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Sensitive magnetometer available to 10-4 EMU using pulsed magnet
Physica B 155 (1989) 389-391 North-Holland. Amsterdam
SENSITIVE K. YAMADA
MAGNETOMETER
AVAILABLE
TO 1O-4 EMU USING PULSED
MAGNET
and M. OKABE
Faculty of Engineering,
Saitama University, 338 Saitama, Japan
We developed a magnetometer capable of measurements down to 10 4 emu in a pulsed field range less than 10T with specially controlled field waveforms. The slow rising behavior of the magnetic fields up to 30T were obtained by a “saturable reactor” connected in series with the pulsed magnet.
1. Introduction In general, magnetization measurements of magnetically weak samples have been performed by vibrating sample magnetometers (VSM) or SQUIDS. The measuring time, however, becomes long when the sample volume gets very small. For samples with a magnetization less than lo-’ emu, approximately 10 min are necessary to obtain a good quality M-H curve. This fact makes it important to stabilize the sample temperature during the measurement, especially when the sample temperature is near the magnetic phase transition. In the pulsed magnetic field method (PMF) this is not a problem, due to the very short pulse duration. However, the signal quality naturally becomes worse not only because of this short measuring time but also by the fast rising behavior of the magnetic field. Consider the case when a high magnetic field with a sinusoidal pulse shape is applied to a ferromagnet. As is well known, a ferromagnet has the largest susceptibility in the weak magnetic field range. Because the signal is generated by the time derivative of the flux inside the pick-up coil, the signals increase in a rapid manner like a delta function. The amplifiers and the integrator for this purpose should have exceptionally large dynamic and frequency ranges because of the divergent behaviors of the signal spectra. In this study, instead of the improvements of electronic devices (ADC, amplifiers, etc.), mainly the waveforms and the geometrical distribu0921-4526/891$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
tion of the applied gated and they netometry.
2. Experimental
magnetic fields were investiwere optimized for mag-
procedures
and results
A saturable reactor (SR) was connected in series to the pulsed magnet composed of multiturned coils to attain a slowly rising pulsed field waveform. A SR has the well-known characteristic that the self-inductance decreases with increasing current and reaches a value of that of the empty core in the large current limit due to the saturation magnetization in the core material. Therefore, if an appropriate SR is connected in series to the main coil, the rising behavior of the magnetic field is controlled by the initial value of the self-inductance (L,,) of the SR. Usually, L,, can be chosen more than 100 times larger than that of the saturated value (L,,). The optimum condition for our purpose is
L,” % L 9 L,, .
(1)
Here L denotes the inductance of the main coil (in the present study L = 1.6 mH). Fig. 1 shows the experimental traces of the magnetic field waveforms at different maximum fields (H,,,,,) up to 20T, and enlargements of the rising parts of H(t) in fig. 2(a) and dH(t)ldt in fig. 2(b). Here, for comparison, the waveforms of those without SR are shown by (a’), (b’), (c’) and (d’) of the same H,,,,, as (a), (b), (c) and (d) respecB.V.
K. Yumudu and M. Okabe
I Mugnetometer
to 10 ’ emu by u pulsed magnet
I
Time
I
t(ms)
Fig. I. Experimental traces of the pulsed magnetic fields generated by discharging a current from a condenser bank (C= 1OmF) through the pulsed magnet (L = 1.6mH) and a saturable reactor connected in series. Maximum fields are (a) 20T: (b) 13T: (c) 7T; and (d) 3.7T.
tively. As seen in these figures, there are no sharp kinks in H(t) and dH(t)ldt in the waveforms with the SR. Improvement of the magnetometry is attained also by a better homogeneity of the magnetic flux within the volume of the pick-up coil which is composed of coaxial winding connected in opposite directions. Fig. 3 shows the magnetic field distribution around the center along the main coil axis. The magnetic field profile in the direction perpendicular to the coil axis was neglected as being constant. It is worth while to note that the flux deviation from the maximum value
0
02
0.4
06
08
0
02
04
06
08
Time
at the position of the pick-up coil is about 0.003% within the length of the pick-up coil (3 mm). The location of the pick-up coil was exactly determined at the center of the magnet after measurement of the flux distribution. Such a small deviation of the flux distribution around the pick-up coil caused little spurious signal due to the main coil vibration. In the present experiment, however, the pulsed magnet started to vibrate when magnetic fields higher than 20T were generated. For suppression of this vibration, a heavy weight of 40 kg was attached to the main coil as a trial. But the vibration originated from the relative movements among the multiturn coils of the pulsed magnet so that this method was not effective in suppressing the magnet vibrations in high fields H,,, > 20 T. As examples of the magnetization measurements, M-H curves of carbonyl powders at room temperatures are shown in fig. 4. The sample weights were determined with an accuracy of 0.1 mg, except for the smallest sample of 4 pg. The weight of this sample is roughly estimated from the particle numbers counted through a
0
t(ms)
Fig. 2. The enlargements of the magnetic field traces their time derivatives in the weak field range. (a) H(t) (b) dH(r)ldr.
