Magnetic Properties of Materials

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Nov 15, 2013 (3 years and 11 months ago)

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Lab Notes

Magnetic Properties


LN 5
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Magnetic Properties of Materials

1.0 Learning Objectives

After successfully completing this laboratory workshop, including the assigned reading, the lab
bluesheets, the lab quizzes, and any required reports, the student will be able to:



Generate
B
-
H plots
experimentally for different materials.



Calculate
the applied field generated from a solenoid.




Identify
the following points on a hysteresis loop of a ferro
-

or ferrimagnetic material:
remanent magnetization (M
R
), remanence (B
R
), coercivity (H
C
), saturat
ion magnetization
(M
sat
), and saturation induction (B
sat
).



Differentiate
between soft and hard magnets using a hysteresis loop.



Explain

the basic classes of magnetic behavior: diamagnetic, paramagnetic, ferromagnetic,
ferrimagnetic, and antiferromagnetic.




Demonstrate

the Meissner effect and relate it to the diamagnetic properties of the cooled
superconductor.



Explain
the origin of the noise observed in the Barkhausen effect.

2.0 Resources

Callister,
Materials Science and Engineering: An Introduction
, Chap
ter 21

IBM’s “How a Hard Drive Works”:
http://www.research.ibm.com/research/gmr/basics.
html

IBM’s Materials used for Storage Systems:
http://www.almaden.ibm.com/sst/

Florida State’s Interactive Tutorials on Electricity, Magnetism, and Transistors:
http://micro.magnet.fsu.edu/electromag/java/index.html

3.0 Materials Applications

The phenomenon of magnetism in materials plays an important part in our everyday experience.
It extends from the permanent magnets used to latch our ref
rigerator doors to the magnetic
memory elements in our most sophisticated computers. In the last few decades our
understanding of magnetism in materials has matured to the point where magnetic devices are
becoming common place in technological application
s.

4.0 Background on Magnetism

The first magnetic phenomenon observed were those associated with naturally occurring
magnets, fragments of iron ore found near the ancient city of Magnesia, Turkey (hence the term
magnet). These natural magnets attract unma
gnetized iron; the effect is most pronounced at
certain regions known as its poles. These poles are termed positive and negative. Like
electricity, opposite poles attract and unlike poles repel each other. It was known as early as 121
A.D. that an iron

rod brought near a natural magnet, would acquire and retain the property of the
natural magnet, in other words the rod had become magnetized. In 1819 Hans Christian Oersted
established a connection between electrical and magnetic phenomena by demonstratin
g a
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magnetic compass needle was deflected when in the presence of a wire carrying current. Later,
and more importantly, it was found that passing a wire through a magnetic field induced a current
in the wire and conversely passing a current through a wire

sets up a magnetic field around the
wire.

4.1 Magnetic Force and Flux

It is now known that all magnetic phenomena result from the forces between electrical charges in
motion. That is, charges in motion set up a magnetic field as well as an electrical fie
ld, and this
magnetic field exerts a force on other charges in the vicinity. This can be summarized as follows:
a moving charge creates a magnetic field in the space around it, and the magnetic field exerts a
force on other moving charges in the field. T
he magnetic field is denoted by the magnetic field
vector
B

(in this article vectors will be denoted in bold letters while its magnitude is regular script).
A charged particle with velocity vector
v

in a magnetic field
B

has a force exerted on it accordin
g
to the following equation:

B
x
v
q
F







(1)

where q is the charge on the particle in coulombs. The units of
B

can be deduced from Equation
1, namely 1 unit of B = 1 N
s
C
-
1
m
-
1
, or since one ampere equals one Cs
-
1
, then 1 unit of B = 1 N m
-
1
A
-
1
. This quantity has been designated as one tesla (1 T), the SI unit of magnetic flux. The cgs
unit of
B
, the gauss (1 G = 10
-
4

T) is also in common use. To summarize:

1
T

1
Nm

1
A

1

10
4
G


(2)

A magnetic field is represented by lines which point in

the direction of
B
; these are called field
lines. In a
uniform field
,
B

has the same magnitude everywhere and the field lines are straight
and parallel. The magnetic flux


through an area A equals the vector component of
B

perpendicular to the area times the area,

d


B

dA


B

dA



(3)

