Image effects and vibrating sample magnetomete

Behrendt, D. R.

Hegland, D. E.

English

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NASA TECHNICAL NOTE 
LOANCOPY: RETURN T 
A W L  (DOGL) 
KIRTLAND AFB,N.M, 
IMAGE EFFECTS A N D  THE VIBRATING 
SAMPLE MAGNETOMETER 
.,@ ,,'4, ' I' 
\ ... 
...,by Donald R. Behrendt and Donald E.  Hegland 
Lewis Research Center 
Cleveland, Ohio 44135 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON,  D. C. MARCH 1971 
https://ntrs.nasa.gov/search.jsp?R=19710011598 2020-03-23T17:32:27+00:00Z
TECH LIBRARY KAFB, NM 
I 1111111111111111lll//I//lllllllIl1111 
__ 0133029 __ -. 
1. 	 Report No. 2. Government Accession No. 3. Recipient's Catalog NO. 
NASA TN D-6253 i I 
4. Title and Subtitle 5. Report Date 
IMAGE EFFECTS AND THE VIBRATING SAMPLE 1 March 1971 
MAGNETOMETER I 6. Performing Organization Code 
7. Authoris) i 8. Performing Organization Report No.. .  I E-6016Donald R. Behrendt and Donald E. Hegland 
10. Work Unit No. 
9. PerformingOrganization Name and Address 129-03 
Lewis Research Center I11. Contract or Grant No. 
National Aeronautics and Space Administration ICleveland, Ohio 44135 
~________ 13. Type of Report and Period Covered 
2. Sponsoring Agency Name and Address Technical Note'I 14. Sponsoring Agency Code
National Aeronautics and Space Administration 
Washington, D. C.  20546 
5. Supplementary Notes 
~­
6. Abstract 
Some anomalous effects observed in  the output signal of the vibrating sample magnetometer have 
been investigated. The variations in amplitude and phase of the Fignal coil voltage are attributed 
to changes in the magnetic images of the sample in the pole pieces of the electromagnet due to 
changes in the permeability of the pole pieces. A set of eight signal coils was designed and built 
which eliminated the effects of the phase variations of the signal coil  voltage on the output signal 
and which also reduced changes in amplitude of the output signal due to these image effects. 
17. Key Words (Suggested by Authoris) I 1 18. Distribution Statement 
Magnetometer Unclassified - unlimited 
Magnetic image effects 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. NO. of Pages 22. Price' 
Unclassified Unclassified 9 $3.00 
IMAGE EFFECTS AND THE VIBRATING SAMPLE MAGNETOMETER 
by Donald R. Behrendt and Donald E. Hegland 
Lewis Research Center 
SUMMARY 
Images of the sample in pole pieces of an  electromagnet produce a spurious voltage 
in  the signal coils of the vibrating sample magnetometer. The magnitude of this spuri­
ous voltage is dependent upon the degree of magnetic saturation of the pole pieces as this 
affects the strength of the images and their position relative to the sample and signal 
coils. In general, the spurious voltage is not in phase with the sample induced voltage. 
The superposition of these two induced voltages produces a field dependent amplitude and 
phase variation in the measured signal voltage which does not correspond to the true sam­
ple moment. It is demonstrated that a se t  of coils consisting of eight signal coils can be 
designed which reduces the effects of the phase variations in the output signal to a tol­
erable  level and reduces changes in  amplitude of the output signal due to these image 
effects. 
INTRODUCTION 
The vibrating sample magnetometer is used to measure the magnetic moment of a 
mater ia l  by sinusoidally displacing the sample along a vertical  axis while a magnetic 
field is applied along a horizontal axis. The resulting magnetization of the sample pro­
duces a dipole field which because of the periodic sample motion induces a time varying 
voltage in  the stationary signal coils. The voltage induced in these coils should be pro­
portional to the magnetic moment of the sample and the amplitude of its vertical  motion. 
However, a spurious additional voltage may be induced in the coils by images of the sam­
ple in the pole pieces of the electromagnet and by eddy currents  induced in these pole 
pieces. The magnitude of this  spurious voltage changes as the permeability of the pole 
pieces changes. As the field of the electromagnet is increased, the permeability of the 
pole pieces decreases ,  the strength of the images decreases ,  and the images retreat 
fa r ther  into the pole pieces. 
There will in general  be a difference in the phase between the voltages induced by 
the sample and its images because of various losses  in the pole pieces. As the image-
induced voltage decreases  because of an increasing applied field, the phase difference be­
tween total  induced voltage and sample-induced voltage will decrease.  This  change in 
phase of the total signal voltage cannot be corrected by simply adjusting the calibration 
of the instrument to compensate for  the change in response with applied field. A study 
was  made to determine if a signal coil geometry could be developed that would reduce or 
eliminate the amplitude and phase changes due to the spurious image voltage. Such an  
arrangement of coils was found and is described in this report .  
Weiss and F o r r e r  (ref. 1) in an  earlier paper discussed the effects of sample images 
in the pole pieces of the electromagnet. Stoner, Herbert ,  and Sill (ref. 2) have reported 
observing image effects using a vibrating sample magnetometer. They point out that 
these effects are present a t  low fields and that they can significantly affect the magnitude 
of the vibrating sample magnetometer output signal. Both Foner (ref. 3) and Case and 
Harrington (ref. 4) have reported finding no evidence of image effects in their  exper­
iments using the vibrating sample magnetometer. It is possible that variations in the 
relative locations of the sample and signal coils with respect  to the pole pieces can ac­
count for  these differences in image effects reported in the l i terature.  
The instrument used in  this investigation was built by a commercial  supplier and was 
patterned after one designed and built by Foner (ref. 3). We have found large changes in 
the output signal of this magnetometer due to changes in image effects. These effects 
are observable even a t  small  applied fields. 
RESULTS A N D  DISCUSSION 
The original coil geometry used in this investigation is shown in figure 1. Each coil 
of these t ransverse pole-tip coils shown f i ts  tightly against a pole piece of the electro­
magnet. The response of the magnetometer was determined by using a high-purity poly­
crystalline iron sphere.  For the coil geometry shown in figure 1, the response is shown 
in figure 2 (curve A ) .  This particular sample of i ron was saturated for  applied fields 
greater  than 1.0 tesla. If no image effects are present or if they do not change with ap­
plied field, the response of the magnetometer should be independent of applied field for a 
sample of constant magnetic moment; under the same conditions of no image effects, the 
response for  a n  iron sample should show a smal l  linear increase for  fields above 1.0 
tesla due to the high field susceptibility of iron; this is shown in figure 2 (curve B). The 
actual response using iron shows a rapid rise of more than 16 percent between 1.8 and 
3.0 teslas. These data were obtained with the demodulator phase balance optimized a t  
low fields. This anomalous response was reduced when the gap of the electromagnet was 
increased while the sample and signal coils were held fixed. This suggested that images 
2 

