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Megagauss magnetic fields — generation and application to magnetism
.fournal
of Magnetism
and Magnetic
MEGAGAUSS N. MIURA,
Materials
MAGNETIC
T. GOTO,
Imtriute for Solid State Physm,
54-57
(19%)
1409
1409-1414
FIELDS - GENER4TION
K. NAKAO.
S. TAKEYAMA,
C’niwrsif~ of Tokw.
RoIpq/.
AND APPLICATION T. SAKAKIBARA
TO MAGNETISM
and F. HERLACH
*
MUUIIO-I&, Tokvo. Jopun
A review is presented on the recently constructed facilities for generating ultra-high magnetic fieldh in the megagauss range. These facilities include electromagnetic flux compression, direct discharge to a single turn coil and conventional non-deatructive pulse magnets. Applications of the high fields to studies of magnetism are discussed.
single-turn coil. The single-turn coil technique has the advantage that the system is so compact that experiments can be carried out very easily, but the available field range is limited to about 200 T. Moreover, the samples are not destroyed. Our new laboratory has also a non-destructive long duration pulse field facility using conventional copper wire-wound-magnets. Fields up to 45 T can be generated non-destructively. These are useful to perform precision measurements as subsidiary or preliminary measurements for megagauss experiments. The purpose of this paper is to present these facilities. techniques and some examples of their use for investigating the magnetic properties of solids.
1. Introduction High magnetic fields are a powerful means for investigating magnetic properties of solids. Much effort has been devoted to the generation of very high magnetic fields to extend the available field range since the pioneering work by Kapitza more than half a century ago [I]. So far, steady fields up to about 30 T are available with hybrid magnets [2] and pulsed fields close to 100 T with helical pulse magnets [3.4]. However, the generation of higher fields in the megagauss range (B > 100 T) is still difficult because of the coil destruction by the large electromagnetic forces. Different techniques have been developed for generating megagauss fields. Fowler et al. first reported the generation of fields up to 1400 T (14 MC) by explosive-driven flux compression technique [5]. A magnetic field in excess of 100 T will invariably destroy the structures in which it is generated and confined. although the degree of destruction may vary from one method to another. As a consequence, the duration of the field pulse is limited to a few microseconds or even much less. Moreover. the field volume is limited by the amount of energy available from a fast pulsed power source. For these and other reasons, it has not been easy to develop experiments with megagauss fields in solid state physics. However, by carefully designing proper measuring systems. many kinds of interesting solid state experiments can be performed with excellent accuracy in megagauss fields [6]. Recently. we have constructed a new magnet laboratory where megagauss fields are conveniently obtained for solid state physics research. Two different methods are employed for generating the megagauss fields. One is the electromagnetic flux compression technique which we have developed from our old system [6]. This technique is suitable for producing extremely high fields. Ry installing a large scale condenser bank (5 MJ). we are aiming to produce fields up to 500-1000 T. Our second technique is a direct fast current discharge into a
2. Electromagnetic flux compression Cnare was the first who reported megagauss field generation by electromagnetic flux compression [7]. Further development of this technique has been made at ISSP for practical application to solid state physics [6,8]. Fig. 1 shows the principle of electromagnetic flux compression. A large pulsed primary current is supplied from the main condenser bank. A secondary current is induced in the liner placed inside the primary coil, and
* Permanent
and present address: Katholieke Univeraiteit Leuven, Lahoratorium voor Lage Temperaturen en HogeVeldenfysika, Celestijnenlaan 200 D, B-3030 Leuven, Belgium.
0304-X853/86/$03.50
ej Elsevier Science
Publishers
Fig. 1. Schematic diagram compression coil system.
B.V.
showing
the electromagnetic
flux
it is squeezed
inward
by a large
repulsive
electromag-
This motion of the liner compresses the magnetic flux initially injected by an outer pair of injection coils. and the magnetic field at the centre becomes very high when the diameter of the liner becomes sufficiently small. In this method, the electric energy stored in the main condenser bank is converted to kinetic energy. compressing the liner. and then to magnetic field energy successively. Because of the limited conversion efficiency. a large condenser bank is required. A fairly large subcondenser bank is also necessary to produce the seed field in the large Initial volume. We have succeeded in producing fields up to 2X0 T using the main condenser bank with a stored energy of 2X5 kJ and a subcondenser bank of 72 kJ [6]. In our new laboratory, the main condenser bank of 5 MJ and a subcondenser bank of 1.5 MJ were installed. Fig. 2 illustrates the new condenser bank and coil systems [9]. The 5 MJ bank consists of 5 units: each of these stores 1 MJ of energy. when charged to 40 kV. Two coil systems were constructed for experiments with different energy requirements. One of these (coil system I) is connected to a 1 MJ unit of the bank. the other 4 MJ is usually connected to another coil system (II); this can he increased to 5 MJ by connecting all units together. The I .5 MJ subcondenser bank can be employed for both coil systems by switching connections. The system is operated by using two independent control panels. A maximum current of up to 6 MA can he obtained from the main bank. A total of 120 pressurized air gap switches are used for starting the primary current. Another 120 air gap switches are used to clamp the current to absorb the unused energy after the current maximum. The primary coil is connected to the condenser bank by means of 240 coaxial cables. The suhnetic
force.
