PROPERTIES_ OF IGNEOUS ROCKS
THESIS Submitted by
M.F.M.FAHIM, B.Sc., D.I.C.
for the
DEGREE OF
DOCTOR OF PHILOSOPHY in the
Faculty of Science UNIVERSITY OF LONDON
Department of
Geophysics, Imperial College ofScience
and Technology, London.
JULY, 1954.
ABSTRACT
Three pieces of equipment, the spinning magnetometet, used for the determination of the direction and magnitude of the remanent magnetization of rocks, the equipment used for investigating the thermal variation of the magnetic
susceptibility of rocks and ferromagnetic minerals, and the thermomagnetic separator used for the separation, at different temperatures, of the ferromagnetic constituents contained in igneous rocks, are described.
Two main experiments have been carried out on rocks, from the Tertiary lava flows of Mull, and which possess
inverse natural remanent magnetization. In one, the variation of thermoremanent magnetization with field has been investi- gated to ensure that the rocks are not of the "Nagata" type.
The results suggest that self reversed thermoremanent magnet- ization is not the cause of the reversed natural magnetization of the Mull rocks.
In the second experiment, an attempt was made to examine the temperature variation of the susceptibility, in weak
fields, of the ferromagnetic constituents separated from the rocks on the basis of their Curie temperatures. The results demonstrate the complex nature of the magnetic properties of the rock, but it appears that the heat treatment modifies the properties of the constituents, that no general conclusion.
Nevertheless, no evidence, incompatible with the idea of a reversal of the earth's field, was found.
(1) CONTENTS
Eage
INTRODUCTION • 1
PART I. - EXPERIMENTAL TECHNIQUE
(a) The Spinning Majnetometer !ft 4
General description 004 4
Modifications of the original apparatus 6
The Detecting Coils 7
Balancing the Coils 8
The Step-up Transformer 9
The Specimen Holder 10
The Low Range Selector 12
The Upper Range Selector 000 12
Experimental Procedure 0.4 14
The Absolute Calibration of the Magnetometer 16 (b) The Susceptibility-Temperature Apparatus 20 The Helmholtz Coils and Detecting Coils 21
The Mobile Furnace ••• 23
The Energizing Current . 24
Experimental Procedure ••• 25
(c) The Thermomagnetic Separator ... 29
The Separator ... ... 29
The Furnace ... . • . 30
The Electromagnet .. 040 31
Experimental Procedure ... 32
Page PART II - THE REMANENT MAGNETIZATION OF
IGNEOUS ROCKS AND ITS ORIGIN
The Natural Residual Magnetism of Igneous Rocks 35 Thermoremanent Magnetization of Igneous Rocks 37 Stability of the Remanent Magnetization of
Igneous Rocks 000 40
Some Aspects of the Thermoremanence of
Igneous Rocks ... 00* 42 Partial Thermoremanent Magnetization 43
The Domain Theory 00* 44
Neel's Theory of Thermoremanence 000 45 (d) The Magnetic Properties of the Tertiary Igneous
Rocks of Northern England 46 PART III - THE THERMOREMANENT MAGNETIZATION
OF IGNEOUS ROCKS IN WEAK MAGNETIC FIELDS
Experimental Procedure 000 000 52 Results
**• •••
57
PART IV - MAGNETIC CHARACTERISTICS OF THE FERROMAGNETIC CONSTITUENTS OF IGNEOUS ROCKS
Rock powdering 0O0 OS. 65
Extraction of the Magnetic Minerals .00 66 The Thermomagnetic Separation ... 69 The Magnetic Susceptibility of the• Constituents 70 The Thermoremanent Magnetization 060 73 The Thermal Variation of Susceptibility of the
constituents SOO 000 77
Contents (continued)
PART V - THE ORIGIN OF THE INVERSE
REMANENT MAGNETIZATION OF ROCKS
NeelsT Theorie8 see 0 • • 86 Discussion of Neels? Mechanisms
• • •
90 Other Possible Explanations
• • •
93
Summary and Conclusions
95 Acknowledgments it • I. •
lb •
98
APPENDICES
Appendix I. The Designs and Calculations of the Optimum Dimensionb of the
Detecting Coils
99 Appendix II. The Isothermal Magnetization of
Igneous Rocks Produced 'by Different
Fields ... ..0 103
Bibliography 107
++++++++
The natural residual magnetization of igneous rocks has drawn the attention of many investigators in different parts of the world during the last few decades. Igneous rooks of
(1) (2)
historic lava flows erupted from Etna and in Japan have shown that they are permanently magnetized in the direction of the present earthls field and the study of older igneous rocks has yielded similar results. These facts suggest that, under certain conditions, the direction of the permanent
residual magnetization of igneous rocks can give information about the geomagnetic field in the past.
However, it has been reported from different parts of (3)-(5)
the world that some igneous rocks possess peculiar magnetiza- tion, They were found to be permanently magnetized in a
direction which is roughly opposite to that of the present geomagnetic field. The classical example of the "inversely"
magnetized rock is found in the tholeiite dykes of North of (6)
England. Field work as well as laboratory tests on oriented rock samples from these dykes have confirmed the reversal of their natural magnetization. Since the tholeiite dykes is the last phase of a series of volcanic activities which took place during the Tertiary times and was centered on the Isle of Mull, a study of the residual permanent magnetization of
(7) the early extrusions of Mull lava flows has been done.
The present thesis is a part of a detailed systematic
2.
study of various magnetic properties of the igneous rocks of Mull and which has been planned and initiated at the Imperial College, London, to obtain more information about the origin and the way in which igneous rocks acquired their natural residual magnetization in relation to the geomagnetic field in the past.
The thesis is divided into five parts. Part I contains a description of the experimental technique used for the present investigation. In Part II, the general aspects of the remanent magnetization of igneous rocks and its origin are discussed, together with a short review of the various magnetic properties of the Tertiary igneous rocks of Northern England. Part III deals with the experimental observations of the thermoremanent magnetization acquired by igneous rocks in different weak fields, part IV deals with the experimental observations of the variation of the magnetic susceptibility with temperature of the magnetic materials which were magnetically separated at different temperatures, while part V is devoted to the discussion of the experimental data in relation to the theories of the origin of the residual magnetization of igneous rocks.
PART I
In this section a description of the equipment used for the present investigation is given. It is composed of three different items under the following headings:
A. The Spinning Magnetometer: used for measuring the magnitude and direction of the remanent magnetization of rocks.
B. The Susceptibility-Temperature Apparatus: for the, measurements of the variation of magnetic susceptibility of rocks and magnetic materials with temperature.