In the case where
B

is uniform and perpendicular to the area,



BdA

B
A


(4)

The Sl unit of flux is 1 NmA
-
1
. I
n honor of Wilhelm Weber (1804
-
1890), 1 NmA
-
1

is called one
weber (1 Wb). Since 1 T = 1 Wb m
-
2
, the magnetic field
B

is sometimes called
flux density
. The
total magnetic flux through an area is proportional to the number of field lines passing through th
e
area. In the cgs system, the unit of magnetic flux is the maxwell and the corresponding unit of
flux density, one maxwell per square centimeter is the gauss (1 G). Instruments that measure flux
density are referred to as gaussmeters.


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You may have noti
ced by now that the two systems of units, SI and cgs, are not simply related by
a multiplicative factor. Instead the units are defined a bit differently and one must be careful to
use a consistent set of units.

4.2 Magnetic Field of Solenoid

By the princ
iple of superposition the total magnetic field caused by several moving charges is the
vector sum of the fields caused by the individual particles. A solenoid (or coil) is a helical winding
of wire, such as that obtained by winding wire around the surface

of a cylinder. The moving
charges within the wire (i.e., current) set up a magnetic field longitudinally within the interior of the
solenoid (remember
B

is perpendicular to the velocity of a particle). Increasing the current in the
wire increases the fie
ld strength
B

inside the coil. The field strength B inside the coil is given by:

B


0
nI

(5)

where
n

is the number of turns per unit length
L

and
I

is the current. The important thing to
remember is that the magnetic field
B

within th
e coil is proportional to the current in the loop as
well as to the number of turns. See Callister Figure 21.3.

4.3 Induced Electromotive Force

We have seen that current in the windings of a solenoid produces a magnetic field inside the coil.
But is the
reverse true; does the application of a magnetic field produce a current in the windings?
The answer is yes, if the field is transient. Transient means that
B

varies with time. This concept
is the basis for Faraday's law that states that the induced ele
ctromotive force


in the circuit is
numerically equal to the rate of change of flux through it:



d

dt


(6)

That is, the induced


is equal to the time rate of change of flux through the circuit. Combining
Equations 4 and 5, the flux thr
ough a solenoid is:



BA


0
nIA

(7)

Thus if the rate of change of flux is given by:



d

dt


0
dI
dt

(8)

Then a time varying magnetic field
B

will induce a time varying current in the solenoid and
therefore produce a ti
me varying voltage across the leads of the solenoid. These principles were
used in the design of the instrument you will use to characterize your material.

4.4 Magnetic Materials and Hysteresis

There are two general types of magnetism in materials. The f
irst of these is a class called
induced magnetism
. In this class, the material is magnetized only when there is an applied
magnetic field. Diamagnetism and paramagnetism are two forms of induced magnetism. See
Callister Figure 21.5. The second general
class of magnetic phenomena is referred to a
s
spontaneous magnetism
. Spontaneous magnetism refers to the ability of a material to retain its
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magnetic state even in the absence of an applied magnetic field. Ferromagnetism,
antiferromagnetism, and ferrimag
netism are the common examples of this type of magnetic
phenomena. Almost all magnetic materials that have important engineering applications belong to
this class. Consequently spontaneous magnetism will be emphasized here.

A material is magnetized when a
tomic dipoles within the material become aligned in the same
direction. These dipoles arise mainly from spin imbalances (unpaired electrons) in an atom that
allow the atom to become polarized. A magnetic property of considerable interest is the intensity

of
magnetization

(
M
). The intensity of magnetization is the density of magnetic moment in a
material, or the vector sum of the individual magnetic moments of the dipoles. This property is a
measure of how much a material may become magnetized (or how man
y and to what degree the
dipoles align). When a material will not become more magnetized (increasing
B

will no longer
increase
M
), it is said to be saturated.

A magnetization curve expresses the relationship between the induced magnetic field strength
B

i
n a ferromagnetic material and the corresponding magnetic intensity of an applied magnetic field
H
, provided the sample is initially unmagnetized and the magnetic intensity is steadily increased
from zero. Strictly speaking
B

is not equal to
M

but they ar
e so closely related, and to avoid over
complication, they will be treated as equivalent.

