In this article we will discuss about the properties and classification of magnetic materials.
Properties of Magnetic Material:
1. Permeability:
Permeability is defined as the ratio of magnetic flux density and magnetic field intensity.
µ = B/H
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Where,
B = Magnetic flux density (Wb/m2)
H = Magnetic field intensity (A/m)
And, µ = µ0µr
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µ0 = Permeability of free space
= 4 × 10-7 = 1.257 × 10-6H/m or wb/Am
µr = Relative permeability, dimensionless
= 1 for vaccum
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2. Magnetic Dipole Moment:
A current loop constitutes a magnetic dipole. Magnetic dipole moment is defined as the product of the area of loop and current through the loop. It is the vector quantity and its direction is normal to the plane containing current loop. Magnetic dipole moment (µm) associated with the current loop-
µm = I × A n̂ ….. (1)
n = Normal unit vector
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Unit of magnetic dipole moment is A-m2.
The torque on a magnetic dipole produced by a flux density
T = µm × B ….. (2)
Magnetic dipole moment nm is also defined as the dipole moment due to the orbital angular momentum of electron owing to quantum numbers and expressed µm = eP/dm
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Where, e = electron charge
m = mass number
P = nw2r = angular momentum
w = angular velocity
r = radius of electron motion
3. Bohr Magnetron:
Atomic unit of magnetic moment is called Bohr Magnetron. Let us consider an electron revolving around a nucleus in a circular orbit.
The current through the loop is:
Larmor’s Angular Frequency:
Therefore, Larmor’s angular frequency is defined as the change in angular frequency of orbital electron when an external magnetic field is applied.
4. Magnetization (M):
It is defined as magnetic moment per unit volume.
Where,
N = Number of dipoles per unit volume.
Unit = A/m = unit of H
The magnetic flux density inside a magnetic material under the influence of external field has two components:
1. One component because of magnetisation
2. Other one because of external field.
5. Permanent Magnetic Dipole:
Whenever a charged particle has an angular momentum, the particle will contribute to permanent dipole moment.
In general there are three contributions to the angular momentum of an atom:
i. Orbital angular momentum- It is due to orbital motion of electron.
ii. Electron spin angular momentum- It is due to self-spin of electron. Since electron has a charge so its spin produces a magnetic dipole moment.
Magnetic properties of material are only effected by electron spin dipole moment (angular momentum). The atoms having completely filled inner shells have zero resultant electron spin dipole moment.
iii. Nuclear Spin Angular Momentum:
It is due to nucleus spin.
Susceptibility: (x): MαH
M = xH
Or, X = M/H
= Susceptibility (Parameter to access magnetic ability of material)
Value means be positive or negative
Higher positive value, better is the magnetic quality of material
Classification of Magnetic Materials:
Magnetic materials can be classified as: 1. Diamagnetic Material 2. Paramagnetic Material 3. Ferromagnetic Material 4. Antiferromagnetic Material 5. Ferrimagnetic Material.
1. Diamagnetic Material:
Materials which lack permanent magnetic dipoles are called diamagnetic. These material have small and negative magnetic susceptibility. These material repel the applied magnetic field. The magnetic flux inside diamagnetic material is zero. i.e., B = 0.
Hence, µr = 0; this relation is for perfect diamagnetism, which is also a necessary condition for a material to be a Super Conductor.
Magnetic susceptibility of these material is independent of temperature.
Example s Si, Ge, diamond, NaCI, Al2O3, Cu, Au (Gold) graphite.
The susceptibility of some diamagnetic material (At room temperature)-
Note that induced dipole moments occur in all materials and also a dipole moment induced by a magnetic field in a particular atomic model, is independent of the magnetic dipole moment present in the absence of the field. In this regard, all materials are diamagnetic.
2. Paramagnetic Material:
There are permanent magnetic dipoles associated with the atoms in a paramagnetic material. But the interaction between the atomic permanent dipole moments is zero or negligible.
When these material are placed in an external magnetic field acquires a weak magnetisation in the same direction of magnetic field. These materials have small positive value of magnetic susceptibility.
In paramagnetic material permanent dipole moment of the atoms and ions has no mutual interaction. However in the presence of field the magnetic moment have a tendency to turn towards the direction of applied field.
