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UNIT-IV
MAGNETIC AND SUPER
CONDUCTING PROPERTIES
HISTORICAL BACKGROUND

Magnets have been known for Centuries. The Chinese
and Greeks knew about the “magical” properties of
magnets.

The ancient Greeks used a stone substance called
“magnetite” a natural magnetic material Fe3O4. They
discovered that the stone always pointed towards
north.

Later, stones of magnetite called “lodestones” were
used in navigation.

Term magnet comes from the ancient Greek city of
Magnesia, at which many natural magnets were found.

Pliny (23-79 AD Roman) wrote of a hill near the river
Indus that was made entirely of a stone that attracted
iron.

Chinese as early as 121 AD knew that an iron rod which
had been brought near one of these natural magnets
would acquire and retain the magnetic property. when
this rod is suspended from a string it would align itself in
a north-south direction.

Use of magnets to aid in navigation can be traced back
to at least the eleventh century.

William Gilbert, an English physician, first proposed in
1600 that the earth itself is a magnet, and he predicted
that the Earth would be found to have magnetic posles.
Magnetism:
Magnetism is the force of attraction or repulsion of a magnetic
material due to the arrangement of its atoms, particularly its electrons.
A large electromagnet
used to lift scrap metal
MAGNETIC DIPOLE:
The ends of a magnet are where the magnetic effect is the strongest.
These are called “poles.” Each magnet has 2 poles – 1 north, 1 south.
Any two opposite poles separated by a finite distance constitute a
magnetic dipole.
For Every North, There is a South
S
N
S
N S
No Monopoles Allowed
S
N
N
Like pole
repels!
Opposite poles attract!
Magnetic dipole moment or magnetic moment (μm):
It is defined as the product of pole strengh (m) and the
distance between dipoles (2l).
m  m  2l
Magnetic moment due to a current (I) carrying circular wire of
area of cross-section (A).
 m  IA
UNITS: Am2
THE CONCEPT OF “FIELDS”
Magnetic field:
The space surrounding the magnet upto which
its influence felt is known as magnetic field.
Michael Faraday
MAGNETIZATION:
It is a measure of how a material respond when magnetic field is
applied to it.
INTENSITY OF MAGNETIZATION (M):
When a material is magnetized, it develops a net magnetic moment.
The magnetic moment per unit volume is called intensity of
magnetization.
Units: Am-1
MAGNETIC FLUX (Φ):
Magnetic flux is the product of the average magnetic field times the
perpendicular area that it penetrates.
The contribution to magnetic flux for a
given area is equal to the area times the
component of magnetic field perpendicular
to the area.
For a closed surface, the sum of
magnetic flux is always equal to zero
(Gauss' law for magnetism).
No matter how small the volume, the
magnetic sources are always dipole
sources (like miniature bar magnets), so
that there are as many magnetic field
lines coming in (to the south pole) as out
(from the north pole).
Magnetic Induction or magnetic flux density (B)
It is defined as the number of lines of force passing through
the unit area perpendicularly.
Units: Wb/m2 or tesla
Permeability (μ)
This the characteristic property of a medium.
It indicates the ease with which the material allows the
magnetic lines of force to pass through it.
OR
It is the measure of the ability of a material to support the
formation of a magnetic field within itself.
OR
In other words, it is the degree of magnetization that a material
obtains in response to an applied magnetic field.
   r 0
μr- relative permeability of the medium
μ0 – permeability of free space or perme 4π×10−7 H.m-1
SI UNITS: Henry per meter(H.m-1), Newton per Ampere square (N.A-2)
Magnetic field intensity (H)
Magnetic field intensity at any point in the magnetic field is
the force experienced by an unit north pole placed at that point.
Magnetic susceptibility ( ):
Magnetic susceptibility is the degree of magnetization of a
material in response to an applied magnetic field.
M H
M H
Where
M is magnetization,
H is magnetic field intensity
The proportionality constant
zero, positive or negative.
is called susceptibility. Its value may be
The magnetic induction and magnetic field intensity are related by
B  0 ( H  M )
In vacuum
B  0 H
In a medium
BH
, Since M=0
Since,   r 0
B  0 ( H  M )
M H
B  0 ( 1   ) H
where
 r  (1   )
Relative permeability
Quantity
Symbol
SI Units
SI Units
CGS Units
(Sommerfeld)
(Kennelly)
(Gaussian)
Field
H
A/m
A/m
Oersteds
Flux Density
(Magnetic
Induction)
B
Tesla
Tesla
Gauss
Flux
f
Weber
Weber
Maxwell
Magnetization
M
A/m
-
erg/Oe-cm3
Conversion between CGS and SI magnetic units.
Origin of magnetic moment
1. Orbital Motion - Orbital magnetic moment
2. Spin motion – Spin magnetic moment
Orbital magnetic moment
The revolving electron in circular orbit establishes a current given by
I= charge/time period= -e/(2π/ω)= - ωe/2 π
The current establishes a magnetic field around the circular orbit, so
that upper surface act as South pole and lower surface acts as North
pole.
Then orbital magnetic moment is given by
2
e
e