Thus, in Figure 1, if the magnetizing current in the windings of a solenoid is steadily increased,
the magnitude of B increases from the origin to its saturation va
lue at point 1, B
s
. If the current in
the windings, and hence the field H is now reduced to zero, the value of B decreases, following
the curve, until B
r

is reached. Even though there is now zero applied field, there is still some
remanent magnetization
(M
r
) which in turn causes a remanent field B
r
. When the current is
reversed, causing a reverse in the applied field H, B decreases to zero at point
-
H
c
. The
magnitude of applied filed required to cause B to be reduced to zero is called coercive field
st
rength, or coercivity, H
c
.

If H is now increased in the negative direction, B will increase to saturation magnetization in the
reverse direction, at point 2. Reducing the applied field to zero will again result in a remanent
field B
r
. Increasing H to the

coercivity value, H
c
, results in bringing the magnetization back to
zero. Thus if an ac current is applied to the solenoid, the value of B will cycle around this
hysteresis loop
, reaching saturation in one direction and then the other. The value of B
s

,

B
r

and
H
c

characterize the material and should be selected with a specific application in mind.


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Figure
1
. Hysteresis loop showing remanence (B
r
), saturation (B
s
) and coercivity (H
c
).

The area inside the loop is directly propor
tional to the energy dissipated in a material as it
traverses its hysteresis loop. A simple way to visualize why energy is dissipated during a
hysteresis cycle is to recognize that atomic dipoles must slide relative to each other during
alignment and duri
ng reversals, and that this sliding causes frictional energy that is dissipated as
heat. Some materials do this more efficiently or have less dipoles which move, and therefore
have loops which are smaller than other materials.

Different magnetic propertie
s are important for different applications. A permanent magnet
should have a large remanent magnetization and, to prevent demagnetization in external fields,
large coercivity. A magnetic material used in data storage applications, such as hard disks,
sho
uld have a large coercivity so that it cannot be accidentally erased, but not so large that it
cannot be changed when storing information.

For iron cores in inductors, transformers, motors, and other devices, it is usually desirable to have
as little hyst
eresis as possible because of the attendant energy loss and heating when the field
undergoes repeated reversals in the presence of alternating current. In such cases the remanent
magnetization and coercivity should be as small as possible.

A material hav
ing a small remanence and coercivity is said to be magnetically soft, while one
having large values is said to be magnetically hard. It is interesting to note that soft iron is soft
mechanically as well as magnetically, while some (but not all) steels are
hard magnetically. See
Callister Figure 21.16 for a comparison of the B
-
H loops of soft and hard magnets.

4.5 Measuring the B
-
H Loop

Now let us turn our attention to how these loops may be measured. Recall that a solenoid
containing a transient magnetic
field has alternating current induced within it, and that a B
-
H loop
is a material's response B to an applied field H. The apparatus shown in Figure 2 can be used to
measure a hysteresis loop. This apparatus is called a B
-
H Looper and consists of a prima
ry coil
(thin lines) carrying alternating current surrounding a secondary coil (thick lines) in the center of
which a sample is placed. Ohm's law states that the voltage across the primary coil is

H (Oe)
B (kG)
Hc
Br
Bs
-Hc
1
0
2
-Br
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proportional to the current through it. Thus from Equatio
n 5 the applied magnetic field is
proportional to the voltage across the coil. Since the primary coil carries alternating current, the
field in the primary coil also varies with time. Thus simply by measuring the voltage across the
primary coil, the field

strength H within the primary coil can be determined.


Figure
2
.

The B
-
H looper apparatus.

When a specimen is placed in the secondary coil, it is within the field of the primary coil and it
becomes magnetized by the primary field
. Since the field varies with time the induced magnetism
within the specimen also varies with time. Because the secondary coil is in the induced field of
the specimen and because this field is transient, current is set up in the secondary coil.
Combining E
quations 6 and 7 and integrating yields an expression showing the induced field
strength is proportional to the integrated voltage across the secondary coil.