If no opposing force act, complete alignment of the dipole will be produced and the specimen would acquire a very large magnetisation. But thermal agitation of atoms opposes this tendency and tends to keep the dipole moment at random.
This result only into a partial alignment in the field direction. Therefore a weak magnetisation and small positive value of susceptibility occurs. The effect of increase in temperature is to increase the thermal agitation and therefore there is decrease in the susceptibility. Some paramagnetic materials follow curie law.
3. Ferromagnetic Material:
In ferromagnetic material, the dipoles interact in such a manner that they tend to line up in parallel.
These are the material which get magnetised in the direction of external field and remain magnetised even after the removal of magnetic field. This property of ferromagnetic material is called Spontaneous Magnetization.
Direction of magnetization can be reversed by reversing the direction of external magnetic field.
The ferromagnetic materials are characterised by parallel alignment of magnetic dipole.
Example: Fe, Co, Ni, Gd (Gadolinium), Dy (Dysprosium) etc.
The ferromagnetic material remains ferromagnetic upto a critical temperature called Curie Temperature. These materials start behaving like Paramagnetic Material above Curie Temperature (θf).
Gadolinium possess this property at low temperature (below 290K)
Europium oxide (EuO) is an example of ferromagnetism.
It’s susceptibility is positive of order 102 to 105.
Above Curie-temperature hysteresis loop merges to a straight line.
(i) T > θf:
In the region above the θf, the behavior of ferromagnetic material is somewhat similar to that of a paramagnetic material.
Curie-Weiss Law:
C = Curie Constant
θ = paramagnetic Curie temperature
The paramagnetic Curie temperature θ ferromagnetic Curie temperature.
(ii) T < θf:
Below the θf ferromagnetic materials exhibit the well-known hysteresis in the B Vs H curve.
4. Antiferromagnetic Materials:
In the atomic configuration, if neighbouring dipoles tend to line up so that they are antiparallel, the material is antiferromagnetic or ferrimagnetic, depending on the magnitudes of the dipoles on the two “sub lattices”.
In the ferromagnetic case, there is large resultant magnetization, whereas in an antiferromagnetic configuration the magnetization vanishes. In the case of ferrimagnetic materials, there may be a relatively large net magnetization resulting from the tendency of antiparallel alignment of neighbouring dipole moments of unequal magnitude.
In ferromagnetic materials, the tendency for parallel alignment of the electron spins was due to quantum mechanical exchange forces. In certain materials, for example when the distance between the interacting atoms is small, the exchange force produce a tendency for antiparallel alignment of electron spins of neighbouring atoms. This kind of interaction is encountered in antiferromagnetic and in ferrimagnetic materials.
In antiferromagnetic materials, net magnetization is zero when no external field is applied but when material is subjected to an external field, the dipole moment starts aligning in the direction of field.
These materials have small positive values of susceptibility.
These materials are antiferromagnetic upto a critical temperature called neel temperature and above this temperature these materials start behaving like paramagnetic.
The most characteristics feature of an antiferro-magnetic material is the occurrence of a rather sharp maximum in the susceptibility-vs-temperature curve.
The temperature for which this maximum susceptibility occurs is called the Neel temperature TN.
Above the Neel temperature, the susceptibility is observed to follow the equation:
where, C = the curie constant, θ = paramagnetic curie temperature
5. Ferrimagnetic Materials:
In ferrimagnetic material dipole moments of adjacent atom are also aligned in opposite direction but they are of unequal magnitude.
Ferrimagnetic materials are similar to ferromagnetic ones in the sense that both kinds may exhibit a large magnetization. On the other hand, ferrimagnetic materials resembles antiferromagnetic materials with respect to the tendency for antiparallel alignment of neighbouring dipole moments.
They are also called Ferrites. They behave as ferromagnetic materials in as much as they show spontaneous magnetization below a certain temperature. As far as their conductivity is concerned, they behave as semiconductors.
The dc-resistivity of ferrites is many orders of ten higher than that of iron; consequently the eddy current problem preventing penetration of magnetic flux into the material is much less severe in ferrites than of iron. Ferrites can therefore be used for frequencies upto microwaves in transformer cores and are of great technical importance in this respect.
Electrical and Magnetic characteristics of Ferrite:
1. High d.c. resistivity
2. Low eddy current losses.
3. High permeability.
4. High dielectric constant.
5. High Curie temperature.
The Chemical formula of simple ferrites is Me2+Fe32+O24– where, Me2+ may represent a variety of divalent metallic, such as Fe2+,Co2+,Mn2+, Zn2+, Cd2+, Mg2+ etc.