r
orbital  IA  
 r 2  
2
2
Angular momentum = linear momentum x radius
=mvr
=mωr2
-
μorb
I
e r 2
orbital  
2
e

(mr 2 )
2m
e

 orbital angular momentum
2m
Angular momentum associated with orbital quantum number l is lh
2
e lh
orbital   
2m 2
  Bl
elh
is the fundamental unit of magnetic moment known as Bohr
4 m
magneton and its value is 9.27 x 10-24 A/m2
B 
Spin magnetic moment
e
 spin  g
S
2m
Where, g is dimensionless number and is called g-factor.
This number depends upon the particle. For electron its value is ~2
μs
- -- -- -
I
 An atom is said to be magnet if it carries a permanent dipole
moment.
 Every substance is formed from an assembly of atoms which can be
either non-magnetic or magnetic.
The magnetic materials are classified into five groups depending on
their response to the magnetic field.
1. Diamagnetic Materials
2. Paramagnetic Materials
3. Ferromagnetic Materials
4. Antiferromagnetic Materials
5. Ferrimagnetic Materials
DIAMAGNETISM
 Diamagnetism characterizes the substances that have only nonmagnetic atoms (lack of permanent diople moment).
 Origin:
• An electron moving around the nucleus results in magnetic
moment.
•
Due to different orientations of various orbits of an atom, the
net magnetic moment is zero in diamagnetic materials.
•
When an external field is applied the motion of electrons in
their orbits changes resulting in induced magnetic moment in a
direction opposite to the direction of applied field.

The magnetization induced by the applied magnetic field is
weak and the magnetic lines of force are repelled.
very

This magnetism is also exist in substances with magnetic atoms,
but very weak and completely masked by the contribution of
magnetic atoms.

Relative permeability is slightly less than unity.

The magnetic susceptibility is independent of applied magnetic
field strength.
Magnitude of Temperature dependence
susceptibility
Small & negative Independent
Intermediate &
negative
Examples
Organic materials,
light elements
Below 20K varies with field and Alkali earhs,
temperature
Bismuth
Large & Negative Exists only below critical
Superconducting
temperature (Meissner effect) materials
PARAMAGNETISM
 The paramagnetic substances consists of magnetic atom that
posses permanent dipole moment
 Origin
 Each electron in an orbit has an orbital magnetic moment and a
spin magnetic moment.
 When the shells are unfilled there is net magnetic moment.
 In the absence of the external field the net moments of the
atoms are arranged in random directions because of thermal
fluctuations. Hence there is no magnetization.
 When external magnetic field is applied, there is tendency for the
dipoles to align with the field giving rise to an induced positive
dipole moment.
 The induced magnetism is the source for paramagnetic behaviour.
 Paramagnetic susceptibility is small
independent of applied field strength.
and
positive
and
is
 Spin alignment is random.
Magnitude of
susceptibility
Small & positive
Large & positive
Temperature dependence
Independent


C
T
C
T 
Curie law
Curie-Weiss law
Examples
Alkali metals,
transition
metals, rare
earths
FERROMAGNETISM

Even in the absence of external applied field, some substances
exhibits strong magnetization.