B
t



1
A
V
t



dt

(9)

An R
-
C circuit can be used to integrate the voltage across th
e secondary coil. When the voltage
across the primary coil and the integrated voltage of the secondary coil is input to an oscilloscope
on the x
-
axis and y
-
axis respectively, a B
-
H loop is displayed. The values plotted on the scope
are only proportional
to the absolute values; therefore this display yields qualitative not
quantitative information about a materials magnetic properties. Even if the proportionalities are
accounted for the absolute precision of these B
-
H loopers is generally low. The advanta
ge of a
B
-
H looper is that it is simple, and fast to operate. As such, they are ideally suited to such a task
as quality control of ferrimagnetic oxides at a magnetic tape factory.

To summarize, alternating current in the primary coil sets up a time varyi
ng magnetic field H in its
core. This field induces an alternating magnetic field B in a sample placed in the H field. The
induced magnetic field in the sample in turn induces alternating current in the secondary coil.
Then, when the integrated voltage
across the secondary coil is plotted against the voltage across
the primary coil on an oscilloscope, an output proportional to the B
-
H loop is displayed.


sample
integrator
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5.0 B
-
H Loop Experiment

5.1 Equipment and Materials

Metal samples (bars)

B
-
H looper circuit

0
-
10 volt
ac. power supply

oscilloscope

5.2 Procedure

Figure 3 shows a schematic of the B
-
H looper that is used for this experiment. The left side of this
looper shows the connections necessary to connect the looper to the power supply and
oscilloscope. The voltag
e input terminal supplies the primary coil current, while the horizontal
and vertical terminals when input to the oscilloscope display the applied magnetic field on the
horizontal axis and induced magnetic field on the vertical axis respectively. The coil

selector
switch is used to select between two different sets of coils. The left switch position operates the
flat specimen coils and the right switch position operates the round specimen coils.


Figure 3.

Schematic of B
-
H looper apparatus.

To operate the

looper:

a)

Connect the voltage supply with the power off and in the O
-
volt position

b)

Connect the oscilloscope leads.

c)

Insert a sample and set the coil selector switch to the coil that is being used.

d)

Carefully increase the input voltage to around 5 volts, th
en adjust the oscilloscope horizontal
and vertical scale controls until the B
-
H loop is visible and is as large as the oscilloscope
screen will allow.

e)

Adjust the input voltage until the specimen is completely saturated as evidenced by the sharp
end point
s of the B
-
H loop. Never apply more voltage than is required to saturate the
specimen as this will cause undue heating of the circuit resistors and coils. Even so, the large
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resistor will become hot and care should be exercised to keep your hands away fr
om it while
the circuit is in operation.

To read the oscilloscope:

a)

Each line is a major grid line and is subdivided into 5 minor divisions by smaller hash lines.

b)

The oscilloscope control block has scale dials for each axis. The divisions on these dials

indicate the scale for each axis in terms of one major division. For example, if the vertical
axis control dial is set for 2 volts than each major division represents 2 volts and the full scale
display (all 10 divisions) represents 20 volts.

c)

Finally, ske
tch the B
-
H loop, then record the voltages which correspond to the remanence
and the coercivity for each specimen on the data table.

d)

A second BH looper has been set up on which the student can measure the value of the
remanence, coercivity and saturation f
ield. The sample is a toroid of carbon steel.

6.0


Background on Superconductivity

(written by Ted Salazar, BSEE ‘96, SJSU)

The Meissner effect was discovered in 1933 by W. Meissner and R. Ochsenfeld when they
observed the exclusion of a magnetic field from a

superconductor. In other words, when a
magnetic field was applied to a superconductor while in its superconducting state, it did not
penetrate the superconductor. See Callister Figure 21.23. In order to give a complete
explanation of the Meissner effec
t, a background in quantum mechanics is required; however,
most of the students participating in this experiment may not have taken quantum mechanics yet,
therefore, a simplified (but also correct) explanation will be given.

Current and magnetic fields are

sources of each other. Ampere’s law states that the magnetic
field intensity, H, around a closed contour is equal to the net current flowing through that contour.
That is why a surface current develops when a magnetic field is in contact with a supercon
ductor
as shown in Figure 4a. However, if the magnetic field is not allowed to penetrate the
superconductor then where is the closed contour? This is why many pages of quantum
mechanics has to be used to completely explain the phenomenon. However, even
the top people
in the superconducting field like to think about it this way, and it is definitely suitable for an
introduction to superconductivity.