Fig. 3. ‘The magnetic field distribution along the main coil axis. The dashed curve shows the enlargements around the center. The size of the pick-up coil is indicated by a bar.
and and
2 H (Tl
L
0
2
4
H IT)
Fig. 4. Examples of magnetometry carhonyl iron powder.
by PMF.
The sample
is
K. Yamada and M. Okabe
H
I Magnetometer
(T)
Fig. 5. Example of the measurement 30T. The sample is a paramagnet Tb20, + 0.9. non-magn. material.
001
01
in a high field up to composed of 0.1
1
of the saturable
The inductance derived by d(L,I)ldt
+ L d/ldt
In the largest of the
SR is experimentally
+ RI + ;
I
Zdt=O,
where R denotes the total resistance in the circuit. L,(Z) is numerically derived from (2) as L,=
-[R/ldt+
#dtdt]/l-
L.
current
Ls(f=0,Z=4kA)=0.3mH.
(2)
100
reactor
derived
from
half at f = 3 kHz and one-tenth at f = 10 kHz. In the small current limit, L, is given by L,(f=0,1=2A)=80mH.
3. Discussion
10
H(T)
Fig. 6. The inductance eq. (3) in the text.
microscope. Fig. 5 shows the magnetometry in high fields up to 30 T. In this case the resolution became worse at lo-’ emu, mainly due to the irreproducible magnet vibrations.
391
to 10 ’ emu by a pulsed magnet
(4) limit (5)
The necessary condition (1) for our purpose is satisfied by (4) and (5) in the present experiment. However, the frequency response of the SR is not enough to suppress the rush current. For our purpose, a ferrite core for the SR might be a better material.
(3) Acknowledgements
Fig. 6 shows the results of L, used in the present experiment. L, shows a slight increase with increasing current in the small current range less than 2 A or 0.02 T, exhibiting the shortage of the high frequency response of iron in the SR. Note here that the inductance of the SR became one-
The authors are indebted to Dr. Sakakibara (ISSP, University of Tokyo) for his helpful discussions to design this magnetometer. The author thanks Dr. Narumiya (TDK, Tokyo, Japan) for providing the ferrite cores.
SENSITIVE K. YAMADA
MAGNETOMETER
AVAILABLE
TO 1O-4 EMU USING PULSED
MAGNET
and M. OKABE
Faculty of Engineering,
Saitama University, 338 Saitama, Japan
We developed a magnetometer capable of measurements down to 10 4 emu in a pulsed field range less than 10T with specially controlled field waveforms. The slow rising behavior of the magnetic fields up to 30T were obtained by a “saturable reactor” connected in series with the pulsed magnet.
1. Introduction In general, magnetization measurements of magnetically weak samples have been performed by vibrating sample magnetometers (VSM) or SQUIDS. The measuring time, however, becomes long when the sample volume gets very small. For samples with a magnetization less than lo-’ emu, approximately 10 min are necessary to obtain a good quality M-H curve. This fact makes it important to stabilize the sample temperature during the measurement, especially when the sample temperature is near the magnetic phase transition. In the pulsed magnetic field method (PMF) this is not a problem, due to the very short pulse duration. However, the signal quality naturally becomes worse not only because of this short measuring time but also by the fast rising behavior of the magnetic field. Consider the case when a high magnetic field with a sinusoidal pulse shape is applied to a ferromagnet. As is well known, a ferromagnet has the largest susceptibility in the weak magnetic field range. Because the signal is generated by the time derivative of the flux inside the pick-up coil, the signals increase in a rapid manner like a delta function. The amplifiers and the integrator for this purpose should have exceptionally large dynamic and frequency ranges because of the divergent behaviors of the signal spectra. In this study, instead of the improvements of electronic devices (ADC, amplifiers, etc.), mainly the waveforms and the geometrical distribu0921-4526/891$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
tion of the applied gated and they netometry.
2. Experimental
magnetic fields were investiwere optimized for mag-
procedures
and results
A saturable reactor (SR) was connected in series to the pulsed magnet composed of multiturned coils to attain a slowly rising pulsed field waveform. A SR has the well-known characteristic that the self-inductance decreases with increasing current and reaches a value of that of the empty core in the large current limit due to the saturation magnetization in the core material. Therefore, if an appropriate SR is connected in series to the main coil, the rising behavior of the magnetic field is controlled by the initial value of the self-inductance (L,,) of the SR. Usually, L,, can be chosen more than 100 times larger than that of the saturated value (L,,). The optimum condition for our purpose is
L,” % L 9 L,, .