Reason:
Zinc ions go preferably into tetrahedral positions, thereby forcing some of the Fe3+ ions from tetrahedral to octahedral sites. Since the Zn2+ ions have no magnetic dipole moment, the net magnetisation increases.
Applications of Ferrimagnetic Materials:
i. Ferrites are compound of two metallic oxides out of which one is invariably an iron oxide. Other metallic oxide may be bivalent element. Such as Ni, Mn, Zn, Cu, Fe, Symbolically ferrite may be designated as (MC+O. Fe2O3). Where Me+ stands for metallic oxide.
ii. Soft Ferrites:
These are used for construction of core of inductors and transformer. These materials have high permeability, low coercive force and low eddy current loss.
Example:
Mn-Zn Ferrites, Ni-Zn Ferrites.
iii. Ni-Zn ferrites are used in audio and TV transformer.
iv. Hard Ferrites:
These are used for construction of permanent magnet. These materials have high permeability, high coercive force and high resistivity.
Example:
Ba & Sr Ferrites.
v. Rectangular Ferrites:
These are used as the core of magnetic memories. These ferrites are having rectangular shape of — hysteresis curve.
Example:
Mn-Mg Ferrites, Mn-Cu Ferrites, Ni-Li Ferrites.
vi. Microwave Ferrite.
vii. These ferrites are used at microwave frequency.
viii. At this frequency the electromagnetic wave interact with the spin magnetic moment of electron. Because of this, the plane of polarization of electromagnetic field gets rotated by some angle when the wave passes through the material. This phenomenon is called as Faraday rotation.
ix. These ferrites are used in microwave devices, Example: Gyrators, Circulators and Isolators etc.
Example:
Mn-ferrites, Co-ferrites, Ni-ferrites, Garnets (used in magnetic bubble memory).
Note:
(i) Barium ferrite (Bao. 6Fe2O3) is used in permanent magnet.
(ii) Hard ferrite such as barium, strontium & lead ferrites show semiconductor behaviour.
(iii) Lithium ferrites are soft ferrites. They possess square hysteresis loop, low dielectric losses. These are low mobility semiconductor, due to their high curie temperature they are used for application at microwave frequencies.
(iv) Ferrite have three different types of crystal.
a. Spinel: Cubic structure, MFe2O4, example: M = Zn, Mg, Co, Cd, Cu, Mn, Ni
b. Garnet: Cubic structure, example: MFe2O12, M = 5m, Tb, Ho, Er, Lu, Tm, Gd, O4.
c. Magnet plumbite: Hexagonal, example: MFe2O19, M = Ba
(v) The spinel ferrite unit cell contains 8 × MFe2O4 unit where M = divalent metal ion. Similar to MgAl2O4. Iron and metal iron occupy octahedral and tetrahedral site of the spinal lattice. This distribution give intrinsic property in terms of magnetic and electric.
6. Soft Magnetic Materials:
These materials are easy to magnetise and demagnetise. In these materials, the direction of magnetization can be altered easily by an applied field.
Soft magnetic materials should have following characteristics:
i. These materials have low retentivity.
ii. Low coercivity
iii. High permeability
iv. High magnetic saturation
v. Hysteresis curve should be tall and thin.
vi. Low hysteresis losses (because of lesser area of hysteresis loop)
These materials are used for transformer and inductor core to minimize energy disipatation (reduction in hysteresis loss).
Soft magnetic materials are desirable for electromagnets. Examples and Applications
1. Silicon-Steel/Soft-Iron/Fe-Si Alloy:
i. They are used in power transformer.
ii. They are used up to power frequency (50 to 60 Hz).
iii. Silicon is added to increase dc resistivity and to reduce the area of hysteresis loop, i.e. for low eddy current loss and low hysteresis loss.
2. Fe-Ni Alloy:
i. 36% Ni → used for high frequency applications such as high speed relay and transformer.
ii. 50% Ni → used for magnetic memory.
iii. 77% Ni → used for precision voltage and current transformers.
iv. 45% Ni → Permalloy
v. 79% Ni → Superalloy
vi. 75% Ni → Mu-metal
7. Hard Magnetic Materials (Permanent Magnet Materials):
Hard magnetic materials are those which retain a considerable amount of their magnetic energy after the magnetizing force has been removed, i.e., the materials which are difficult to demagnetize. These materials are also called permanent magnet materials.