When placing a magnet on top of a superconducting pellet, you are changing the magnetic field
with respect

to time across the surface of the superconductor. Anytime there is a change in the
magnetic field (flux to be exact) with respect to time, an equal and opposite current will be
induced as to oppose that change. This statement is known as Lenz’s law and

we shall label the
opposing current and magnetic field intensity as I
L

and H
L

respectively as shown in Figure 4b.

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(a)








(b)

Figure 4. (a)

A surface current that develops by Ampere’s Law.
(b)

An equal but opposite
magnetic field, H
L
, develops due

to Lenz’s law.

Contrary to all normal conductors, this opposing current, I
L
, never dissipates because of the zero
resistance of the superconductor. Therefore, a magnet can be levitated by this opposing
magnetic field, H
L
, as long as the material remains
in its superconductive state.

Superconductivity was discovered in 1911 in a sample of mercury metal that was cooled to 4K.
Since it is relatively difficult and expensive to perform experiments at such low temperature (it
requires liquid He), superconducti
vity has been primarily a curiosity or used only in critical
applications until the recent discovery of the so
-
called high
-
temperature superconductors (in
1986), which are ceramic oxides. These superconductors operate at or just above the
temperature of l
iquid nitrogen, 77K. Liquid nitrogen is relatively inexpensive and easy to handle,
so applications for superconductors have dramatically increased in recent years. The two main
effects are zero resistance and the Meissner effect, described above. The sup
erconductive state
occurs at low temperature and the sample’s resistance abruptly drops to zero below the critical
temperature.

Superconductors are perfect conductors of electricity which means they offer zero resistance.
How do they have zero resistanc
e? In 1957, J. Bardeen, L. Cooper, and J. Schrieffer published a
theory known as the BCS theory based on quantum mechanics. Basically what their theory said
was that there are two type of current carriers. The current carrier that carries current without
r
esistance is called a Cooper pair. A Cooper pair is made of two electrons of opposite spin bound
by long range interaction. The Cooper pairs act as a group with a single momentum. This single
momentum is what explains the zero resistance. However, the BCS
theory only explains the low
temperature superconductors which are metals and alloys. Currently, there is no accepted theory
that completely explains the zero resistance of high temperature superconductors.

i
I
H
I
L
H
L
i
I
H
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7.0 Superconductivity Experiment

7.1 Materials



s
uperconducting pellet



tweezers



safety glasses and safety gloves



dewar of liquid nitrogen



superconductor disk holder ( upside
-
down cup)



hot air dryer ( common hair dryer)

7.2 Safety

Liquid nitrogen (LN
2
)exists at 77K, or about

200
o
C. Even though it is col
d, its effect on your skin
is similar to a burn. Do not ever place your fingers in a container of liquid nitrogen. The “smoke”
you see above the LN
2

is actually water vapor, and it will not harm you. However, in a LN
2

container it is difficult to see th
e liquid surface through the water vapor so never place your hand
or fingers inside the container.

When LN
2
is poured, small droplets form and scatter over the table and floor. You do not want
any droplets in your eyes; if a droplet touches your skin you’
ll feel a slight burn but, it probably will
not harm you. However, it is best to wear gloves when you pour LN
2

to prevent any contact.
Wear safety glasses whenever you pour LN
2

from one container to another.

Never leave an open container of LN
2

unattende
d, because someone else may come upon it and
mistakenly touch it. If you have a
small amount
of excess LN
2

(say, some extra in your petri
dish), either wait for it to evaporate, or carefully pour it on the floor away from other people or any
equipment and

it will evaporate quickly.

7.3 Observing the Meissner Effect

1.

Gently place superconductor disk on top of disk holder.

2.

Place rare earth magnet in a separate place. BE CAREFUL, THE RARE EARTH MAGNET IS
VERY SMALL AND CAN BE EASILY MISPLACED.

3.

Fill disk holder

with a liquid nitrogen so that disk is completely submerged.

4.

Wait for disk to cool to 77K (1
-
4 Minutes); periodically keep adding liquid nitrogen.

