidential 27 • 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.
  • 28.
    4/30/2015 M VV K Srinivas Prasad Confidential 28
  • 29.
    4/30/2015 M VV K Srinivas Prasad Confidential 29
  • 30.
    4/30/2015 M VV K Srinivas Prasad Confidential 30 Ferromagnetic Materials
  • 31.
    4/30/2015 M VV K Srinivas Prasad Confidential 31 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.
  • 32.
    4/30/2015 M VV K Srinivas Prasad Confidential 32 aligned aligned No Applied Magnetic Field (H = 0) Applied Magnetic Field (H)
  • 33.
    • Magnetic susceptibilityis as high as 106. • So H << M. thus B = µoM 4/30/2015 M V V K Srinivas Prasad Confidential 33 MagneticinductionB(tesla) Strength of applied magnetic field (H) (ampere-turns/m) Ferromagnetic
  • 34.
    4/30/2015 M VV K Srinivas Prasad Confidential 34 • 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.
  • 35.
    4/30/2015 M VV K Srinivas Prasad Confidential 35 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.
  • 36.
    4/30/2015 M V VK Srinivas Prasad Confidential 36 Thermal energy can randomize the spin Ferromagnetic Paramagnetic Tcurie Heat Tc for different materials: Fe=1043 K, Ni=631 K, Co=1400 K, Gd= 298 K
  • 37.
    4/30/2015 M VV K Srinivas Prasad Confidential 37 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.
  • 38.
    4/30/2015 M VV K Srinivas Prasad Confidential 38 Antiferro magnetic Material
  • 39.
    4/30/2015 M VV K Srinivas Prasad Confidential 39 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. N m TT C  
  • 40.
    4/30/2015 M VV K Srinivas Prasad Confidential 40
  • 41.
    4/30/2015 M VV K Srinivas Prasad Confidential 41
  • 42.
    4/30/2015 M VV K Srinivas Prasad Confidential 42 Ferrimagnetic Materials
  • 43.
    4/30/2015 M VV K Srinivas Prasad Confidential 43 Classification of Ferrimagnetic Materials Ferrimagnetic Materials Cubic Ferrites MeFe2O4 Hexagonal Ferrites AB12O19 Garnets M3Fe5O12
  • 44.
    4/30/2015 M VV K Srinivas Prasad Confidential 44 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
  • 45.
    4/30/2015 M VV K Srinivas Prasad Confidential 45 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
  • 46.
    4/30/2015 M VV K Srinivas Prasad Confidential 46 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
  • 47.
    4/30/2015 M VV K Srinivas Prasad Confidential 47
  • 48.
    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. 4/30/2015 M V V K Srinivas Prasad Confidential 48
  • 49.
    4/30/2015 M VV K Srinivas Prasad Confidential 49
  • 50.
    4/30/2015 M VV K Srinivas Prasad Confidential 50
  • 51.
    4/30/2015 M VV K Srinivas Prasad Confidential 51 Domain Theory of Ferromagnetic Materials
  • 52.
    4/30/2015 M VV K Srinivas Prasad Confidential 52
  • 53.
    4/30/2015 M VV K Srinivas Prasad Confidential 53 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?
  • 54.
    4/30/2015 M VV K Srinivas Prasad Confidential 54 Ferromagnetism • Materials that retain a magnetization in zero field • Quantum mechanical exchange interactions favour parallel alignment of moments • Examples: iron, cobalt
  • 55.
    4/30/2015 M VV K Srinivas Prasad Confidential 55 • 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.
  • 56.
    4/30/2015 M VV K Srinivas Prasad Confidential 56 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
  • 57.
    4/30/2015 M VV K Srinivas Prasad Confidential 57 Magnetic domains • Applying a field changes domain structure • Domains with magnetization in direction of field grow • Other domains shrink
  • 58.
    Domain Structure andthe Hysteresis Loop 1. 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 4/30/2015 M V V K Srinivas Prasad Confidential 58
  • 59.
    4/30/2015 M VV K Srinivas Prasad Confidential 59  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.
  • 60.
    4/30/2015 M VV K Srinivas Prasad Confidential 60 Magnetic domains • Applying very strong fields can saturate magnetization by creating single domain
  • 61.
    4/30/2015 M VV K Srinivas Prasad Confidential 61 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).
  • 62.
    4/30/2015 M VV K Srinivas Prasad Confidential 62
  • 63.
    Domain Structure andthe 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) Magnetic induction(B) 0 Bsat H H H H H ferromagnetic or ferrimagnetic material initially unmagnetized 4/30/2015 M V V K Srinivas Prasad Confidential 63
  • 64.
