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High sensitive magnetometer using a pulsed magnet
783
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 783-784 North-Holland
High sensitive magnetometer using a pulsed magnet T. Yosida, A. Yamagishi
a
and M. Date
b
Low Temperature Center, Osaka University, Toyonaka 560. Japan Q
b
The Research Center for Extreme Materials, Osaka University, Toyonaka 560. Japan Department of Physics, Faculty of Science, Osaka University, Toyonaka 560, Japan
Strong field gradient produced by a pulsed magnet is applied to the magnetic susceptibility measurement. The principle and practical applications are shown by using paramagnetic materials. An extension to get high sensitivity with the accuracy of lO- t2 emu/em) is discussed .
Much' work has been reported on the magnetic susceptibility measurements under pulsed magnetic field. Principle of the method is to measure the differential magnetization dM/dH by using a pick-up coil placed in the uniform field region. Another method .is to evaluate the force acting on the specimen placed in the gradient field as has been done in the static Faraday method. The first application to the pulsed field was done by Kapitza [1] who measured the force by using an electromagnetic oscillograph. We propose a different method of measuring the susceptibility in pulsed magnetic field gradient. The method is to observe the free oscillation of the specimen after a shot of pulsed magnetic field. Consider a paramagnetic or diamagnetic specimen which.is located near the end of the pulsed magnet where the field gradient dHz/dz is large. The z axis is defined along the axial direction of the vertical solenoid magnet. The force acting on the paramagnetic specimen is given by
The maximum gradient of the magnet (type 150(1L)60) [2] is approximately 90 kOe/cm with the intensity of 210 kOe when the maximum field 350 kOe is generated at the center. The obtained velocity after a shot of pulse field (w::::: 10 4 and a::::: 10 4 with a » 0.15 em-I) is of the order of crnys with the displacement less than 10- 3 em. Therefore it is not difficult to observe the free oscillation of the specimen at the maximum gradient position hung by a soft rubber string. After the pulsed field generation, the specimen begins to oscillate with the resonant frequency of the rubber string lower than 2 Hz and slowly damps. A video TV camera is used to monitor the amplitude of the oscillation II, which is proportional to the velocity i or Hg. Typical results of II
(1) where X is the susceptibility per unit volume. Neglecting the changes in Hz and dHz/dz due to the displacement of the specimen, motion of the specimen under an oscillating field Hz(t) ex H o sin w( is described by
/
~....•......•..
(2) where a and p are the coefficient of field gradient normalized by the maximum field Ho and the density of the specimen, respectively. The motion is calculated as shown in fig. 1. After one pulse shot (r = ';T/ w), the velocity and displacement are given by
t
~--
o z
(3),(4) Practical measurements are carried out in the Research Center for Extreme Materials of Osaka University. The pulsed magnet with a single-layer winding can produce a strong field gradient near the ends of the coil.
o
TYw
Fig. 1. Magnetic field generation and the associated motion of the specimen at the field gradient.
0304-8853/90/503.50 © 1990 - Elsevier Science Publishers B.Y. (North-Holland) and Yamada Science Foundation
784
T. Yosida et al. / High sensitive magnetometer using a pulsed magnet
h Cern J
5 4
3
Z CuS04' 5HP
o
3
2
m.2.89
1---+---1--
--I
4
5
6
xl0 4
HaZ C(kOe)2] Fig. 2. Amplitude of free oscillation vs.
HJ. Two paramagnetic samples are illustrated.
vs. HJ for paramagnetic crystals (MnCI 2 • 4H 20 : X = 1.6 X 10- 4 emuycrn', p = 2.0 g./cm', CUS04' 5H 20: X = 1.4 X 10- 5 ernuycrrr', p = 2.3 g/cm3 ) are shown in fig. 2. In case of diamagnetic specimens, the repulsive movement just after a pulse is observed in this setup. The sensitivity is estimated as 2 X 10- 6 emuycrrr' with Ho of 350 kOe. A high sensitive magnetometer can be designed by extending the present method. The most important point is to use a long duration pulse-magnet. The amplitude of the free oscillation after the shot is proportional to i and is enhanced by decreasing w as seen in eq. (3). However, the maximum field H o which is included in a in eq. (3), necessarily decreases as the pulse width increases because the total energy of the capacitor bank is limited. Considering these conditions, the following system is planned and under construction. The capacitor bank, composed of 900 electrolytic capacitors, stores energy of 375 kJ with the capacitance of 0.68 F charged to 1050 V. A thyristor is used as the switching element. The pulsed magnet is a multi-layer type to get a larger inductance of about 0.68 mll, The solenoid, consisted of the hard copper wire with the rectangular cross section of 3.2 X 2.2 mm", has a size of 85 mm in bore diameter and 100 mm in length. When the solenoid is used in liquid nitrogen, the system is expected to generate the semi-sinusoidal pulse with maximum magnetic field of 140 kOe with duration of 0.65 s (w '" 5). The
maximum field gradient is expected to be about 13 kOe/cm and the intensity 100 kOe (0'" 0.075). The maximum sensitivity expected in the susceptibility measurement is 1O- t 2 ernuycrrr' which is about 100 times higher than that of the SQUID magnetometer. When such a high sensitivity is achieved, the important problem is how to remove the background signals coming from the magnetic susceptibility due to the equipment. This may be done by making a magnetic bridge balance in the suspended network of the specimen. In conclusion, we have proposed the new method to measure the susceptibility and shown the preliminary results and experimental system. Improvements to the magnetometer with sensitivity of 10- 12 emuycnr' are also proposed.
The authors would like to thank Mr. T. Terao for the assistance of the experiments.
References [1) P. Kapitza, Proc. Roy. Soc. (London) A131 (1931) 224, 243. (2) A. Yamagishi and M. Date, in: High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983) p. 289.