5.

Place rare earth magnet on top of disk. If the magnet does not float you may have to add
more liquid nitroge
n.

6.

The magnet will fall on top of the disk once the temperature becomes higher than the
superconductor’s critical temperature. Use the hot air dryer to dry up all the moisture that has
accumulated on the superconductor. If you don’t dry up all of the mois
ture, the
superconductor will get damaged.

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7.4 Zero Resistance Measurement

See your lab instructor for experimental procedure.

8.0 Barkhausen Effect

(contributed by T. Westmore, M.S. MatE 1997)

8.1 Background on the Barkhausen Effect

The Barkhausen effec
t is indirect evidence for the existence of magnetic domains within
ferromagnetic materials. When domains grow, under an applied magnetic field, the movement of
the domain walls occurs by discontinuous and abrupt Barkhausen jumps. A domain is a region
with
in a crystal where the magnetic fields, of electron spins, align in the same direction. See
Callister Figure 21.11. The “wall” separates regions with different magnetization directions. See
Callister Figure 21.12. The number of domains within a ferromag
netic material is large, each with
a random orientation. The jumps in magnetization of a ferromagnetic material can induce a
voltage in a winding that in turn can produce Barkhausen noise through a speaker.

8.2 Experimental Procedure

It is possible to demo
nstrate the Barkhausen effect with very simple apparatus. The parts include
a mini
-
amplifier with speaker (Radio Shack part number 277
-
1008C), 300 to 400

turns of 30

AWG
magnet wire, a 9

V battery, a permanent magnet, and a drinking straw. The magnet wire
is wound
around the straw, across a length of approximately 5

cm, and connects to the amplifier and
speaker. An illustration of the set
-
up appears in Figure 5. A readily available specimen to test for
Barkhausen noise is a straightened steel paper clip.

Th
e magnetic material is placed inside the straw and within the turns. Starting from a relatively
large distance away, the permanent magnet is brought closer to the straw. With the field parallel
to the long axis of the specimen, this increases the applied f
ield on the specimen. The action of
moving the magnet closer will result in noise in the speaker. When the permanent magnet is
close to the specimen, the magnetic material approaches saturation and the noise from the
speaker stops. When the permanent magne
t is withdrawn, the noise in the speaker occurs again
but is not as loud. This is the hysteresis effect and a discussion appears in the following section.
To continue, it is necessary to turn the permanent magnet and reverse the polarity. Bringing the
per
manent magnet closer again is equivalent to magnetizing the specimen along the lower half of
an M
-
H loop.






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Figure 5.

Experimental set up for the Barkhausen effect.

One can experiment with other magnetic materials of different coercivity. Examples ma
y include
wires of nickel, cobalt, or nickel
-
iron alloys (permalloy, orthonol). Low coercivity materials will
produce softer speaker noise because they are easy to magnetize. Vicalloy wires that receive
special processing will have only a single Barkhause
n jump. These Wiegand wires have their
domain walls pinned such that forcing the walls to move requires more energy.

8.3 Barkhausen Noise

A schematic illustration of domain wall motion appears in Figure 6 (and Callister Figure 21.13).
Domains, with favora
ble magnetic orientation, will grow when a ferromagnetic material is in an
increasing magnetizing field. The growth of favorable domains is at the expense of others that are
not in close alignment with the applied field. Growth takes place by rotation of s
pins, in adjacent
regions, towards the direction of the growing domain.

The M
-
H loop in Figure 6 illustrates the change in magnetization (M) of the material under an
applied field (H). The magnetization initially follows the circuit ABC. Eventually, at h
igh applied
fields, the material contains only a single domain. An additional increase in the applied field will
cause rotation of the magnetization direction into complete alignment with the field (from C to D
on the M
-
H loop). This is the condition of ma
gnetic saturation. Subsequent decrease in the
applied field results in the magnetization moving along DCEFG of the M
-
H loop. This is the
hysteresis effect of magnetization.





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Figure 6.

Schematic of domain wall motion and magnetic hysteresis.