C. The Thermomagnetic Separator employed for the separation of magnetic materials of different Curie temperatures.
4
.A. THE SPINNING MAGNETOMETER General Description
This is a slightly modified form of an apparatus made and used by Vincenz for the measurements of the intensity of magnetization of rocks. A schematic diagram of the equipment is shown in fig. (1). The rock, in the form of a 2 cm. cube
,
is rotated between two symmetrically placed pick-up coil systems A and B in which it induces an alternating e.m.f.
The coil systems are in series and in series with the primary of a step-up transformer the output of which is balanced and measured on a potentiometer.
Each pick-up coil system consists of two coils wound on separate formers and placed coaxially a short distance apart.
The coils of each system have the same effective areas and are connected in series opposition so, the net e.m.f. induced in them by an external variation of a uniform magnetic field, such as that of the earth, is zero. If the system is set with forced angular vibrations by the spinning mechanism, the induced e.m.f. is also compensated. The rotating specimen, however, induces a bigger e.m,f. on the nearer coil, the main coil, than in the further one, or compensating coil, giving rise to a differential e.m.f. The two sets of pick-up coils are connected in such a way that the differential e,m.f's in them add.
The rock specimen is contained in a plastic holder which
.s
ro
VIBRATION G ALV O.
POTENTIOM TER
M: MAGNET
Si: UPPER RANGE SELECTO Sz : LOWER RANGE SELECTO R A
0 SPECIMEN DETECTING COILS
REFERENCE VOLTAGE GENERATOR
FIG. I
5.
is rotated by a long horizontal shaft joined to a distant synchronous motor. The speed of rotation of the shaft is kept fixed at 30 cycles per second.
On the same shaft and fixed in it, an Alnico steel magnet is rotated between two symmetrically placed coils connected in series. This, produces the reference voltage feeding the potentiometer on which the signal voltage, due to the rotating specimen, is balanced and measured. The reference voltage generator (R.V.G.) coils are mounted on a plastic drum which can be rotated coaxially with the shaft and its position is read on a circular scale attached to ane end of the drum.
This is to bring the two voltages, namely, the R.V.G. and signal voltage into phase with each other. The angular position of the R.V.G. coils is related to the direction of magnetization, while the balancing reading on the potentiometer gives a measure of the intensity of magnetization of the
specimen in the plane perpendicular to the axis of rotation.
By spinning the rock cube about three perpendicular axes, the total intensity and direction of magnetization can be
calculated, following the same method adopted in the original apparatus.
To detect a balance on the potentiometer, a high gain, low noise selective amplifier is employed. It is tuned to the frequency of rotation, its output being connected to a Campbell vibration galvanometer which is also sharply tuned to
the frequency of rotation. This system eliminates any interference from extraneous fields of different frequency and reduces the noise to that of the narrow band of frequency employed. The details of the various parts of the amplifier have been described in detail by Vincenz. (7)
Modifications of the original apparatus
The original Bruckshaw-Vincenz apparatus was mainly made and used satisfactorily for the measurements of the natural residual magnetization of igneous rocks which have, in general, fairly high intensities of magnetization. In its original form, the main and compensating coils each had the same dimensions and number of turns (about 30,000 turns of a 44 s.w.g. copper wire).
With such a design, the resistance of the detecting coils was high and with it the coil noise arising from its resistance.
This was the main source of noise in the equipment and limited the smallest magnetic moment which could be detected. With the high values of the intensity of remanent magnetization found in igneous rocks, however, the coil noise was so small that it had no effect on the measurements.
To enable the equipment to measure the smaller intensities of weakly magnetized igneous rocks, such as some granites and sedimentary rocks, it was necessary to reduce the coil noise by reducing its resistance. The coils were wound with wire of greater diameter and the number of turns decreased. Both
7.
the signal from a given rotating magnet and the noise was reduced, the latter by a greater factor. The magnitude of these two e.m.ffs was then augmented by a step-up transformer of high turns ratio and it was the output of this transformer which was balanced on the potentiometer. The overall gain, although not as high as anticipated, was satisfactory.
The detecting coils
The number of turns on the main coil was 2,000 compared with 30,000 of the old system. A corresponding decrease in the number of turns of the compensating coil was made, but this was further reduced by winding it on a former of diameter bigger than that of the main coil. To obtain the maximum
possible flux linkage with the detecting coils, optimum radial dimensions were calculated for the least possible distance of the specimen from the coils and for a given width of the winding space. These calculations were carried out assuming the specimen to behave as a magnetic dipole.
A small gain was obtained here since although the turns were reduced by a factor of 15, the flux linkage was only reduced by a factor of 12.
The details of the relevant calculations are given in Appendix 1. A 30 s.w.g. enamelled copper wire was used for both the main and compensating coils. The following table shows the actual details of the coils.
the signal from a given rotating magnet and the noise was reduced, the latter by a greater factor. The magnitude of
these two e.m.ffs was then augmented by a step-up transformer of high turns ratio and it was the output of this transformer which was balanced on the potentiometer. The overall gain, although not as high as anticipated, was satisfactory.
The detecting coils
The number of turns on the main coil was 2,000 compared with 30,000 of the old system. A corresponding decrease in the number of turns of the compensating coil was made, but this was further reduced by winding it on a former of diameter bigger than that of the main coil. To obtain the maximum
possible flux linkage with the detecting coils, optimum radial dimensions were calculated for the least possible distance of the specimen from the coils and for a given width of the winding space. These calculations were carried out assuming the specimen to behave as a magnetic dipole.
A small gain was obtained here since although the turns were reduced by a factor of 15, the flux linkage was only reduced by a factor of 12.
The details of the relevant calculations are given in Appendix 1. A 30 s.w.g. enamelled copper wire was used for both the main and compensating coils. The following table shows the actual details of the coils.
8 .
Table (1)
1
Main coil Compensating Coil 'No. of turns for system A
Inner radius Outer radius Radial width Axial width Resistance
1984 turns 805 turns
1980 800
2.6 ems. 6.2 ems 5.1 n 6.8 n 2.5 I, 0.6 n
1.0
110 ohms 70 ohms
it
1.5 rt
The total resistance of the whole assembly of the detecting coils was 360 ohms.
Balancing the coils
The matching of the effective areas of the main and
compensating coils of each system was carried out by balancing them in a uniform alternating field, the balancing being
effected by taking off turns. The two coils were connected in series opposition and placed in the homogeneous alternating
systerr,
field of a large Helmholtz coil/. They were connected to a low noise amplifier and balance was detected by a Campbell Vibra- tion galvanometer. This was tuned to the frequency used, the mains frequency, and connected to the output stage of the amplifier. By this process, the two coils could be balanced to the nearest turn.