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 783-784 North-Holland
High sensitive magnetometer using a pulsed magnet T. Yosida, A. Yamagishi
a
and M. Date
b
Low Temperature Center, Osaka University, Toyonaka 560. Japan Q
b
The Research Center for Extreme Materials, Osaka University, Toyonaka 560. Japan Department of Physics, Faculty of Science, Osaka University, Toyonaka 560, Japan
Strong field gradient produced by a pulsed magnet is applied to the magnetic susceptibility measurement. The principle and practical applications are shown by using paramagnetic materials. An extension to get high sensitivity with the accuracy of lO- t2 emu/em) is discussed .
Much' work has been reported on the magnetic susceptibility measurements under pulsed magnetic field. Principle of the method is to measure the differential magnetization dM/dH by using a pick-up coil placed in the uniform field region. Another method .is to evaluate the force acting on the specimen placed in the gradient field as has been done in the static Faraday method. The first application to the pulsed field was done by Kapitza [1] who measured the force by using an electromagnetic oscillograph. We propose a different method of measuring the susceptibility in pulsed magnetic field gradient. The method is to observe the free oscillation of the specimen after a shot of pulsed magnetic field. Consider a paramagnetic or diamagnetic specimen which.is located near the end of the pulsed magnet where the field gradient dHz/dz is large. The z axis is defined along the axial direction of the vertical solenoid magnet. The force acting on the paramagnetic specimen is given by
The maximum gradient of the magnet (type 150(1L)60) [2] is approximately 90 kOe/cm with the intensity of 210 kOe when the maximum field 350 kOe is generated at the center. The obtained velocity after a shot of pulse field (w::::: 10 4 and a::::: 10 4 with a » 0.15 em-I) is of the order of crnys with the displacement less than 10- 3 em. Therefore it is not difficult to observe the free oscillation of the specimen at the maximum gradient position hung by a soft rubber string. After the pulsed field generation, the specimen begins to oscillate with the resonant frequency of the rubber string lower than 2 Hz and slowly damps. A video TV camera is used to monitor the amplitude of the oscillation II, which is proportional to the velocity i or Hg. Typical results of II
(1) where X is the susceptibility per unit volume. Neglecting the changes in Hz and dHz/dz due to the displacement of the specimen, motion of the specimen under an oscillating field Hz(t) ex H o sin w( is described by
/
~....•......•..
(2) where a and p are the coefficient of field gradient normalized by the maximum field Ho and the density of the specimen, respectively. The motion is calculated as shown in fig. 1. After one pulse shot (r = ';T/ w), the velocity and displacement are given by
t
~--
o z
(3),(4) Practical measurements are carried out in the Research Center for Extreme Materials of Osaka University. The pulsed magnet with a single-layer winding can produce a strong field gradient near the ends of the coil.
o
TYw
Fig. 1. Magnetic field generation and the associated motion of the specimen at the field gradient.
0304-8853/90/503.50 © 1990 - Elsevier Science Publishers B.Y. (North-Holland) and Yamada Science Foundation
784
T. Yosida et al. / High sensitive magnetometer using a pulsed magnet
h Cern J
5 4
3
Z CuS04' 5HP
o
3
2
m.2.89
1---+---1--
--I
4
5
6
xl0 4
HaZ C(kOe)2] Fig. 2. Amplitude of free oscillation vs.
HJ. Two paramagnetic samples are illustrated.
vs. HJ for paramagnetic crystals (MnCI 2 • 4H 20 : X = 1.6 X 10- 4 emuycrn', p = 2.0 g./cm', CUS04' 5H 20: X = 1.4 X 10- 5 ernuycrrr', p = 2.3 g/cm3 ) are shown in fig. 2. In case of diamagnetic specimens, the repulsive movement just after a pulse is observed in this setup. The sensitivity is estimated as 2 X 10- 6 emuycrrr' with Ho of 350 kOe. A high sensitive magnetometer can be designed by extending the present method. The most important point is to use a long duration pulse-magnet. The amplitude of the free oscillation after the shot is proportional to i and is enhanced by decreasing w as seen in eq. (3). However, the maximum field H o which is included in a in eq. (3), necessarily decreases as the pulse width increases because the total energy of the capacitor bank is limited. Considering these conditions, the following system is planned and under construction. The capacitor bank, composed of 900 electrolytic capacitors, stores energy of 375 kJ with the capacitance of 0.68 F charged to 1050 V. A thyristor is used as the switching element. The pulsed magnet is a multi-layer type to get a larger inductance of about 0.68 mll, The solenoid, consisted of the hard copper wire with the rectangular cross section of 3.2 X 2.2 mm", has a size of 85 mm in bore diameter and 100 mm in length. When the solenoid is used in liquid nitrogen, the system is expected to generate the semi-sinusoidal pulse with maximum magnetic field of 140 kOe with duration of 0.65 s (w '" 5). The
maximum field gradient is expected to be about 13 kOe/cm and the intensity 100 kOe (0'" 0.075). The maximum sensitivity expected in the susceptibility measurement is 1O- t 2 ernuycrrr' which is about 100 times higher than that of the SQUID magnetometer. When such a high sensitivity is achieved, the important problem is how to remove the background signals coming from the magnetic susceptibility due to the equipment. This may be done by making a magnetic bridge balance in the suspended network of the specimen. In conclusion, we have proposed the new method to measure the susceptibility and shown the preliminary results and experimental system. Improvements to the magnetometer with sensitivity of 10- 12 emuycnr' are also proposed.
The authors would like to thank Mr. T. Terao for the assistance of the experiments.
References [1) P. Kapitza, Proc. Roy. Soc. (London) A131 (1931) 224, 243. (2) A. Yamagishi and M. Date, in: High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983) p. 289.