As domains

grow, the domain walls move. This motion is intermittent and occurs in abrupt jumps
(the Barkhausen effect). With a sense coil around a sample providing voltage to a speaker
(Figure 5), it is possible to hear the motion of magnetic domains. Increasing a
n applied field on
the ferromagnetic material will produce popping, crackling, and hissing noises in the speaker. The
discontinuous domain wall motion produces a changing flux that, in turn, induces a voltage in the
sense coil. The voltage is proportional

to the changing flux according to Faraday’s law (Equation
10). The parameter
V

is the induced voltage,
N

is the number of turns in the sense coil, and
d
dt

/

is the changing flux. Amplifying the voltage and feeding it into the speaker wi
ll cause the
crackling noises.

V
N
d
dt




(10)

Williams and Shockley [1] demonstrate the direct relation between changing magnetization and
domain wall motion. Direct observation of domain wall movement, in single crystals of iron +
3.8

% si
licon, shows that the wall position is linearly proportional to the magnetization. Crystal
imperfections temporarily held up the movement of the domain walls causing the abrupt
movements.

The crystal imperfections that can interrupt the movement of domain

walls are inclusions and
residual stresses [2]. Inclusions, as impurities, voids, or other phases, are regions in a material
with different magnetization. Large inclusions impede wall motion because they create domains
to lower their magnetostatic energ
y. When these inclusions are in the path of moving domains,


Tech 025

San Jose State University

Lab Notes

Magnetic Properties


LN 5
-
14

the inclusion domains attach to the moving wall and increase the walls magnetostatic energy.
Small inclusions delay the movement of walls by lowering the overall surface energy.

Residual stress
in a material may vary over short distances; less than or equal to the grain size.
The causes are crystal imperfections such as dislocations. When a domain wall moves through a
region of changing residual stress there is an increase in the wall and magnet
ostriction energy
[2]. When either residual stresses or inclusions hamper the movement of domain walls they do
so by creating a restoring force. This is a result of the increase in energy of the domain and wall
system. When there is sufficient energy to

overcome the restoring force, the domain wall can
take irreversible jumps to equivalent energy levels ahead.

8.4 Applications of the Barkhausen Effect

Since dislocations are a cause of abrupt and discontinuous jumps, it is possible to use the
Barkhausen e
ffect to examine stresses in a material [3,4,5]. Specific applications include
evluating near surface stress in crankshafts, fatigue testing of steels, and examining stress
dependent piezoelectric properties of metallic galsses. Point defects, dislocations
, grain
boundaries, and impurities all play a part in domain wall movement. With appropriate reference
samples or standards, Barkhausen noise can provide a measure of the material’s structure. In
turn, the structure can determine properties of a material,
thus providing an non
-
invasive and
non
-
destructive method of microstructure analysis [4].

8.5 References

1.

Williams, H.J. and Shockley, W., Phys.Rev.
75

(1949) 178.

2.

Cullity, B.D.,
Introduction to Magnetic Materials
, Addison
-
Wesley Publishing Company,
M
assachusetts


1972.

3.

D.A. Kaminski and D.C. Jiles, J. Appl. Phys.
79
, 8 (1996) 4749.

4.

L.B. Sipahi, J. Appl. Phys.
75
, 10 (1994) 6978.

5.

L. Malkinski, et al., J. Mag. and Mag. Matls.
112

(1992) 323.


Tech 025

San Jose State University

Lab Notes

Magnetic Properties


LN 5
-
15

Bluesheet 1: Meissner Effect


Date


Section




Gro
up Leader









Safety Expert









Materials Manager









Other Group Members








A superconductor repels any applied magnetic field. Microscopic dipoles are induced in the
material that oppose the external field. This repels the source of

the external field, in this case
the magnet. For the magnet to be levitated the force must be strong enough to overcome the
force of gravity. The small rare earth magnets we use have very strong M
sat,

and hence produce
a very strong field. This levitat
ion of the magnet is called the Meissner Effect.

The Styrofoam platform at each station can be used to hold the LN
2
.

SAFETY PRECAUTIONS when handling Liquid Nitrogen (LN):



Wear safety glasses or a face shield when near the LN.



Wear safety glasses and glo
ves while pouring the LN into the dish.



Do not pour more than you need and do not pour the LN onto the floor or other
surface.