This is due to a special form of interaction called exchange coupling
between adjacent atoms that results in spontaneous magnetization
of the substance.

When placed inside a magnetic field, it attracts the magnetic lines
of force very strongly.

Each ferromagnetic material has a characteristic temperature
called the ferromagnetic Curie temperature θf. Below this
temperature the spontaneous magnetization exists.
 Spin alignment is parallel.
 Ferromagnetic materials exhibit Hysteresis.
 They Consists of a number of small regions which are called domains.
Magnitude of
susceptibility
Very large &
positive
Temperature dependence
C

T 
For T>θf paramagnetic behavior
For T<θf ferromagnetic behavior
Examples
Fe, Co, Ni, Gd
ANTI-FERROMAGNETISM
 Antiferromagnetism macroscopically similar to paramagnetism, is
a weak form of magnetism.
 In certain materials when the distance between the interacting
atoms is small the exchange forces produce a tendency for
antiparallel alignment of electron spins of neighboring atoms.
 The magnetic susceptibility increase with the increase of
temperature and reaches maximum at a certain temperature.
This temperature is known as Neel temperature (TN). Above this
temperature the susceptibility again decreases.
Spins are aligned antiparallel
Magnitude of
susceptibility
small & positive
Temperature dependence Examples
C

T 
when T>TN
 T
when T<TN
Salts of
transition
metals
FERRIMAGNETISM
 This is a special case of antiferromagnetism.
 The net magnetization of magnetic sublattices is not zero, since
antiparallel moments are of different magnitudes.
 Hence ferrimagnetic materials possesses a net magnetic moment.
 This moment disappears above a Curie temperature analogous to the
Neel temperature.
 Above TC, thermal energy randomizes the individual magnetic
moments and the material becomes paramagnetic.
 Ferrimagnetic domains become magnetic bubbles to act as memory
elements.
 Spin alignment is antiparallel of different magnitudes.
Magnitude of
susceptibility
Temperature dependence
Very large &
positive

C when T>T
N
T 
when T<TN behaves as paramagnetic
material
Examples
Ferrites
HYSTERESIS
Hysteresis of ferromagnetic materials refers to
magnetization behind the magnetizing field.
 A hysteresis loop is a curve
showing the change in magnetic
induction
of
a
ferromagnetic
material with an external field.
 When the external magnetic field
is
increased
the
induction increases.
magnetic
the lag
of
 Once magnetic saturation has been achieved, a decrease in the
applied field back to zero results in a macroscopically permanent
or
residual
magnetization,
known
as
remanance,
Mr.
The
corresponding induction, Br, is called retentivity or remanent
induction of the magnetic material. This effect of retardation by
material is called hysteresis.
 The magnetic field strength needed to bring the induced
magnetization to zero is termed as coercivity, Hc. This must be
applied anti-parallel to the original field.
 A further increase in the field in the opposite direction results in
a maximum induction in the opposite direction. The field can once
again be reversed, and the field-magnetization loop can be closed,
this loop is known as hysteresis loop or B-H plot or M- H plot.

Below the ferromagnetic Curie
substances exhibit hysteresis.
temperature
ferromagnetic

The phenomenon of hysteresis can be explained with domain
theory.

A region in a ferromagnetic material where all the magnetic
moments are aligned in the same direction is called a domain.

Each of these domains is separated from the rest by domain
boundaries / domain walls.