The important applications of permanent magnets are in meters, transducers, electron tubes, motors, focusing magnets in TV tubes.
Materials for use as permanent magnets should have the following characteristics:
i. High permeability ensured by a large content of magnetic atoms or ions.
ii. High coercive force, generally, above 104 A/m.
iii. Appreciable remanent flux density
iv. High Curie temperature, to minimize easy demagnetization.
v. Low cost.
They have high hysteresis losses (because of large area of B-H curve).
Hard Magnetic Materials are:
i. Hardened Steels and Iron.
ii. Carbon Steel – 98% Fe, 1% Mn, 0.9% C, used as magnet for latching relays or compass middle.
iii. Tungsten Steel – Fe, W, C, and Cr used as magnets in dc motors.
iv. Alnico – Al, Ni, Co- Their magnetic properties are very stable with time and temperature, (retain 70% magnetization at 600°C)
v. Platinum Cobalt – 77% Pt, 23% Co
vi. Cunite (Cu, Ni, Fe)
vii. Remalloy
viii. Ba – ferrite
ix. Na2Fe14B
x. Powder ceramic permanent magnetic material such as COOFe2O3 & PbO.6Fe2O3
xi. MnBi
For metallic magnet Br > 2Hc & BHmax = 1/2BrHr.
For Ceramic magnets Br < Hc & BHmax = Br2/4.
Aelomax (55% Fe, 11% Ni, 22% Co, 8% Al, 4% Cu) & Hycomax (50% Fe, 21% Ni, 20% Co, 9% Al) are found as hard magnetic material.
8. High Energy (Product) Hard Magnetic Materials (HEHMMs):
Hard magnetic materials whose energy products (Br, Hc)max are in excess of about 80 kJ/m3 are called high energy HMMs. Conventional HMMs generally possess an energy product of about 3 to 80kJ/m3. The high energy HMMs are recently developed materials having the structure of intermetallic compounds. These are available in different compositions.
Example:
i. Samarium-cobalt rare earth (SmCo5), and
ii. Neodymium-Iron-Boron alloy (Nd2Fe14B)
Samarium-Cobalt Rare Earth:
It belongs to the family of alloys which are formed by combining iron or cobalt with a rear earth element. These are produced by sintering method of powder metallurgy technique. Magnetic Properties of High Energy HMMs and Their Comparison with a Conventional (Non-High Energy) Hard Magnetic Material.
i. Composition/Formula:
Samarium-cobalt rare earth: SmCo5
Neodymium-iron-boron alloy: Nd2Fe14B
Alnico 8: 7% Al, 15% Ni, 35% Co, 4% Cu, 5% Ti, rest Fe
ii. Maximum Energy Product (kJ/m3):
Samarium-cobalt rare earth: 170
Neodymium-iron-boron alloy: 255-400 (maximum)
Alnico 8: 36 (minimum)
iii. Remanence (Tesla):
Samarium-cobalt rare earth: 0.92-1.0
Neodymium-iron-boron alloy: 1.16 (maximum)
Alnico 8: 0.76 (minimum)
iv. Coercivity (kA/m):
Samarium-cobalt rare earth: 200
Neodymium-iron-boron alloy: 220-345 (maximum)
Alnico 8: 125 (minimum)
v. Resistivity (ohm m):
Samarium-cobalt rare earth: 5 × 10-7
Neodymium-iron-boron alloy: 16 × 10-7
vi. Curie Temperature (°C):
Samarium-cobalt rare earth: 725
Neodymium-iron-boron alloy: 310 (minimum)
Alnico 8: 860 (maximum)
vii. Fabrication Technique:
Samarium-cobalt rare earth and Alnico 8: Sintering
Neodymium-iron-boron alloy: Sintering, or rapid solidification
viii. Cost:
Samarium-cobalt rare earth: Costlier
Neodymium-iron-boron alloy: Cheaper
Alnico 8: Cheaper
ix. Uses:
Samarium-cobalt rare earth: As magnets for fractional horse power motors
Neodymium-iron-boron alloy: As magnets for fractional horse power motors
Alnico 8: As magnets for large horse power motors