Don’t let the LN
2

touch you. The extreme cold (78K) can cause permanent damage
(cold burns).



Don’t touch the holder or the sampl
es or the magnets while they are below room
temperature.



Use the tweezers to handle the magnets.

Things to try:

Stack up superconductor disks to increase the effect. The magnet will be levitated higher.



Notice the flux pinning: this is the effect that ca
uses the magnet to stay in one place. The
magnetic lines of flux developed in the superconductor are pinned in place by defects in the
material, which then pins the magnet.



If you spin the magnet it will keep rotating for a very long time. This is an e
xample of the
frictionless magnetic bearing. You can use a drinking straw to blow a stream of air at the
magnet and increase its spin speed.


Tech 025

San Jose State University

Lab Notes

Magnetic Properties


LN 5
-
16

Bluesheet: 2: B
-
H Looper


Date


Section




Group Leader









Safety Expert









Materials Manager









Calcularions Expert









Other Group Members








Objective:

Calculate the remanent magnetization,
M
r
, the saturation magnetization,
M
S
, and the coercive
field
H
C

for various samples.


1. Determine n, the number of turns per unti length, on th
e drive coil (the larger coil). Determine
N, the total number of turns on the sense coil (where the sample goes).


2. Generate the hysteresis loop for annealed permalloy ( sample is marked).


3. Locate remanent magnetization, saturation magnetization, a
nd coercive field on hysteresis
loop and record the corresponding voltages on your data table.


4. Repeat Steps 2 through 6 for all the other samples and fill in the data table. Make your
calculations after taking all the data so other groups can use the
instrument.



5. Calculate
M
R

,
M
S
, magnetization using

s
e
c
t
i
o
n

c
r
o
s
s

s
a
m
p
l
e
2
A
G
N
V
M
o
r
r


, where
V
r

is the remanent

magnetization voltage,
N

is the total number of turns of sense coil,

G

= 15,079 Hz, and µ
0
= 4

x 10
-
7

Henry/meter.

Note sample dimensions: Thickness
= .002 in, width = 0.2 in


6. Prove that this formula gives M in A/m.

(Note that Henry/meter can also be expressed as Wb/Am, and 1 Wb = 1 V s).


7. Convert the X
-
voltage obtained for coercivity into magnetic field using the following
expression:
H

=
ni

=
nV/R

=
nV

since
R

= 1 Ohm.


8. Repeat Steps 2 through 6 for all the other samples and fill in the data table, after taking your
measurements.

Tech 025

San Jose State University

Lab Notes

Magnetic Properties


LN 5
-
17


Table 1.

Magnetic material type identification.


Color/Number

Material

Hard

Brown/White

Steel guitar strin
g

Soft

Brown

Amorphous metal

Annealed

5

Orthonol (annealed 900
o
C 1hr)

Hard
-
worked

4

Orthonol

Ferromagnetic

3

Supermalloyed (annealed 925
o
C 1hr)

Ferrimagnetic

1

Fe
-
Ni
-
Zn(slow quenched) (amorphous)

Ferrimagnetic

2

Fe
-
Ni
-
Zn (fast quenched) (amorphous
)


Table 2.

Magnetic properties data

Material

V
r

M
r

(Am
-
1
)

V
S

M
S

(Am
-
1
)

V
C

H
C
(Am
-
1
)

Annealed
permalloy







Orthonol
(annealed 900
o
C 1hr)







Orthonol







Supermalloy
(annealed 925
o
C 1hr)







Fe
-
Ni
-
Zn (slow
quenched)

(amorphous)







Fe
-
Ni
-
Zn (fast
quenched)

(Amorphous)







Questions

1.

How do annealed and un
-
annealed samples differ in their properties?

2.

Which of the samples is the hardest magnetically? Which is the softest?

3.

Which has the highest saturation magnetization? Which has t
he lowest?

Some values from literature are given below. Our results do not seem to match these all that
well; we may have a calibration problem. How well do your results match?

Permalloy:

H
c
: 4 A/m

M
s
: 10.8 MA/m

Iron:

H
c
: 80 A/m

M
s
: 21.5 MA/m

Superma
lloy:

H
c
: 0.2 A/m

M
s
: 8 MA/m