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The 40-T facility of the University of Amsterdam
Physica B 155 (1989) SWXI North-Holland. Amsterdam
THE 40-T FACILITY L.W. ROELAND, Natuurkundig
OF THE UNIVERSITY
R. GERSDORF
Laboratorium,
Universiteit
OF AMSTERDAM
and W.C.M. van Amsterdam.
MATTENS
Valckenirrstraat
6S, 1018 XE
Amstudam.
The Netherlands
An overview is given of the high-field installation of the University of Amsterdam now in operation; this installation can generate fields up to 40 T, constant within 8 mT for about X0 ms. Recent improvements of the regulation of the field arc discussed, as well as some examples of recent measurements.
The high-field installation of the University of Amsterdam is of the semi-continuous type. It is continuous in the sense that a coil is energized by a rectifier, which is fed from the mains. The pulses are sufficiently long to allow the magnetic field to be electronically regulated. However, the pulses are so short that the heat which is generated cannot be removed; the coil heats up and the next pulse can only be made after it has been cooled to the initial temperature (25 K), which may take about 3 h. At the High Field Conference in Osaka in 1983 [l] an extensive description of the equipment was presented. The feedback system controlling the field has. however, been changed considerably in the last 5 years; the new. computerized version was presented in Boston at the MT-10 Conference in 1987 [2]. In this paper we discuss some new experiences we gained in the past year with this regulation system. Moreover, we present the results of measurements that are of current physical interest and which illustrate the experimental possibilities of the facility.
lates a number which is converted to a voltage, which is then used to regulate the rectifier in a feedback loop. In ref. 2 it has already been indicated that some parameters, used in the regulation algorithm mentioned above, do not behave entirely as expected. The “solution” presented in [2] was to use an empirical value for one of these parameters, based on the (predicted) resistance of the coil at that moment. A new situation has. however, been found where this method does not work. namely when regulating a linearly increasing held. Rather than trying to understand the detailed behaviour of the system, it was observed that a misadjustment of this parameter had, as a consequence, an undamped oscillation of the system with a typical frequency (75 Hz or 1.50 Hz). The onset of these oscillations is now detected by the computer, which immediately acts by decreasing or increasing one of the regulation parameters. In the next section a symmetrical pulse, going up to 29 T with 40 T/s and down again to zero field with the same slope, will be shown (fig. 1); this pulse owes its realization to this new mode of regulation.
2. Regulation
3. Experimental
1. Introduction
of the field
A particular pulse-shape (magnetic field as a function of time) is stored in the memory of a computer. During the pulse the actual field in the coil is compared with the stored reference values. From the differences the computer calcu0921-4526/89/$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
possibilities
and some results
In ref. 1 the experimental methods for measuring magnetization, magnetoresistance, Hall effect and the de Haas-van Alphen (dHvA) effect have been reviewed. In principle the methods have not been changed with the advent of the B.V.
L. W. Roeland
et al. I The Amsterdam
computerized version of the regulation. The electronic amplifiers use more advanced components. which has resulted in a lower noise level. Another new feature is that, except for the dHvA measurements, data are taken at every ignition, e.g. 300 times each second. Magnetization measurements
3.1.
Magnetizations are measured by integrating the output voltage of an elaborately compensated coil system. Recently, much research is being done on the high-T, superconductors. When possible, the magnetization measurements were done in a slowly ascending field, followed by a descending field with the same slope. In fig. 1 such a field pulse, going up to 29 T, is demonstrated, together with the magnetization of a sample of YBa,Cu,O, single crystals oriented with the c-axis along the field. These measurements will be presented more elaborately elsewhere at this conference [3].
3.2.