Boundaries, also called Bloch walls, are narrow zones in which the
direction of the magnetic moment gradually and continuously
changes from one domain to that of the next.
Block wall transition (B) between domains (A) and
(C) with 180° difference
The increase in the value of the resultant magnetic moment of the
specimen under the action of the applied field can be attributed to
1. The motion of domain walls
2. Rotation of domains
 When a weak magnetic field is applied, the domains that are aligned
parallel to the field and in easy direction of magnetization grow in size
at the expense of less favorably oriented ones. This results in the
Bloch wall movement.
 When the weak field is removed the domains reverse back to their
original state. Shown by the curve OA.
Domain rotation
B
A
irreversible wall
displacement
O
Reversible wall
displacement
 When the field becomes stronger the Bloch wall movement
continues and it is mostly irreversible movement. Shown by the
curve AB.
 At the point B all domains have got magnetized along their easy
directions.
 Field is further increased the domains rotate in the field direction
which is away from the easy direction thereby storing anisotropy
energy.
 Once the domain rotation is complete the specimen is saturated.
 On the removal of the field the specimen tend to attain the
original configuration by the movement of Bloch walls. But this
movement is hampered by impurities, lattice imperfections etc.

A Coercive field is required to reduce the magnetization of the
specimen to zero.

The amount of energy spent in this regard is a loss.