The two coil systems were mounted on a "Tufnol" base with
26 5.
SPECIMEN COMPENSATING COIL
_T1
N
N
rA"
V
68 6.2
9
MAIN COIL
•I 44-103 _ _
18.4 16.9 12.I
ALL FIGURES ARE IN CMS.
FIG. 2.
N
F I G. 3
the common axis of the coils making an angle of about 63° with the horizontal. This made the axis of the coils parallel to the direction of the total intensity of the earthfs magnetic field in the laboratory. Thus, any differential e.m.f.,
arising from any forced angular vibrations of the coils in the earthTs field, in one turn mismatch in the effective areas of the main and compensating coils would be reduced to a minimum.
In any case, the frequency of this induced e.m.f. due to these angular vibrations would be twice the frequency of rotation and would be undetected.
The e.m.f. developed in the coils by the rotating sample is given by :
e = 2
Tr f#
10-8 sin (21rft14 ) Volts
(1) where is the maximum flux threading the coils and arising from the sample, f is the frequency of rotation of the sample (30 cycles per second) and S depends on the direction ofmagnetization.
The assembly of one system of the detecting coils is shown diagrammatically in fig. (2) and a photograph of the whole system of the pick-up coils with the specimen holder in position is shown in fig. (3)
The step-up transformer
The reduction in the number of turns of the detecting coils eventually reduced the e.m.f. produced by the rotating specimen and so, it reduced the sensitivity of the apparatus.
10.
This was compensated and increased by the construction of a low frequency transformer with a step-up ratio 1:350.
The output of the detecting coils was connected to the primary coil of the transformer, the secondary of which was connected to the potentiometer and the detecting amplifier gee fig.(1):7
The primary consisted of 96 turns of 30 s.w.g, enamelled copper wire, as one layer, wound on a core made of thin mu- metal sheets. Its impedance matched that of the detecting coils with an inductance of 1.91 henry.
The secondary had 35,000 turns of 44 s.w.g., double silk covered copper wire with a total resistance of 24,000 ohms.
The core of the transformer was made of mu-metal plates .005" thick. They were coated with shellac and thin sheets of mica were embedded between the plates to ensure better insulation.
In addition, the whole transformer was shielded by a double walled iron box and placed as far as possible from the rotating part of the apparatus. This was done to cut off any
effect from the reference voltage generator magnet we the of rotating specimen on the transformer. All the wire leads to and from the rest of the apparatus were shielded and the shield was earthed.
The Specimen holder (and its electrostatic shielding)
The holder is the same one made and used in the initial
apparatus. It was made of one piece of Tufnol with long projecting axial rods fitting into the recessed ends of the brass shafting. This was made to ensure that any magnetic properties of, or eddy currents arising from, the rotating brass shaft had a negligible effect on the detecting coils.
In the absence of the specimen, when the holder was spun empty, a small variable background was detected. This might be due to the accumulation of static charges, perhaps from dust particles, on the insulating surface of the holder. To reduce this effect, the holder at first was coated with very thin gold foils and the coating was earthed. This was not satisfactory as it did not reduce the "zero" reading appre- ciably and moreover, the coating came off with the rotation of the holder. Another attempt was tried unsuccessfully by
polishing the holder with a detergent (Stergene) as it was anticipated that the hydrophilic detergent would retain (19) sufficient moisture to form a continuous conducting layer.
Lastly, a colloidal solution of graphite "Aquadag" was used for coating and found to be more satisfactory. But in spite of this, the electrostatic shielding was not perfect, giving rise to a very small zero reading which was neglected when measuring the magnetization of igneous rocks having a high intensity of magnetization. On the other hand, when very
weakly magnetized rocks, such as sediments, are measured, this zero reading must be taken into account.
/0 •
•
TRANSFORMER It MI
crY667666-65-6150raMirr
TO POTENTIOMETER
(a) LOW RANGE SELECTOR
a b c
rot 20 50
111,,,,1„,\„.
TO POTENTIOMETER
/01 20 50 loo 6 c
100
1/14 41 4114 d I
(i) UPPER RANGE SELECTOR
FIG.4
12.
The low range selector
See
fig.(4)a7Each of the two coils of the reference voltage generator is composed of 100 turns of 28 s.w.g. enamelled copper wire and of resistance 3.49 ohms. In order to reduce the potential drop across the potentiometer to some fractions of its normal
value, four different tappings were made on the two coils, namely, at 10, 20, 50 and 100 turns of each coil. The leads to the potentiometer from the R.V.G. coils could be plugged into the desired tapping to reduce the potential to the
desired range. In this way weakly magnetized rock specimens could be measured with greater accuracy. A photograph of the R.V.G. and the low range selector is shown in fig.(5), where the ranges a, b, c and d correspond to the tappings at 10, 20, 50 and 100 turns of each coil respectively0
The upper range selector (gee fig.(4)bJ
On the other hand, strongly magnetized samples could be measured by means of the upper range selector. It consists of a number of non-inductive stable resistances connected across the secondary of the transformer. Fractions of the total potential drop.developed across it,could be tapped off for measurements. The total resistance of this selector is 800,000 ohms and tappings are made at 80,000; 200,000 and
800,000 ohms giving ranges of 40, and 1 of the total potential drop respectively.
Each range in these two selectors has its conversion
factor, so any reading, in any range, on the potentiometer could be transferred into the corresponding value of intensity in c.g.s. units. These factors were determined by an absolute calibration of the apparatus to be described later.
Determination of the sense and zero of the angular scale Before using or calibrating the instrument, it was
necessary to determine the sense and zero of the angular scale attached to the drum carrying the R.V.G. coils.
(i) Sense of the scale
This was done by using a rock cube which has been
deliberately magnetized approximately parallel to one of its edges. It was inserted in the holder with its magnetic vector pointing to a reference mark on the holder and a balance was obtained by adjusting the potentiometer and the angular position of the R.V.G. coils. The circular scale reading was recorded. Then the specimen was turned in the holder through 90° in the positive direction (direction of rotation of the holder) and balance obtained again. In this case, the angular reading has been readjusted without adjusting the potentiometer. The angular reading actually increased by 90° in the second position. So, increasing positive directions of magnetization corresponded to increases in the reading.of the circular scale.