of the sample in  the pole pieces were responsible since increased distance between the 
sample and images would be expected to reduce the contribution of the images to the s ig­
nal  voltage. 
A second and more ser ious problem encountered is that along with the increase in 
response the phase of the signal voltage was observed to change with applied field. The 
induced voltage in the signal coils depends on the displacement amplitude of the sample, 
on the magnetic moment of the sample,  and on the image effects. The output signal of 
the magnetometer is made independent of the displacement amplitude of the sample by 
utilizing a reference voltage derived from capacitor plates attached to the sample rod. 
The response then depends only on the sample magnetic moment and image effects if any. 
The proper operation of the magnetometer requires  that the reference voltage and the sig­
nal coil voltage be matched in phase. Because the phase of the signal coil voltage var ies  
with applied field, this phase match cannot be maintained and large e r r o r s  result .  These 
phase variations can be attributed to changes in the strength and position of the sample 
images in  the pole pieces and to changes in the eddy currents  in the pole pieces produced 
by the moving sample. 
Stoner, Herbert ,  and Sill (ref. 2)  have made a theoretical analysis using a magnetic 
dipole sample and i t s  images in two infinite-plane, parallel  pole pieces for  which they 
calculate the field distribution for  two values of the permeability of the pole pieces. 
They show that the response of the t ransverse pole-tip coils should increase as the per­
meability of the pole pieces decreases .  For the axial pole-tip coils shown in figure 3 
their  analysis shows that the response should decrease with decreasing permeability of 
the pole pieces. This agrees  with both their experimental resul ts  and ours  for  these two 
coil configurations. By combining the voltages of the t ransverse and axial pole-tip coil 
sets (figs. 1 and 3), the response should depend less on the image effects since the 
changes in the two signal coil voltages will compensate one another. However, the addi­
tional s e t s  of coils between the sample and pole pieces would require  that the air gap of 
the electromagnet be increased and lead to an  undesirable reduction in maximum field. 
This  objection can be overcome by using two additional pa i r s  of axial coils (the axial side 
coils), one pair  on the positive x-axis and the other on the negative x-axis, as shown in 
figure 3.  The observed response for  this set of coils is s imi la r  to the t ransverse pole-
t ip  coils in  that their response increases  with increasing applied field. The analysis of 
reference 3 indicates (figs. 8 and 9 of ref. 3) such coils should show an increase in re­
sponse with decreasing permeability of the pole pieces in  agreement with our 
observations. 
Four pa i r s  of coils, two pairs  of axial pole-tip coils and two pairs  of axial side coils, 
were made using 32 B & S enameled wire and installed on the pole pieces of the electro­
magnet. The four axial pole-tip coils were wound with an inside diameter of 1 . 2 7  cen­
t imeters  ( 0 . 5  in.)  and an  outside diameter of 4 . 4 1  centimeters (1 .73  in.)  and were 0 . 6 3 5  
centimeter (0.25 in. ) thick. Each pair  of coils was supported by placing them in holes 
3 
I I 1 I 