Hysteresis loss is the loss of energy in taking a ferromagnetic
body through a complete cycle of magnetization and this loss is
represented by the area enclosed by the hysteresis loop.
Based on the area of the hysteresis loop the magnetic materials are
classified into two types
1. Hard magnetic materials
2. Soft magnetic materials
Hard magnets
soft magnets
Hard magnets
o
Hard magnets are characterized by high remanent inductions and high
coercivities.
o
These are also called permanent magnets or hard magnets.
o
These are found useful in many applications including fractional
horse-power motors, automobiles, audio- and video- recorders,
earphones, computer peripherals, and clocks.
o
They generally exhibit large hysteresis losses.
o
Ex.: Co-steel, Tungsten steel, SmCo5, Nd2Fe14B, ferrite Bao.6Fe2O3,
CuNiFe (60% Cu 20% Ni-20% Fe), Alnico (alloy of Al, Ni, Co and
Fe), etc.
SOFT MAGNETS
 Soft magnets are characterized by low coercive forces and high
magnetic permeabilities; and are easily magnetized and demagnetized.
 They generally exhibit small hysteresis losses.
 Application of soft magnets include: cores for electro-magnets,
electric motors, transformers, generators, and other electrical
equipment.
 Ex.: ingot iron, low-carbon steel, Silicon iron, superalloy (80% Ni-5%
Mo-Fe), 45 Permalloy (55%Fe-45%Ni), 2-79 Permalloy (79% Ni-4%
Mo-Fe), MnZn ferrite / Ferroxcube A (48% MnFe2O452%ZnFe2O4), NiZn ferrite / Ferroxcube B (36% NiFe2O4-64%
ZnFe2O4), etc.
Bubble memory is a type of non-volatile computer memory that uses a
thin film of a magnetic material to hold small magnetized areas, known
as bubbles or domains, each of which stores one bit of data.
SUPERCONDUCTIVITY
Definitions
• Tc: This is the critical temperature at which the resistivity of a
superconductor goes to zero. Above this temperature the material is
non-superconducting, while below it, the material becomes
superconducting.
• Bc: The scientific notation representing the "critical field" or maximum
magnetic field that a superconductor can endure before it is "quenched"
and returns to a non-superconducting state. Usually a higher Tc also
brings a higher Bc. Type II superconductors have lower Bc1 and upper
Bc2 critical fields.
INTRODUCTION
For some materials, the resistivity vanishes at some low temperature: they
become superconducting.
Superconductivity is the ability of
certain materials to conduct electrical
current with no resistance. Thus,
superconductors
can
carry
large
amounts of current with little or no
loss of energy
In July 1909, Heike Kamerlingh Onnes found that the resistance of mercury dropped suddenly to an
immeasurable small value when it is cooled below 4.2K. Onnes termed the new electrical state that
the mercury had entered the superconducting state.
PROPERTIES VIS-A-VIS
SUPERCONDUCTIVITY
Properties not affected :
 Elastic properties
 Thermal expansion behaviour
 Photoelectric properties
 Internal arrangement of crystal lattice as confirmed by X-ray diffraction pattern before and after
such a transition
Properties affected :
 Magnetic properties change
 Electrical properties : as the electrical resistivity tendo to zero at T=Tc
 All thermo electric effects disappear for T  Tc
 Specific heat shows a discontinuous change
 Some latent heat of transition may also be involved
 The transition in zero magnetic field from the superconducting state to the nornal state is not
exactly involving latent heat , though there is discontinuity in the heat capacity – temp2 graph
 Entropy shows a decrease for T  Tc
NON SUPERCONDUCTOR
SUPERCONDUCTOR
Bint = 0
Bint = Bext
MESSINER EFFECT
MEISSNER EFFECT : Meissner and Ochsenfeld discovered that when a
superconductor is cooled in a magnetic field to below the value of transition
temperature corresponding to that field, then the lines of magnetic induction
B are pushed out of the bulk superconductor.
TYPE I SUPERCONDUCTORS
Type I superconductors : These are superconductors which exhibit
Complete Meissner effect.
 There are 30 pure metals which exhibit
zero resistivity at
low temperature.
 They are called Type I superconductors
(Soft
Superconductors).
 The superconductivity exists only below
their critical temperature and below a
critical magnetic field strength.
Type I
Superconductors
Mat.
Tc (K)
Mat.
Tc (K)
Be
0
Gd*
1.1
Rh
0
Al
1.2
W
0.015
Pa
1.4
Ir
0.1
Th
1.4
Lu
0.1
Re
1.4
Hf
0.1
Tl
2.39
Ru
0.5
In
3.408
Os
0.7
Sn
3.722
Mo
0.92
Hg
4.153
Zr
0.546
Ta
4.47
Cd
0.56
V
V
5.38
U
0.2
La
6.00
Ti
0.39
Pb
7.193
Zn
0.85
Tc
7.77
Ga
1.083
Nb
9.46
TYPE II SUPERCONDUCTORS
Type II superconductors : These are superconductors which do not
exhibit Meissner effect strictly
 Starting in 1930 with lead-bismuth alloys, were found which exhibited
superconductivity; they are called Type II superconductors (Hard
Superconductors).
 They were found to have much higher critical fields and therefore could carry
much higher current densities while remaining in the superconducting state.
Type II
Superconductors
CHARACTERISTICS PROPERTIES OF A
SUPERCONDUCTOR : FACTOR AFFECTING
Temperature : If a ring made of superconducting material is cooled in a magnetic field from
ordinary temeprature to a value below its critical temperature and then the magnetic field is removed,
an induced current is set up in the ring. The resistance in the superconducting state being practically