(ii) Determination of the zero of the circular scale
This was done with the same artificial specimen used in
(i) or it could be done with any other cubic rock. Balance was obtained with the specimen in the holder in position (a) of fig. (6) and the circular scale reading recorded. Then the specimen was set in the holder in position (b) - by turning it through 1800 about an axis perpendicular to the axis of rotation and the main N face of the cube - and a new balance was obtained by readjusting the angular position of the R.V.G. coils. In one case the direction of the magnetic vector, in the plane perpendicular to the axis of rotation was + 4) and in the other .16 with respect to the true zero of the scale. Hence the mean of the two readings on the circular scale gave the true zero direction. The position of the scale on the rotating drum was then adjusted so as to have the true zero position correspond to the zero reading of the scale.
This procedure, as indicated above, can be carried out on any specimenino matter how it is magnetized.
Experimental 3rocedure
In measuring the magnitude and direction of magnetization of a rock, the system of reference axes used before in the
(7)
original apparatus was adopted. This was a right handed
system of rectangular axes fixed to the specimen and parallel to the edges of the cube. In other words, if the x axis was in the direction of the North, the y axis pointed to the East and the z axis, looking from above the x-y plane, was downward.
FIG,
7
SIGN CONVENTION
THE SPECIMEN IS S PUN CLOCKWISE
RIGHT LEFT
ROTATION ABOUT Z AXIS.
1/ X
(2) (3)
ft
REFERENCE MARK
15.
The rock cube was spun about each axis and a balance was attained in each case by adjusting the potentiometer and the R.V.G. coil, as indicated by the vibration galvanometer,
If the potentiometer readings corresponding -to rotations about the z„ x, y axes are rz , rx , ry and the corres- ponding angular scale readings are 0 z x 0 then the components X , Y , Z of the total magnetic vector R , are given by the following Csee fig.(7)2
X 7. rz cos z)
)position (l),for rotation about Y a rz sin ef z) z axis
( 2 ) Y r. r cos x
e )
x)position Z - r
x sin
e x
))
Z a r
7 cos 9 Y) ) position X = ry sin
e,)
(2), for rotation about x axis
(3), for rotation about y axis
Thus, each component was measured twice and provided a check on errors. Taking the mean values of X, Y and Z, the total magnetic moment of the specimen was then expressed by the magnitude R of the resultant vector, its "azimuth" 4, and
"dip" I where :
2 2 2 2 R = (X +Y + Z ) IS ' tan- 1
( X )
in potentiometer units
-1 I tan
( x2+ )7
16.
The magnitude of the total intensity of magnetization J expressed in c.g.s. units was deduced using the follow—
ing expression:
= A.R .10-6 c.g.s. units (6) V
where V is the volume of the specimen and A the calibration factor for the range used and which vas determined by an absolute calibration of the equipment.
(see below). As for the sign of 0 and I, if x is taken to be along the North direction, the positive value of 0 implies an easterly direction and a positive I a downward dip.
The absolute calibration of the magnetometer
A plastic cube, of 2 ems. edge, was used as a former round which a small coil of thin wire was wound. This coil was placed in the rotating holder in place of the rock
specimen. When a direct current was passed through it, the coil would act as a magnet whose moment could be ca]culated from the dimensions of the coil, number of turns and the current. Fig.(6) shows a diagram of the circuit that was used for the calibration. The coil consisted of 60 turns of
CONDUCTING RING
SHAFT
CARBON BRUSH
</A
RINGS & BRUSH
CALIBRATING CO IL
FIG,
8
DETECTING COILS
17.
44 s.w.g., double silk covered copper wire wound on the plastic former as one layer having a resistance of 14.8 ohms.
If A is the cross section area of the coil, n number of turns /cm. of the coil and 1 the length of the coil, then for a current i (e.m.u.) passing through the coil, the effective magnetic moment will be = i A n 1
And the intensity of magnetization J = magnetic moment/unit volume
then J = iAAn11 = i n
or J = n I c.g.s. units (7 ) 10
where I is measured in amps.
Rings and brushes
To feed the calibrating coil with a continuous supply of direct current while rotating with the specimen holder, two pairs of circular rings were placed around the brass shaft on both sides of the holder. Each is composed of two concentric rings firmly attached to each other, The inner ring was of an insulating material pinned to the main shaft by a counter sunk screw. The terminals of the calibrating coil were attached to the outer brass rings which were in contact with stationary carbon brushes. They provided a continuous contact to the coil during rotation. The current, measured by a microammeter, was drawn from a 6 volts battery through a set of high
resistances.
18,
For the calibration of one range of the magnetometer, a small current was passed through the calibrating coil and balance was attained. The current was increased gradually in steps and in every case the corresponding balance observed.A graph was drawn between the potentiometer reading and the corresponding moment, giving a linear relation, from which the calibration factor of the magnetometer could be determined.
Separate graphs were drawn for each range and these are shown in figures (9) - (14).
The calibration factors of the magnetometer for the different ranges are summarized in table (2) and correspond to the constant A of formula (6) (p.16)
Table (2)
Upper Range Lower Range !Factor A !Standard deviation i
1
1 a 1 3,96 1 +0.05
1 b 6,76 + 0.14
1 c 14.44 + 0.18
1 d ! 21.92 4-0.29
1
d i 209.70 ± 1.20 1 a 1 35.50 4- 0.05 10
10
Thus, the apparatusi in its present form,can measure intensities of magnetization in the range from 5 x 10.7
to
CVO ,.710
FIG. 10
L.RANGE
b
600 800 400
200 1·2
1·6 2·4
0.8
0400 600
FIG.9
L.RANGE
a
o 200 1'0
UPPER RANGE: I
R
600 Boo 400
o 200
2 8
R
400 600 o 200
.6
\·8 6
FIG. II
L.RANGE C FIG.12
L.RANGE
d
1·2 4
2·4
19.
3 x 10-2 c.g.s. units.
It is worth noting -,flat the effect of rotating the calibration coil in the earth's field on the coil itself is negligible. The amplitude of the voltage generated by rotating the coil in this field is given by
2 7-rf n A H . 10-8 volts
where f is the frequency of rotation, n the number of turns, A the effective area of the coil and H the earth's magnetic field.
Taking f = 30 c/s n 60 turns, A a 2.6 cm.2 and
H m 0.5 gauss the voltage produced fe 16 x 10-5 volts which is negligible in comparison with the e.m.f. of the accumulator used in the calibration.
B. THE SUSCEPTIBILITY-TEMPERATURE APPARATUS This instrument is a form of an inductive method of measuring the magnetic susceptibility adopted to investigate the change of susceptibility with temperature of rock
samples or rock minerals. Its main principle, which has been (6)
used elsewhere, is the fact that a rock, when placed in an alternating magnetic field, becomes an alternating magnet that produces a secondary alternating field proportional to its magnetic susceptibility. The apparatus used here is a modification of a previous one which has been described by
(16) Manley.