drilled into 0.632-centimeter - (0.25-in. -) thick G-10 epoxy boards. Each of the G-10 
boards was mounted flush against one of the pole pieces of the electromagnet. The two 
pairs of axial side coils were wound on forms having an  inside diameter of 1.27 centi­
me te r s  (0.5 in.) and a n  outside diameter of 3.81 centimeters (1.5 in. ), each coil being 
1.58 centimeters (0.625 in. ) thick. The coils and fo rms  were then mounted between the 
G-10 boards which contained the four pole-tip coils. The placement of these side coils 
relative to the sample and applied magnetic field is shown in figure 3. 
The four axial pole-tip coils were connected in series in such a manner that their 
sample induced voltages were in phase. The same  was done for  the four axial side coils. 
The voltages from the two sets of four coils were then combined in parallel; a res i s tor ,  
R1, was added in series with the four axial pole-tip coils to reduce their sensitivity to 
that of the axial side coil combination as shown in figure 4. In order  to produce an  elec­
t r ica l  zero near the geometric center of the eight-coil combination, each coil was shunted 
with a large value res i s tor .  The electrical zero is the position of the sample between the 
coils such that vertical  displacements of the sample (positive or negative displacements 
along the z -direction) decrease the output signal and displacements along either positive 
or negative x- or  y-axies increase the output signal. By varying the res i s tor ,  R1, in 
series with the set of axial  pole-tip coils, the response could be made to rise o r  fall in 
the range of applied fields in which the large changes due to  image effects were observed. 
It has already been mentioned that the large image effects using the t ransverse pole-
tip coils produced such large changes in the phase of the pickup coil voltages that the sys­
tem was not accurate above 2.0 teslas. With the eight-coil combination the phase of the 
pickup coil voltage was almost constant, that is, the smal l  variations in phase that do oc­
cur  are not large enough to detune the demodulator. 
The magnitude of the residual image effects for  the eight-coil combination is shown 
in figure 2 (curve C) .  The t ransverse pole-tip coils alone showed an  amplitude change of 
over 16 percent due to image effects.  The combination of axial pole-tip and axial side 
coils shows a smaller  change of less than 4 .5  percent over the same field range. 
In order  to  investigate the low field response of the eight-coil combination, the iron 
sample had to be replaced since the iron is not sufficiently saturated at these low fields. 
A smal l  coil of wire 0.955 centimeter (0.375 in . )  in diameter by 0.955 centimeter (0.375 
in. ) long was attached to  the sample rod. The constant magnetic moment produced by 
passing a direct  current  through the coil could be used to  examine the response at low 
fields. For fields above 1.2 teslas the response was determined by using the coil in con­
junction with the iron sample along with published values of the high field susceptibility 
of iron (ref. 5). Figure 5 shows the response as a function of applied field for the eight-
coil combination. This curve shows the relative change in the calibration constant of the 
system with applied magnetic field relative to zero  applied field. Even at relatively 
smal l  applied fields the image effects cannot be neglected. A t  0 .7  tesla the response is 
4 