    • Notice thepermeability 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 4/30/2015 M V V K Srinivas Prasad Confidential 64
  • 65.
    4/30/2015 M VV K Srinivas Prasad Confidential 65  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!
  • 66.
    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 4/30/2015 M V V K Srinivas Prasad 66
  • 67.
    4/30/2015 M VV K Srinivas Prasad Confidential 67 remanent magnetization = M0 coercivity = Hc
  • 68.
    4/30/2015 M VV K Srinivas Prasad Confidential 68 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).
  • 69.
    4/30/2015 M VV K Srinivas Prasad Confidential 69 Hysteresis, Remanence, & Coercivity of Ferromagnetic Materials
  • 70.
    4/30/2015 M VV K Srinivas Prasad Confidential 70 “hard” ferromagnetic material has a large M0 and large Hc. “soft” ferromagnetic material has both a small M0 and Hc.
  • 71.
    Hard versus SoftMagnets  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: 4/30/2015 M V V K Srinivas Prasad Confidential 71
  • 72.
    Importance of SoftMagnetic 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. 4/30/2015 M V V K Srinivas Prasad Confidential 72
  • 73.
    Hard versus SoftMagnets 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:
  • 74.
    Importance of Hardmagnetic 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
  • 75.
    Who to getlarger 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.
  • 76.
    Baskar, Naren &G.Srinivas Hard Magnetic Material Soft Magnetic Material Have 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.
  • 77.
    4/30/2015 M VV K Srinivas Prasad Confidential 77 Applications of Magnetic Materials
  • 78.
    4/30/2015 M VV K Srinivas Prasad Confidential 78 Simulation of hard drive courtesy Martin Chen. Reprinted with permission from 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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    4/30/2015 M VV K Srinivas Prasad Confidential 79 --Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy)
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    4/30/2015 M VV K Srinivas Prasad Confidential 80 --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10- 30nm! (hard drive)
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    4/30/2015 M VV K Srinivas Prasad Confidential 81
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    4/30/2015 M VV K Srinivas Prasad Confidential 82
  • 83.
    Magnetic hard drives 4/30/2015M V V K Srinivas Prasad Confidential 83
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    4/30/2015 M VV K Srinivas Prasad Confidential 84 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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    4/30/2015 M VV K Srinivas Prasad Confidential 85
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    4/30/2015 M VV K Srinivas Prasad Confidential 86
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    4/30/2015 M VV K Srinivas Prasad Confidential 87
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    4/30/2015 M VV K Srinivas Prasad Confidential 88 RELAYS
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    4/30/2015 M VV K Srinivas Prasad Confidential 89 • 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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    4/30/2015 M VV K Srinivas Prasad Confidential 90 • 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.
  • 91.
    4/30/2015 M VV K Srinivas Prasad Confidential 91 Applications • Telecommunication system • Computer interfaces • Domestic appliances • Air conditioning • Traffic control • Control of motors • Business machines • Electric power control
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    4/30/2015 M VV K Srinivas Prasad Confidential 92 • 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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    4/30/2015 M VV K Srinivas Prasad Confidential 93 • 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
  • 94.
    4/30/2015 M VV K Srinivas Prasad Confidential 94 • 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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    4/30/2015 M VV K Srinivas Prasad Confidential 95 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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    4/30/2015 M VV K Srinivas Prasad Confidential 96 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 low 5) The low energy product will drive up the volume of magnet 6) magnetic flux can be lost rapidly with the introduction of small demagnatising forces.
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    4/30/2015 M VV K Srinivas Prasad Confidential 97 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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    4/30/2015 M VV K Srinivas Prasad Confidential 98 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
  • 99.
    4/30/2015 M VV K Srinivas Prasad Confidential 99 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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    4/30/2015 M VV K Srinivas Prasad Confidential 100 Samarium – Cobalt Family • This family of magnets was developed in the 1970’s. • Applications requiring high magnetic energy with little volume were 1) 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
  • 101.
    4/30/2015 M VV K Srinivas Prasad Confidential 101 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.
  • 102.
    4/30/2015 M VV K Srinivas Prasad Confidential 102 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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    4/30/2015 M VV K Srinivas Prasad Confidential 103 SENSORS
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    4/30/2015 M VV K Srinivas Prasad Confidential 104 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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    4/30/2015 M VV K Srinivas Prasad Confidential 105 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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    4/30/2015 M VV K Srinivas Prasad Confidential 106 Magnets can be used to sense – Position – Force – Torque – Speed – Rotation – Acceleration – current and magnetic field