The circuit employed is illustrated diagrammatically in fig.(15). Two balanced pick up coils PI and P2, in series opposition, are fixed coaxially inside a pair of big Helmholtz coils which is energized by an alternating current and
produces the magnetizing field. In the absence of the rock specimen, the resultant e.m.f. in the pick up coils is about zero, since they have the same mutual inductance with the
Helmholtz coils. When the rock specimen is introduced near to the pick up coils, it produces a bigger e.m.f. in the nearer coil than in the other and the differential e.m.f. is a direct measure of the magnetic moment of the specimen. This e.m.f.
is balanced and measured on a potentiometer which is fed by a standard e.m.f. produced by a small auxiliary coil (the neutralizer) P3 wound on P2. Since this standard e.m.f. is
H
AMPLIFIER
VIBR. GALV.
H
MILLIAMMETER.
"'MAINS
50 WATT 'AMPLI FIEf\
OSCILLATOR
FIG. 15
==771
I-
I
I
HH.: HELMHOLTZ COILS P2 PICK UP COILS
P3 NEUTRALIZER COIL F : FURNACE
W : WATER JACKET T : THERMO-COUPLE
S SPECIMEN C :CARRIAGE G : GROOVE
B :BRIDGE
Fl G. 1 6
21.
proportional to the exciting field, the balance on the potentiometer gives a measure of the ratio of the induced moment to the field, i.e. a measure of KV where K is the susceptibility and V the volume of the specimen. To detect the balance, a three stage, low noise amplifier is used and whose output stage is coupled to a Campbell Vibration
galvanometer tuned to the frequency of the exciting field.
To study the variation with temperature of the susceptibil- ity, the specimen is placed inside a non-magnetic, non-inductive
electric furnace which is situated inside the Helmholtz coils, in front of the pick up coils. The furnace draws its heating current from a 110 V., D.C. supply through a rheostat and is surrounded by a water jacket. This is necessary to reduce heat radiation to the pick up coils which would be warped by any slight change of their temperature. The temperature of the furnace is measured by means of a non-magnetic thermocouple made of Platinum-Platinum Rhodium and connected to an
"Electroflon temperature recorder.
Details and improvements of the apparatus
(a) The Helmholtz coils and detecting coils
gee
fig.(16)2The Helmholtz coils consist of 50 turns each, of 0.20 cm.
cotton covered copper wire wound with a mean radius of 32 cms.
The wooden formers are accurately spaced by rods with
insulating nuts at their ends. To hold these formers rigid, they rest on a big wooden stand and are fixed from outside to
two big supports made of thick wood. These supports are, in turn, fixed to the floor to attain maximum rigidity of the system without the use of metal. This was done because it was found that any mechanical vibration or very small displace- ment would alter the electric balance of the system.
The pick up coils P1 and P2 are rigidly attached to one of the Helmholtz coil formers by means of a tufnol spindle through their centres with plastic spacers separating them and the whole column is clamped between two big tufnol nuts. The pick up coils were balanced to the nearest turn as regard
their mutual inductances with the Helmholtz coils, in the same way as described above in the spinning magnetometer.
On the coil P2 there is a third coil P3, consisting of 10 turns of wire which, through its mutual inductance with
the Helmholtz coils, provides the e.m.f, feeding the potentiometer.
Although the pick up coils were balanced to the nearest turn, there is, in the absence of any specimen, a small
differential e.m.f. giving rise to a "zero" reading on the potentiometer. It is the difference between the balancing reading, when the specimen is present, and this zero reading,
that is a measure of the magnetic susceptibility of the rock sample.
The pick up coils are connected, through thin parallel wires to a bank of variable condensers situated outside the Helmholtz coils, the condensers being used to bring the two
23.
e.m.f.s in the coils into phase with each other. These condensers are sensitive to "hand capacity" and so are
manipulated by long plastic rods fitted into the plastic heads of the condensers.
In table (3) the actual details of the coils are given.
Table (3)
P1 P2 P3
Internal diameter External diameter Width of winding Number of turns Resistance
3,86 CITIS •
:11.17 ft
1.40 It
22.965 turns 11,500 ohms
3.96 ems) 11.28 If
1.40 1!
22,900 turns 11,500 ohms1
)11.44 ems
10 turns 1 ohm.
(b) The mobile furnace
The main source of trouble in the original apparatus was that the zero reading was not constant but irregularly
changing. It might be due to slight changes of room tempera- ture, mechanical vibrations or to some influence of the
(16)
furnace temperature. As a result, Manley was unable to
trace accurately the susceptibility-temperature curves of rock specimens. To overcome this, it became necessary to observe the zero reading just before or after any reading taken with the specimen at any temperature.
In its present form, the furnace rests on a wooden ft carriage" to which it is firmly secured. The carriage has two parallel wooden "tongues" on its base and these can slide in two parallel grooves cut in a horizontal wooden bridge.
The bridge is firmly fixed to the floor with its length
perpendicular to the axis of the Helmholtz coils and extending outside them.
When the furnace, with the specimen inside, is moved outside the Helmholtz coils a zero reading can be taken, whereas when it is returned in front of the pick up coils it gives rise to a second reading corresponding to the presence of the specimen. In order to have the furnace in exactly the same position in front of the pick up coils, for each reading, there are two wooden stops fixed on the bridge between which the furnace can slide. By moving the furnace until its carriage is against either the inner stop or the outer stop, standard positions relative to the pick up coil system are achieved.
No ferromagnetic material was used in fixing the different parts of the system and, whenever possible, the minimum amount of brass screws were used so as not to have any serious or detectable eddy current effects on the pick up coils. A
photograph of the whole system is shown in fig. (17).
(c) The energizing current
In the original apparatus, the energizing current for the
FIG. 17
Helmholtz coils was drawn from the mains through a 12 V transformer. This was found to be unsatisfactory since any slight change in the frequency of the mains gave a troublesome interference and balancing the apparatus could not be done accurately. Also, extraneous magnetic fields resulted in poor balancing. Accordingly, a frequency of 65 cycles per second produced by an oscillator was used. This was a
standard DAWE oscillator, type 4000, fitted with a time base to adjust the frequency employed. The output of the oscillator was coupled to the input of a 50 watt VORTEXION amplifier to produce the necessary energizing current. The output was Connected in series with a milliammeter and a small slide resistance to the Helmholtz coils. The output impedance of
the amplifier was made to match that of the Helmholtz coils through a matching transformer fitted to the amplifier. The energizing current was kept at a constant value of 0.415 amps.