40-T facility
59
Magnetoresistance
and Hall effect
A conventional four-point measuring technique is used for magnetoresistance and the Hall effect measurements; recently measurements have been done on small single crystals of the high-T, superconducting compounds, the results will be published elsewhere [4]. In fig. 2 an example of a magnetoresistance measurement is shown. The sample is GdIn,, an antiferromagnet which undergoes a spin-flop transition at a field of 32T. A change in magnetoresistance is clearly visible [5]. The Hall voltage is measured in essentially the same way, only the voltage leads are connected in a different way. 3.3.
de Haas-van
Alphen
measurements
Since 1976 useful de Haas-van Alphen measurements have been made with the high-field installation. Since the dHvA signals are very
40
30
5-
_\ \ ‘\
8.ry0
I,,
c.11,, 5
,1/ 1. 0 Time in
I, 14:. seconds
5 iii:
2. 0
Fig. 1. The lower trace of this plot represents the direct recording of a field pulse, which rises linearly with 40T/s to 29T and then falls with -4OTis. The upper trace is the magnetization of a number of YBa,Cu,O, single crystals, oriented with the c-axis along the field. The magnetization is irreversible, characteristic of the superconducting state. The rectangular pulse is for calibration purposes and corresponds to 4.67 mA m’. At the end of this recording the sample, and with it the trapped flux, is pulled out of the measuring coil system. The temperature is 20 K.
\ OLJ’~““~.J..‘J 0 500
1000
t
1500
tmsl
Fig. 2. Longitudinal magnetoresistance of a single crystal of GdIn, at 4.2 K. The lower trace is the magnetic field, which is regulated to go linearly down from 38 T to zero. The upper curve is the output voltage of a conventional four-lead device. From magnetization measurements it is known that at 32 T a spin-flop transition occurs. This manifests itself clearly in the present data also. The rectangular pulse at the end of the resistance recording is for the calibration, and corresponds in the present case with switching off the current altogether.
60
L. W. Roeland
000
005
010 ttme
015 I”
et al.
0.20
seconds
Fig. 3. Part of a recording of a de Haas-van Alphen measurement of GdIn,. The almost straight line represents the magnetic field, which is very smooth because the current decays freely through a diode parallel to the magnet coil. The wobbly curve is the almost true dHvA signal. At about 30T the same spin-flop transition as in fig. 2 can be seen. The frequencies on both sides of the transition are different.
small the magnet cannot be used in a regulated mode. Instead the rectifier is switched off when a certain maximum field is reached, and the current is allowed to decay through a diode battery. The magnetic moment of a sample is detected by a simple, compensated coil system with a large number of windings. The output voltage is amplified by a special electronic device, which is insensitive to dc voltage and acts like an integ-
I’hr Amrterdarn
30-T futility
rator for the frequencies of interest. The data are digitized and stored at a much higher rate than for magnetization measurements, up to 50 000 points per second. The results can be analyzed in terms of the well-known theoretical expression for the dHvA amplitude derived by Lifschitz and Kosevitch. In fig. 3 part of a recent measurement on GdIn, at 1.35 K is shown. The almost straight line is the decaying field, the wobbly trace the dHvA signal, It has already been indicated that this compound has a spin-flop transition at 32T. It is interesting to see that the dHvA signal can be observed on both sides of the transition. The frequencies are different, a Fourier analysis gives for the main components 3016T and 1290T below the transition and 3070T and 1142 T above it [5].
References [ 11 R. Gersdorf, F. R. de Boer. J.C. Wolfrat, F.A. Muller and L.W. Roeland. in: High Field Magnetism. M. Date. ed. (North-Holland, Amsterdam, 1983). p. 277. L.W. Roeland. R. Gersdorf and W.C.M. Mattens. in: Proc. Mt-lb. (Boston, 1987). F.R. de Boer. J.N. Li. K. Kadowaki. A. Menovsky, Y.K. Huang, M.J.V. Menkcn. P.H. Kes and L.W. Roeland. Physica B 155 (1989) 136 (these Proceedings). R.J. Wijngaarden, K. Heeck, R. Griessen. A. Menovsky and M.J.V. Menken. in: Proc. HTSC-MMS (Interlaken. 1988). J. Miiller and J.L. Olsen. eds.. Physica C 15331SS (1988) 1329. A. Tal. to be published.