increased by 0.1 percent over its zero-field value. By using a calibration constant 
which is a function of the applied field, the smal l  residual image effects can be compen­
sated to  yield an overall accuracy of *O. 2 percent fo r  the system. 
Lewis Research Center,  
National Aeronautics and Space Administration, 
Cleveland, Ohio, November 27, 1970, 
129-0 3. 
REFERENCES 
1. Weiss, Pierre; and F o r r e r ,  R. : Magnetization and the Magnetocaloric Phenomena of 
Nickel. Ann. Phys. (Paris), vol. 5, 1926, pp. 153-213. 
2. 	Stoner, R. E. ; Herbert, R. H. ; and Sill, L. R. : Image Effects in Vibrating Sample 
Magnetometer Systems. J. Appl. Phys. , vol. 41, no. 9, Aug. 1970, pp. 3706-3712. 
3. 	Foner,  Simon: Versatile and Sensitive Vibrating-Sample Magnetometer. Rev. Sci. 
Instr .  , vol. 30, no. 7, July 1959, pp. 548-557. 
4. Case ,  W. E. ; and Harrington, R. D. : Calibration of Vibrating-Sample Magnetom­
eters. 	 Nat. Bur. Standards J. Res. , vol. 70C, no. 4, Oct. -Dec. 1966, pp. 
255-262. 
5. 	 Herring, C . ;  Bozorth, R. M. ; Clark, A. E . ;  and McGuire, T. R. : High-Field Sus­
ceptibilities of Iron and Nickel. J. Appl. Phys. , vol. 37, no. 3, Mar. 1, 1966, 
pp. 1340-1341. 
5 

W 
W 
W 
Figure 1. - Or ig ina l  co i l  geometry. Transverse pole-tip coi ls exhibi t  increase 
in  signal  voltage as pole pieces become saturated. Sample located at or ig in.  
Curve  
A 
B 
C 
Transverse pole-tip coi ls shown in fig. 1 
I r o n  sample in absence of image effects ideal 
response 
Eight-coil combination of axial pole-tip and axial 
side coi ls shown in fig. 3 
VI
c 

g 
m
I 

c 

.95  
.85
.90L,J 
I I I I.80 
. 8  L 2  16 2.0 2.4 2.8 3.2 3.6 
Applied magnetic field, T 
F igu re  2. - Response (relat ive ou tpu t  signal) of v ibrat ing 
sample magnetometer fo r  two coil geometries f o r  var ious 
applied magnetic f ields u s i n g  i r o n  sample. 
6 

Figure 3. - Placement of f i na l  eight-coil combination, f ou r  axial pole-tip coi ls 
and four axial side coils. 
Axial pole-tip coils 
of detection 
Isystem 
I I I 
Axial side coi ls I 
R9 
Figure 4. - Schematic w i r i n g  diagram for eight-coil com­
bination. Resistors R2 to R9 a re  large value resistors 
w h i c h  balance indiv idual  co i l s  of each s e t  Resistor R 1  
i s  used to produce a balance between axial pole-tip coi ls 
and axial side coils. 
7 

8 NASA-Langley, 1971 -26 E-6016 


Image effects and the vibrating sample magnetometer

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