(R.M.S.) to provide an alternating magnetizing field of the same order as the earthts field (0.5 gauss),since the magnetic
(11) susceptibility is a function of the exciting field. The vibration galvanometer was tuned, consequently, to this
frequency. A circuit diagram of the whole equipment is shown in fig. (15).
Experimental procedure
The amplifier, used to detect the balance, and the
exciting current supply were turned on and allowed to warm up
26.
until the energizing current reached its steady value of
0.415 amps. Starting with a small amplification, the condensers and the potentiometer were altered, in turn, until, with
maximum amplification balance was reached - the furnace being at the "in" position. The specimen was then slowly pushed into the furnace to its optimum position, with respect to the pick up coils,where the specimen produced a maximum deflection on the vibration galvanometer. The two ends of the furnace were then closed by plugs made of plaster of Paris and through one of them the thermocouple was introduced till its end
touched the specimen.
The furnace was then moved to the "out" position and balance attained to give a zero reading on the potentiometer.
Then it was slowly moved to the "in" position and a second balancing reading observed corresponding to the presence of the specimen. This was repeated several times, at room temperature, as a check of the stability of the apparatus.
The furnace current was then switched on and the cooling water circulation started and the former was adjusted to give a constant slow rate of heating as observed on the temperature recorder. At intervals of 10-150C the furnace was moved in and balance obtained and immediately the temperature was read.
This was followed by moving the furnace out to note the corresponding zero reading.
In this way, continuous readings of the magnetic
susceptibility could be observed every 4 minutes at intervals
of about 10-15°C. The heating of the specimen to the Curie point (600°C approx.) could take half an hour, but in practice it was arranged to take a total of 3 hours for heating and cooling, both being approximately at the same rate. This was done for the double purpose of enabling as many readings as possible to be taken, so as to have a well defined shape of the K-T curve, and to enable the rock to attain the temperature of the furnace. If the heating was done quickly, the temper- ature of the rock would be less than that of the furnace, while during cooling it would be more due to the bigger thermal
capacity of the rock over that of the thermocouple.
After making these modifications and before using the apparatus for actual observations, the changes in the zero reading were investigated by two tests.
In one, and at room temperatures balance on the potentio- meter was obtained for the two standard positions of the
furnace without a specimen. The mean difference, of a set of observations, between the two corresponding readings, was very small (less than 4 potentiometer units). This indicated that the furnace and its thermocouple had no appreciable
magnetic material in them.
In the second test, the furnace was heated slowly to 60000 and allowed to cool down to room temperature, without a specimen, and readings recorded during the run. With the furnace in the "out" position, the zero reading changes slowly but irregularly during heating and cooling. Similarly, the
28.
readings with the furnace "in", showed similar changes but the difference between them was small and constant so that, for practical purposes, the furnace could be regarded as non- magnetic.
C. THE THERMOMAGNETIC SEPARATOR The principle of this apparatus is as follows:
Suppose we have two magnetic materials A and B which have two different Curie temperatures, and assume that the Curie point of A is higher than that of B. If a mixture of A and B is heated to a temperature intermediate between the two Curie points, the latter will become non-magnetic while A will still be magnetic. Exposing the mixture to a magnet at this temper- ature will then enable the separation of A from B.
In general, when a mixture of materials, having different Curie points, is heated, the material with the lowest Curie
temperature could be isolated from the rest when the temperature of the mixture reaches this point. Thus, by making a magnetic separation at various temperatures the different constituents could be separated.
The equipment is composed of three parts, namely, the separator, the furnace and the electromagnet.
The separator takes the form of an airtight copper tube; of a rectangular cross section, 60 cms. long, 3.8 cm. wide and 0.8 cm. inner height. A copper partition running down the centre of the tube divided it into two sections except near the middle where a break in the partition about 2,5 cm. long gave a join between the sections. These grooves were made as smooth as possible and at both ends of each they tapered into circular copper tubes projecting from the separator so that
"4-- 16
16.5
28
F
3- 8 csr.
TO VACUUM
SYSTEM FROM RESERVOIR
29 60 Cm. Cm.
MAGNE C NON MAGNETIC
MATERIAL MATERIAL
(a) SEPARATOR
FIG. 18
(b)
ELECTRO MAGNET(SECTIONS SEPARATED)
there was no shoulder at the ends. Joints were all made
using silver solder to stand high temperatures. The inlet tube A is connected through rubber pressure tubing to a spherical glass reservoir containing the powdered magnetic material to be separated. The outlets C and D are connected in a similar way to two receivers, made of glass. The end B is connected to a vacuum system to prevent oxidation of the heated materials inside the separator. A sketch diagram of the separator is shown in fig. (18a).
The furnace
The separator is situated with its middle part inside a non-magnetic electric furnace which draws its heating current from the A.C. mains through a variac transformer. The furnace with the separator is inclined to the horizontal at an
appropriate angle (about 400) to allow the fine powder to slide very slowly, under gravity, from A down the passage. The
reason for the slow feed of the powder is to allow it to
acquire the temperature of the furnace before reaching the gap where the separation occurs. The furnace, of rectangular
shape, consists of 4 flat electric fire bars containing the heating element and mounted together to form the walls of the furnace. They are mounted in a box made of "Sindanyo"
(compressed asbestos sheets) and the space between them is packed with magnesium oxide powder to provide further thermal insulation. At both front and back faces of the furnace
*C6
rectangular openings are cut to permit the introduction of the separator and electromagnet. These openings are sealed by sliding partitions which fit closely the shape of the separator and magnet.
Again, the temperature of the furnace is measured by a platinum - platinum rhodium thermocouple connected to the
"Electroflo" temperature recorder giving a continuous record of the temperature. The thermocouple fits tightly into a hole in the front window of the furnace and all gaps are plugged with asbestos wool to ensure adequate thermal insulation.
The electromagnet
The core of the electromagnet is made in two sections, each U shaped, so that it can be assembled round the furnace.
Figs. (18b) and (19). It is made of Stalloy strips 3.8 ems.
wide and 0.16 cm. thick stacked on top of each other and over- lapped at the corners of each section to give a strong assembly.
Each half carries a skew pole piece of soft iron and these are one on top of each other when the mating ends of the U sections are in contact. The separator is positioned between these skew poles and keeps them apart with the break in its
central partition lying between the poles. The distance between the two arms of the U is made fairly large (20 ems.) to reduce magnetic leakage. In an earlier model used in some preliminary experiments this was found to be an important factor reducing
FIG. 20
1 ill
iii
32.
seriously the field between the poles.