THE 40-T FACILITY L.W. ROELAND, Natuurkundig
OF THE UNIVERSITY
R. GERSDORF
Laboratorium,
Universiteit
OF AMSTERDAM
and W.C.M. van Amsterdam.
MATTENS
Valckenirrstraat
6S, 1018 XE
Amstudam.
The Netherlands
An overview is given of the high-field installation of the University of Amsterdam now in operation; this installation can generate fields up to 40 T, constant within 8 mT for about X0 ms. Recent improvements of the regulation of the field arc discussed, as well as some examples of recent measurements.
The high-field installation of the University of Amsterdam is of the semi-continuous type. It is continuous in the sense that a coil is energized by a rectifier, which is fed from the mains. The pulses are sufficiently long to allow the magnetic field to be electronically regulated. However, the pulses are so short that the heat which is generated cannot be removed; the coil heats up and the next pulse can only be made after it has been cooled to the initial temperature (25 K), which may take about 3 h. At the High Field Conference in Osaka in 1983 [l] an extensive description of the equipment was presented. The feedback system controlling the field has. however, been changed considerably in the last 5 years; the new. computerized version was presented in Boston at the MT-10 Conference in 1987 [2]. In this paper we discuss some new experiences we gained in the past year with this regulation system. Moreover, we present the results of measurements that are of current physical interest and which illustrate the experimental possibilities of the facility.
lates a number which is converted to a voltage, which is then used to regulate the rectifier in a feedback loop. In ref. 2 it has already been indicated that some parameters, used in the regulation algorithm mentioned above, do not behave entirely as expected. The “solution” presented in [2] was to use an empirical value for one of these parameters, based on the (predicted) resistance of the coil at that moment. A new situation has. however, been found where this method does not work. namely when regulating a linearly increasing held. Rather than trying to understand the detailed behaviour of the system, it was observed that a misadjustment of this parameter had, as a consequence, an undamped oscillation of the system with a typical frequency (75 Hz or 1.50 Hz). The onset of these oscillations is now detected by the computer, which immediately acts by decreasing or increasing one of the regulation parameters. In the next section a symmetrical pulse, going up to 29 T with 40 T/s and down again to zero field with the same slope, will be shown (fig. 1); this pulse owes its realization to this new mode of regulation.
2. Regulation
3. Experimental
1. Introduction
of the field
A particular pulse-shape (magnetic field as a function of time) is stored in the memory of a computer. During the pulse the actual field in the coil is compared with the stored reference values. From the differences the computer calcu0921-4526/89/$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
possibilities
and some results
In ref. 1 the experimental methods for measuring magnetization, magnetoresistance, Hall effect and the de Haas-van Alphen (dHvA) effect have been reviewed. In principle the methods have not been changed with the advent of the B.V.
L. W. Roeland
et al. I The Amsterdam
computerized version of the regulation. The electronic amplifiers use more advanced components. which has resulted in a lower noise level. Another new feature is that, except for the dHvA measurements, data are taken at every ignition, e.g. 300 times each second. Magnetization measurements
3.1.
Magnetizations are measured by integrating the output voltage of an elaborately compensated coil system. Recently, much research is being done on the high-T, superconductors. When possible, the magnetization measurements were done in a slowly ascending field, followed by a descending field with the same slope. In fig. 1 such a field pulse, going up to 29 T, is demonstrated, together with the magnetization of a sample of YBa,Cu,O, single crystals oriented with the c-axis along the field. These measurements will be presented more elaborately elsewhere at this conference [3].
3.2.