(1)
Here L denotes the inductance of the main coil (in the present study L = 1.6 mH). Fig. 1 shows the experimental traces of the magnetic field waveforms at different maximum fields (H,,,,,) up to 20T, and enlargements of the rising parts of H(t) in fig. 2(a) and dH(t)ldt in fig. 2(b). Here, for comparison, the waveforms of those without SR are shown by (a’), (b’), (c’) and (d’) of the same H,,,,, as (a), (b), (c) and (d) respecB.V.
K. Yumudu and M. Okabe
I Mugnetometer
to 10 ’ emu by u pulsed magnet
I
Time
I
t(ms)
Fig. I. Experimental traces of the pulsed magnetic fields generated by discharging a current from a condenser bank (C= 1OmF) through the pulsed magnet (L = 1.6mH) and a saturable reactor connected in series. Maximum fields are (a) 20T: (b) 13T: (c) 7T; and (d) 3.7T.
tively. As seen in these figures, there are no sharp kinks in H(t) and dH(t)ldt in the waveforms with the SR. Improvement of the magnetometry is attained also by a better homogeneity of the magnetic flux within the volume of the pick-up coil which is composed of coaxial winding connected in opposite directions. Fig. 3 shows the magnetic field distribution around the center along the main coil axis. The magnetic field profile in the direction perpendicular to the coil axis was neglected as being constant. It is worth while to note that the flux deviation from the maximum value
0
02
0.4
06
08
0
02
04
06
08
Time
at the position of the pick-up coil is about 0.003% within the length of the pick-up coil (3 mm). The location of the pick-up coil was exactly determined at the center of the magnet after measurement of the flux distribution. Such a small deviation of the flux distribution around the pick-up coil caused little spurious signal due to the main coil vibration. In the present experiment, however, the pulsed magnet started to vibrate when magnetic fields higher than 20T were generated. For suppression of this vibration, a heavy weight of 40 kg was attached to the main coil as a trial. But the vibration originated from the relative movements among the multiturn coils of the pulsed magnet so that this method was not effective in suppressing the magnet vibrations in high fields H,,, > 20 T. As examples of the magnetization measurements, M-H curves of carbonyl powders at room temperatures are shown in fig. 4. The sample weights were determined with an accuracy of 0.1 mg, except for the smallest sample of 4 pg. The weight of this sample is roughly estimated from the particle numbers counted through a
0
t(ms)
Fig. 2. The enlargements of the magnetic field traces their time derivatives in the weak field range. (a) H(t) (b) dH(r)ldr.
Fig. 3. ‘The magnetic field distribution along the main coil axis. The dashed curve shows the enlargements around the center. The size of the pick-up coil is indicated by a bar.
and and
2 H (Tl
L
0
2
4
H IT)
Fig. 4. Examples of magnetometry carhonyl iron powder.
by PMF.
The sample
is
K. Yamada and M. Okabe
H
I Magnetometer
(T)
Fig. 5. Example of the measurement 30T. The sample is a paramagnet Tb20, + 0.9. non-magn. material.
001
01
in a high field up to composed of 0.1
1
of the saturable
The inductance derived by d(L,I)ldt
+ L d/ldt
In the largest of the
SR is experimentally
+ RI + ;
I
Zdt=O,
where R denotes the total resistance in the circuit. L,(Z) is numerically derived from (2) as L,=
-[R/ldt+
#dtdt]/l-
L.
current
Ls(f=0,Z=4kA)=0.3mH.
(2)
100
reactor
derived
from
half at f = 3 kHz and one-tenth at f = 10 kHz. In the small current limit, L, is given by L,(f=0,1=2A)=80mH.
3. Discussion
10
H(T)
Fig. 6. The inductance eq. (3) in the text.
microscope. Fig. 5 shows the magnetometry in high fields up to 30 T. In this case the resolution became worse at lo-’ emu, mainly due to the irreproducible magnet vibrations.
391
to 10 ’ emu by a pulsed magnet
(4) limit (5)
The necessary condition (1) for our purpose is satisfied by (4) and (5) in the present experiment. However, the frequency response of the SR is not enough to suppress the rush current. For our purpose, a ferrite core for the SR might be a better material.
(3) Acknowledgements
Fig. 6 shows the results of L, used in the present experiment. L, shows a slight increase with increasing current in the small current range less than 2 A or 0.02 T, exhibiting the shortage of the high frequency response of iron in the SR. Note here that the inductance of the SR became one-
The authors are indebted to Dr. Sakakibara (ISSP, University of Tokyo) for his helpful discussions to design this magnetometer. The author thanks Dr. Narumiya (TDK, Tokyo, Japan) for providing the ferrite cores.