The exciting coil is wound on a Tufnol former with a rectangular hole to accommodate the two halves of the core.
It is made of 2000 turns of 20 s.w.g., double cotton covered wire wound along the former in six layers. Its total resist- ance is 47 ohms and the electric current is drawn from a 110 volts D.C. supply through a slide wire resistance. The maximum current which can pass through the coil is 1.8 amps. giving a field in the gap between the two poles, which are separated by 1.3 ems. when in operation, of about 200 gauss. This field was found satisfactory. Any larger field resulted in
the magnetic material adhering with the walls of the separator and not travelling through the system as required.
The electromagnet surrounds the separator with its poles placed on opposite sides of the gap, inside the separator, one above and the other below it as shown in fig. (19).
The idea of making the core of the electromagnet of two pieces is to facilitate fitting the whole assembly in the furnace. It is more easy to fit the separator with the electromagnet inside the furnace in this way, than building the furnace around a one piece electromagnet. A photograph of the components of the electromagnet is shown in fig. (20), while fig. (21) shows a photograph of the whole assembly.
Experimental procedure
The procedure of thermomagnetic separation was as follows:
1. The separator was cleaned thoroughly from inside by means of ether or carbon tetrachloride to get rid of any material that remained after previous experiments,
2. The separator and electromagnet was then introduced in the furnace such that its middle part was situated inside the furnace with the two poles of the electromagnet on both sides of the gap of the separator.
3. The ends of the furnace were closed and the thermocouple introduced through a hole in one of the ends.
4. The material was poured in the glass reservoir which was attached to the upper inlet A of the separator with the pressure tubing. The receiving containers were similarly attached at the lower ends and the vacuum pump was connect- ed to the other tube at the upper end.
5. The furnace was heated and the vacuum pump started. The heating current of the furnace was adjusted so that when the required temperature of separation was reached it remained constant within 4- 5°C.
6. When the vacuum reached a reasonable high value, about 3 mm. of mercury, the current of the electromagnet was put on and the powder was fed through the separator in small quantities. It was necessary to feed it very slowly to allow it to acquire the temperature of the furnace
before reaching the gap.
34.
7. The electromagnet, suitably energized, separated the magnetic materials which, when the container was tapped, moved into the second channel. The particles which were non-magnetic at the temperature of separation passed
straight through to the reservoir C.
PART II
THE REMANENT MAGNETIZATION OF IGNEOUS ROOKS AND ITS ORIGIN
The natural residual magnetism of igneous rocks
The fact that rocks can possess permanent residual
magnetization has been known for several centuries, in fact, since the discovery of the magnetized 'lodestone' which is a mass of natural magnetite.
Since
this early discovery, many improvements have been
made in the experimental technique for examining the magnetic properties of rocks so that, now, natural intensities ofmagnetization of 10-6 c.g.s. units can be measured. As a result, natural magnetization have been examined by many
(8) different investigators. The results obtained led Mercanton
(9)
and Chevallier to suggest that this can be used as a tool for the determination of the earth's magnetic field in the past geological times.
The study of the magnetic polarizations of recent (1), (2)
historic lava flows has shown that they are magnetized in a direction which is very near to the direction of the present earth's field, the small deviations between the two directions being accounted for by the secular variations which have taken place. So, in the light of these facts, the direction of the residual magnetization of the older rocks could yield some information about the direction of the geomagnetic field
36.
in the past.
The extensive study of the permanent magnetism of pre- historic igneous rocks showed that, while some of the rocks are !normally? magnetized roughly in the direction of the present earthfs field, the polarization of others were found to be approximately in the opposite direction. Such "inverse"
magnetization of igneous rock masses have been reported in Germany, France, Japan, in many places in the North Atlantic
(2)-(5) (6)
area and in the tholeiite dykes of North of England.
It may be significant that most of these rock masses, which have this anomalous magnetization, are of Tertiary age, although some are very much older as, for example, the
(22), (23)
Pilansburg dykes in South Africa. Further, these rock masses have not been subjected to any earth movement and they are in the same position that they possessed at the time of their formation. So, any explanation of these reversals by the displacement of the rock bodies, after acquiring their remanent magnetization, is inadequate.
There are, in general, two possible explanations of the phenomenon of inverse magnetization:
(6)
In one, the earthfs magnetic field, either locally or on a world wide scale, was in the opposite direction to its present one. This view is supported by a mass of experimental evidence, including the study of magnetization of recently ejected igneous rocks as well as laboratory investigations
of collected samples.
, (14)
The second explanation was advanced by Neel and the reverse magnetization was attributed to some intrinsic property of the rocks and to certain special physical and chemical conditions in the rock. In support to this
(15)
hypothesis, Nagata in Japan has reported that a particular rock does acquire reverse magnetization under certain
conditions. These two views will be discussed in detail later.
The stability of the residual magnetization of igneous (10)
rocks was studied exclusively by K3enigsberger who stated a number of conditions which an igneous rock formation must possess in order to give information about the geomagnetic field in the past. These conditions are :
(a) Since its formation, the rock must not have been subject- ed to any appreciable movement.
(b) During the cooling of extruded magma, no internal displacement must have occurred below a certain critical temperature.
(c) The rock must possess a high value of coercivity, at normal temperatures, in order to retain its original magnetization for long periods of time.
Thermoremanent magnetization of igneous rocks
There are two ways in which a rock can acquire permanent magnetization. When a rock is subjected to a magnetic field at room temperature it acquires permanent magnetization which
36.
is called "isothermal magnetization". If an intensity of magnetism JH is produced by a magnetizing field H then, when the field is removed, the isothermal intensity of magnetiza- tion Jo remaining is less than JH. On the other hand, when the rock is cooled down in a magnetic field through its Curie point, it acq uires "thermoremanent magnetization".
There can be little doubt that the natural residual magnetization of igneous rocks is of thermoremanent origin.
This has been demonstrated in different ways as follows:
(i) The ratio of the natural magnetization Jn of the rock to the magnetization which could be acquired in the earthts field, namely, JJK H Qn is called the K8enigsberger ratio. The Qn values of igneous rocks have been measured by different investigators and it has been found that, in general, they have values much greater than 1, ranging
(2),(7),(10),(20).
from 2 - 100.
(ii) The study of heated samples from the igneous rocks of (7)
Mull has shown that, when the rock is cooled in the
present earthts field through the Curie temperature, its thermoremanent magnetization is of the same order as
its natural residual magnetization. Generally, the latter was lower than the former and this could be accounted for by the demagnetization effect on rocks since the time of their formation.