40-T facility
59
Magnetoresistance
and Hall effect
A conventional four-point measuring technique is used for magnetoresistance and the Hall effect measurements; recently measurements have been done on small single crystals of the high-T, superconducting compounds, the results will be published elsewhere [4]. In fig. 2 an example of a magnetoresistance measurement is shown. The sample is GdIn,, an antiferromagnet which undergoes a spin-flop transition at a field of 32T. A change in magnetoresistance is clearly visible [5]. The Hall voltage is measured in essentially the same way, only the voltage leads are connected in a different way. 3.3.
de Haas-van
Alphen
measurements
Since 1976 useful de Haas-van Alphen measurements have been made with the high-field installation. Since the dHvA signals are very
40
30
5-
_\ \ ‘\
8.ry0
I,,
c.11,, 5
,1/ 1. 0 Time in
I, 14:. seconds
5 iii:
2. 0
Fig. 1. The lower trace of this plot represents the direct recording of a field pulse, which rises linearly with 40T/s to 29T and then falls with -4OTis. The upper trace is the magnetization of a number of YBa,Cu,O, single crystals, oriented with the c-axis along the field. The magnetization is irreversible, characteristic of the superconducting state. The rectangular pulse is for calibration purposes and corresponds to 4.67 mA m’. At the end of this recording the sample, and with it the trapped flux, is pulled out of the measuring coil system. The temperature is 20 K.
\ OLJ’~““~.J..‘J 0 500
1000
t
1500
tmsl
Fig. 2. Longitudinal magnetoresistance of a single crystal of GdIn, at 4.2 K. The lower trace is the magnetic field, which is regulated to go linearly down from 38 T to zero. The upper curve is the output voltage of a conventional four-lead device. From magnetization measurements it is known that at 32 T a spin-flop transition occurs. This manifests itself clearly in the present data also. The rectangular pulse at the end of the resistance recording is for the calibration, and corresponds in the present case with switching off the current altogether.
60
L. W. Roeland
000
005
010 ttme
015 I”
et al.
0.20
seconds
Fig. 3. Part of a recording of a de Haas-van Alphen measurement of GdIn,. The almost straight line represents the magnetic field, which is very smooth because the current decays freely through a diode parallel to the magnet coil. The wobbly curve is the almost true dHvA signal. At about 30T the same spin-flop transition as in fig. 2 can be seen. The frequencies on both sides of the transition are different.
small the magnet cannot be used in a regulated mode. Instead the rectifier is switched off when a certain maximum field is reached, and the current is allowed to decay through a diode battery. The magnetic moment of a sample is detected by a simple, compensated coil system with a large number of windings. The output voltage is amplified by a special electronic device, which is insensitive to dc voltage and acts like an integ-
I’hr Amrterdarn
30-T futility
rator for the frequencies of interest. The data are digitized and stored at a much higher rate than for magnetization measurements, up to 50 000 points per second. The results can be analyzed in terms of the well-known theoretical expression for the dHvA amplitude derived by Lifschitz and Kosevitch. In fig. 3 part of a recent measurement on GdIn, at 1.35 K is shown. The almost straight line is the decaying field, the wobbly trace the dHvA signal, It has already been indicated that this compound has a spin-flop transition at 32T. It is interesting to see that the dHvA signal can be observed on both sides of the transition. The frequencies are different, a Fourier analysis gives for the main components 3016T and 1290T below the transition and 3070T and 1142 T above it [5].
References [ 11 R. Gersdorf, F. R. de Boer. J.C. Wolfrat, F.A. Muller and L.W. Roeland. in: High Field Magnetism. M. Date. ed. (North-Holland, Amsterdam, 1983). p. 277. L.W. Roeland. R. Gersdorf and W.C.M. Mattens. in: Proc. Mt-lb. (Boston, 1987). F.R. de Boer. J.N. Li. K. Kadowaki. A. Menovsky, Y.K. Huang, M.J.V. Menkcn. P.H. Kes and L.W. Roeland. Physica B 155 (1989) 136 (these Proceedings). R.J. Wijngaarden, K. Heeck, R. Griessen. A. Menovsky and M.J.V. Menken. in: Proc. HTSC-MMS (Interlaken. 1988). J. Miiller and J.L. Olsen. eds.. Physica C 15331SS (1988) 1329. A. Tal. to be published.