zero, the decay of thie induced current will take infinitely long time.
Magnetic field : Application of magnetic field to a superconducting specimen brings a stage
when for H=Hc, the critical field, the superconductor behaves like a normal material i.e., the
superconductivity disappears.
Current : If the magnetic field around the superconductor is increased beyond the critical field the
superconductivity is destroyed and the sample behaves as a normal material. Therefore the
supercurrent will flow only up to its critical value .Once the field exceeds Hc(T) the current becomes just
the ordinary current.
Stress : Application of stress increases the transition temperature. As Hc(T) is temperature
dependent, increased stress is found to result in a slight change of Hc(T).
Size : Size of specimen exhibiting superconductivity is an important parameter for its behaviour.
Impurity : The presence of impurities changes almost all properties of a superconductor especially
its magnetic behaviour.
Isotopic Constitution of the Specimen : The critical temperature of a specimen
depends on the isotopic mass. The presence of various isotopes in a given specimen decided what its
average isotope mass will be. The dependence of Tc on such a mass is also called Isotope Effect.
MaTc = constant or Tc  M-1/2
THERMODYNAMICAL PROPERITES
OF SUPERCONDUCTING STATE
Entropy : Going from Normal state to superconducting state the entropy decreases, so the
superconducting state is more ordered than the normal state.
Specific Heat : The specific heat of the normal metal obeys the relation ship, Cn(T) =  T + βT3
where as for the superconducting state the specific heat is Ces(T) = A exp (-/kβT) where A is some
constant and  is superconducting energy group which is equal to one half of the minimum value of
energy for destroying a cooper pair.
Energy Gap : The energy gap of superconductors is of entirely different nature than the energy gap in
insulators. In superconductor the energy gap is due to electron-electron interaction in fermi gas
whereas in insulator or semiconductor the energy gap is caused by electron lattice interaction. In
insulators the gap prevents the flow of electrical current. Energy must be added to lift electrons from the
valence band to conduction band before the current can flow. In a superconductor, on the other hand,
the current flows despite the presence of energy gap . In a superconductor the electrons in the excited
state above the gap behave as normal electron. The transition in zero magnetic filed from
superconducting state to normal state is observed to be a second–order phase transition.
BCS THEORY OF
SUPERCONDUCTIVITY
The microscopic theory put forward by Bradeen , Cooper and Schruffier (BCS) forms the
basis of quantum theory of Superconductivity. The fundamental postulate of BCS theory
is that when an attractive interaction between two electrons by means of phonon
exchange dominates the repulsive coulomb interaction then the superconducting state
is formed.
Electron-phonon-electron interaction : During an interaction of an electron with a positive ion
of the lattice through electrostatic coulomb force, some electron momentum get transferred. As
a result, these ions set up elastic wave in the lattice due to distortion. If another electron
happens to pass through this region then the interaction between two occurs which in its effect
lowers the energy of the second electron. The two electrons interact via the lattice distortion or
the phonon field resulting in the lowering of energy of the electron which implies the force
between two electrons is attractive. This interaction is strongest when two electrons have equal
and opposite moments and spin and this pair is known as cooper pair.
COOPER PAIR
When the temperature of the specimen is lowered, if the attractive
force between two electrons via a phonon exceeds coulomb repulsion
between them, then a weakly bound cooper pair is formed having the
binding energy of the order of 10-3 eV. The energy of Cooper pair is
less than the energy of the pair in free state. The binding energy of
cooper pair is called energy bang gap, Eg. When h  Eg strong
absorption occurs as the cooper pairs break apart.
The electrons in cooper pair have opposite spins so the total spin of
the pair is zero. As a result cooper pairs are bosons whereas electrons
are fermions.
APPLICATIONS OF
SUPERCONDUCTIVITY
Superconductors are used to make the powerful electromagnets,
including those used in MRI machines, beam steering magnets used in
particle accelerators.
 Superconductors have also been used to make digital circuits and
microwave filter for mobile phone base stations.
 Promising future applications include high performance transformers,
power storage devices, electric power transmission, electric motors and
magnetic levitation devices.
Definitions
• Jc: The scientific notation representing the "critical current
density" or maximum current that a superconductor can carry
without becoming non-superconductive.
• Meissner Effect: Exhibiting diamagnetic properties to
the total exclusion of all magnetic fields. (Named for Walter
Meissner.) This is a classic hallmark of superconductivity and
can actually be used to levitate a strong rare-earth magnet.
Superconductor Types
• Type I
Exhibits perfect diamagnetism below transition temperature Tc and has only