The ratio of the natural remanent magnetization to the
thermoremanent magnetization, produced in the present•
earthts field, was calculated for both recent and pre- historic lava flows. For recent lavas, this ratio was 1.01+0.12 in the case of Japanese rocks, while it was 0.90 -1- 0.04 for the Icelandic lavas. On the other hand, the ratio was 0.67-'1- 0.23 for the much older rocks in
(20) Iceland (million years old).
(i1i) Another experiment, described in detail in appendix II, supports the hypothesis of thermoremanent magnetism.
After initially demagnetizing some igneous rock samples, the isothermal magnetization in known magnetic fields were measured. Thus, it was possible to estimate the field necessary to give the observed natural intensity
of magnetization. To obtain this natural intensity requires, in the cases examined, fields ranging from 21 gauss up to 75 gauss. Since such large values of the intensity of the earthts field in the past are most improbable, it would appear that the thermoremanent origin of the natural magnetization is most plausible.
Another significant feature which has been demonstrated by different investigators is that, with cne well known
(15),(17)
exception, whenever an igneous rock is cooled through its Curie temperature in the geomagnetic field, the acquired thermoremanent magnetization is always in the direction of the field.
40.
The mechanism of thermoremanent magnetization has been studied by a number of investigators. It has been shown that the magnetic properties of rocks result from the presence in them of ferromagnetic constituentwainly magnetite.
(10) (12)
K8enigsberger„ Thellier and Nagata have shown that the remanent magnetization in igneous rocks is acquired by the process of cooling in the geomagnetic field from above their ferromagnetic Curie points.
Stability of the remanent magnetization of igneous rocks The fact that igneous rocks are stable, as far as their magnetization is concerned, is evident from the following considerations:
(1) An evidence of the stability is the fact that the inversely magnetized rocks still have their inverse magnetization after many millions of years. It is
worth noting that, on geological evidence, igneous rocks have been once heated to 6000, the Curie point of
magnetite, namely, at the time of their injection and it"
only at this time that they could acquire thermoremanent magnetization sufficient to account for the present
observations. If they are not stable, they should have been demagnetized under the influence of the present earthls field and acquired a polarization parallel to this field, with the result that their present intensity should be much loss than that normally observed.
Vineenz and Hospers found that both normally and
inversely magnetized rocks have intensities of the same order. In Hosper6 results, while the "normal" rocks have mean intensity = 3.02 x 10-3 c.g.s. units, the
"
reverse" ones have 4.27 x 10-3 c.g.s. units. Again, as has been noted above, when the rock is cooled from above its Curie point in the present earth's field, the thermoremanent intensity is of the same order as their natural magnetization.(ii) A less obvious argument arises from an examination of normally magnetized rocks in which the actual direction of magnetization may differ by a few tens of degrees from the present field. It is apparent, on the basis
of modern theories of the geomagnetic field, that over long periods of time, the average direction of the field should be that of an axial dipole. The average direction of a large number of lava flows does, in fact, confirm with this statement and so it would appear that the influence of the present field is negligible.
(iii) The directions of magnetization of boulders in a conglomerate of the early quaternary age (less than 106
years old) were found to be at random. This indicates that their directions of magnetization have been stable since deposition. For, if they are unstable they should all have a uniform direction parallel to the present field.
42:
The effect of "unfolding" on the scatter of magnetization in a folded stratum was studied by Hospers.(20)
He compared the scatter of the directions of remanent magnetization of
specimens, taken from such a bed, before and after correcting for the folding of the stratum. If the removal of the
influence of the folding decreases the scatter their magnetiza- tion was, in all probability, acquired before the tectonic
movements occurred. The effect of the unfolding was, in fact, a decrease in the scatter. He concluded, from this, that
the remanent magnetization of the normally magnetized Tertiary specimens originated before the tectonic movements, i.e.,
before the early Pliocene age, and so, the magnetization has been stable in direction over several millions of years.
From all these arguments, it is quite evident that, in the main, the igneous rocks retain their natural magnetization from the time of their formation.
Some asp.ects of thermoremanent magnetization
As has been indicated, the thermoremanence is the magnetization acquired by a rock when cooling from a high temperature to ordinary temperature in a magnetic field.
(24)
Nagata has shown that the thermoremanent magnetization of a rock depends on the temperature T from which it cooled, as well as on the magnetic field in which cooling takes place.
For a constant field, the thermoremanence increases with T till it attains saturation at a certain temperature, a critical
temperature which corresponds to the Curie point. The saturated or total thermoremanent magnetization is the residual magnetization after cooling from above the Curie
point to room temperature in a magnetic field. In this
thesis, whenever thermoremanent magnetization is mentioned it means the saturated thermoremanence, since in all heat treat- ments, the rock has been cooled from above its Curie point.
Partial thermoremanent magnetization
If during the cooling of a rock, the magnetic field is applied only in a temperature range Tl to T2 (T1 > T2), while in other temperature ranges above T1 and below T2 the field is kept zero, the specimen acquires what has been defined as
(25)
partial thermoremanent magnetization. Nagata, also, has proved experimentally that the partial thermoremanence acquired in the different ranges of temperature between a temperature T and room temperature are subjected to an addition law. In other words, the thermoremanence acquired between T and room temperature is equal to the sum of partial thermoremanent magnetizations that have been acquired in the different ranges between these two temperatures. However, it has been proved experimentally that most of the thermoremanence is acquired in a range of about 100 C below the Curie point.
The partial thermoremanence acquired between temperatures T1 and T2 in a field H during cooling disappears when heated to above T1, independent of the field H. On the other hand, it
44.
is hardly disturbed by heating so long as the temperature does not exceed
T2 (Ti, T2).
The domain theory
In order to explain the way in which a ferromagnetic body acquires residual magnetization, Weiss put forward his concept of ferromagnetic domains. In this theory, the body consists of a large number of domains, each of which is magnetized to saturation. In the demagnetized state the
magnetization vectors of the domains are at random and so, the resultant magnetic moment is zero. The specimen will possess a magnetic moment when some of these domain vectors are made to align themselves in one direction.
Thus, when an external field is applied in a certain direction, at constant temperature, it will tend to change the direction of some of these vectors in its direction. This will be increased by increasing the external magnetic field and when all the domain vectors are along the field, saturation occurs. By this way, the hysteresis loop of the isothermal magnetization could be explained.
(26)
Extensive work has been done by Bates on this subject and it has been possible to observe the different patterns of the domains under different conditions, by photographing the Bitter patterns. This was done by spreading a colloidal or semi-colloidal