one critical magnetic field Bc.
• Type II
Totally expels and excludes magnetic flux below lower critical field Bc1 and
partially does so between Bc1 and upper critical field Bc2; all
superconductors except elements are Type II. This type has a larger Tc than
that of a Type I superconductor.
QuickTime™ and a
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A Brief History of Superconductors
•
•
•
In 1911 superconductivity was first observed in mercury by Dutch
physicist Heike Kamerlingh Onnes of Leiden University. When he
cooled it to the temperature of liquid helium, 4 degrees Kelvin, its
resistance suddenly disappeared!
In 1933 Walter Meissner and Robert Ochsenfeld discovered that a
superconducting material will repel a magnetic field. This
phenomenon is known as perfect diamagnetism and is often referred
to as the Meissner effect.
Since then major developments have been made in both the
discovery of higher temperature superconductors as well as progress
in the theory of superconductivity. In 1957 the 1st major
advancement in the theory was made by American physicists John
Bardeen, Leon Cooper, and John Schrieffer. Their Theories of
Superconductivity became known as the BCS theory - abbreviated
for the first letter of each man's last name - and won them a Nobel
prize in 1972. BCS theory explained superconductivity at
temperatures close to absolute zero for elements and simple alloys.
However, at higher temperatures and with different superconductor
systems, the BCS theory has become inadequate to fully explain how
superconductivity is occurring.
History continued…
•
•
In 1962 Brian D. Josephson, a graduate student at Cambridge
University, predicted that electrical current would flow between 2
superconducting materials - even when they are separated by a nonsuperconductor or insulator! His prediction that superconductors would
exhibit this quantum effect on a macro scale was later confirmed and
won him a share of the 1973 Nobel Prize in Physics. This tunneling
phenomenon is today known as the "Josephson effect" and has been
applied to electronic devices such as the SQUID (Superconducting
Quantum Interference Device), an instrument capable of detecting even
the weakest magnetic fields.
More recently scientists have made improvements in the area of
predicting and engineering new types of superconductors. In the 80’s
carbon based superconductors as well as ceramic superconductors
were developed. These superconductors have fantastic magnetic
properties as well as high critical temperatures, but their mechanical
properties are poor.
Josephson effect
(see also hand-out)
In 1962 Josephson predicted Cooper-pairs can
tunnel through a weak link at zero voltage
difference. Current in junction (called
Josephson junction – Jj) is then equal to:
J  J c sin 1   2 
Electrical current flows between two SC
materials - even when they are separated by
a non-SC or insulator. Electrons "tunnel"
through this non-SC region, and SC current
flows.
Brian D. Josephson
The Discovery of Tunnelling Supercurrents
The Nobel Prize in Physics 1973
JJ’s essential in Superconducting Interferen
The SQUID may be configured as a magnetometer to
detect incredibly small magnetic fields - small enough to
measure the magnetic fields in living organisms.
Threshold for SQUID: 10-14 T
Magnetic field of heart: 10-10 T
Magnetic field of brain: 10-13 T
•
Many uses in everyday life
•Making measurements using SQUIDs
(magnetic susceptibility, static nuclear susceptibility, Nuclear Magnetic resonance...)
• Biomagnetism
(magnetoencephalography [MEG], magnetocardiogram)
• Scanning SQUID microscopy
• Geophysical applications of SQUID
(oil prospecting, earthquake prediction, geothermal energy surveying)
• Higher Temperature SQUIDs
(nondestructive testing of materials...)
Fig.2 Neuromag Ltd.122
sensor array
Fig.1 Neuromag Ltd.122
MEG system
Arrays of gradiometer dc SQUID detectors are
contained within a helmet surrounded by a liquid
helium reservoir for cooling
Fig. MRI scan of a human scull
Uses of SC magnets
Transmission Lines
• 15% of generated
electricity is
dissipated in
transmission lines
• Potential 100-fold
increase in capacity
• BNL Prototype:
1000 MW
transported in a
Pirelli Cables & Systems
Telecommunications
• Superconductors are used as efficient
filters in cellular telephone towers (now 700
worldwide)
• Separate signals of individual phone calls.
• Because of electrical resistance,
conventional interference filters eat away
part of the signal.
Conductus Clearsite system
Superconducting magnets
An electrical current in a wire creates a magnetic field around a wire.
The strength of the magnetic field increases as the current in a wire
increases. Because SCs are able to carry large currents without loss of
energy, they are well suited for making strong magnets. When a SC is
cooled below its Tc and a magnetic field is increased around it, the
magnetic field remains around the SC. If the magnetic field is increased
to a critical value Hc the SC will turn normal.
A typical Nb3Sn SC magnet.
It produces 10.8T with a current
of 146A. Bore diameter is 3.8 cm.
• Support a very high current density
with a very small resistance
• A magnet can be operated for days or
even months at nearly constant field
Cross-section of multifilament
Nb-Ti of 1mm overall
diameter,
consisting from 13255 5-m
filaments