condenser bank uses ignitron switches, 5 to start and 20 for a crowbar circuit. The coil systems are enclosed in steel chambers to protect the measuring apparatus from the scattered liner fragment\. to shield electrical measuring instruments form excessive noise and to confine the sound from the liner explosion. For a 1 MJ experiment, the primary coil is made of an iron plate with a hole at the center and slits for guiding the current [6]. This type of primary coil structure is very strong and convenient to clamp from every side to avoid deformation. which would otherwise be caused by the current pulse. A typical example of the field pulse ix shown in fig. 3. A copper liner 80 mm in outer diameter, 25 mm in width and 2 mm in thickness wax employed. The magnetic field reached 220 T at its maximum and started to decrease until the probe was broken by the impact of the liner at the point shown by the arrow. This phenomenon is called the turn-around effect. It takes place if the liner movement is slowed down due to the high magnetic pressure inside. The turn-around of the field is actually caused by the field diffusion into the liner in relation to the particle speed of shock wave generated within the liner [IO]. Therefore, the maximum field is mainly determined by the liner velocity. The liner velocity can he measured by taking framing pictures of the liner motion with :I high speed camera. In fig. 3 the ratio of the tine1 radius I’ to its initial value (1 is plotted. From such measurements. the liner velocity is estimated to be 1.0 km/s. When thinner liners are used. the velocity iticreases. Fig. 3 also shows the experimental trace ol Faraday rotation in GaP. While the magnetic field is
^^ 4c
Magnetlc t
0
Primary
IO
25
30 Time
Fig. 2. Block-diagram of the condenser bank and the coil hyhtemh for the electromagnetic flux compression field generation techmque using a 5 MJ main bank and a 1.5 MJ subbank.
F!eld
Current
40
50
60
( ps)
Fig. 3. Waveforms of the primary current and the magnetic field generated by electromagnetic flux compression with the I MJ bank. The optical signal of the Faraday rotation m GaP i\ also shown. It was measured at 632.8 nm *avelength for a 0.287 mm thick sample. The open circles denote the ratio of the liner radius r to its initial value a. measured by high speed photography. The time scale i5 changes at I = 25 ps. The arrn\h indicates the break point where the pick-up prohe wa\ broken by the impact of the liner.
N. MIWU et al. / Megugauss
usually measured by integrating the voltage induced in a pick-up coil, Faraday rotation is a useful secondary means to calibrate the generated field, because the rotation angle is proportional to the magnetic field. The magnetic field measured by a pick-up probe and the Faraday rotation are in good agreement with each other with an accuracy better than l%, as is shown in fig. 3. The reproducibility of the field generation is excellent. For 4 MJ or 5 MJ experiments, the use of a primary coil made of an iron plate as used in the 1 MJ experiment is no longer adequate, because the primary current is so large (4-6 MA) that the clamping system for the primary coil cannot endure the strong electromagnetic force. The clamping system itself will be destroyed every time. So we are using a primary coil with a shape as shown in fig. 1. The idea of the coil design is essentially the same as that of the single-turn coil system described in the next section. We do not clamp the coil except for the wide flat part at the electrodes, and we let it expand freely during the shot. The electrodes are connected to the collector plates with a contact pressure supplied by a hydraulic press. The explosion of the primary coil is stopped by a massive steel box which encloses the coil. With such a device, we can safely supply a current up to 4.4 MA to the primary coil. The energy conversion efficiency using this type of primary coil was found to be much better than using one made of iron plates because of the more concentrated current path. With this system the liner velocity can reach 2.5 km/s. Experiments are in progress to achieve the highest possible field using this system.
magnetic fields
0
1411
4
2 Time
6
8
(fis)
Fig. 4. An example of current I and magnetic field B waveforms generated by the single turn coil technique. The value of B/I is also shown as a function of time. The inset shows schematically the principle of the single-turn coil technique.
position by a hydraulic press. The clamping system provides a straight current path from the collector to the coil. To catch the fragments of the coils and to protect
Insulating
3. Single-turn coil system In addition to the electromagnetic flux compression, another technique is employed for generating megagauss fields. That is direct discharge of a fast current to a single-turn coil, as schematically shown in the inset in fig. 4. This technique relies on a fast current discharge which allows the megagauss field generation before the coil destruction. The single-turn coil technique was first attempted by Furth et al. in 1957 [ll]. Later, Herlach et al. built a laboratory instrument for practical use in various experiments, using small thin-walled coils [12]. We built an improved and larger megagauss generator of this type for general use in solid state experiments u31. For this method, a very fast condenser bank is required to provide a fast discharge. A 40 kV, 100 kJ condenser bank with 20 pressurized air gap switches was installed. The internal inductance of the bank is 18 nH, and the condenser bank delivers 2.78 MA with a rise time of 2.42 ~LS(quarter period) when short-circuited at the terminal where the coils are clamped. The design of the coil system is shown in fig. 5. The single-turn coil is made by cutting and bending copper sheet of 3 mm thickness. The coil is firmly clamped in
Fragment catchlyg
box
:or
Fig. 5. Drawing of the coil and its supporting system for the single-turn coil system. A perspective view of the coil is shown in the inset.
the other parts of the device, two nose pieces on the clamping device and a steel box with a wood lining are set around the coil. The entire coil system including all these devices is mounted in a sturdy steel compartment, A maximum field of 150 T is obtained when we use a coil with 10 mm inside diameter d, and 10 mm axial length 1. This is our standard coil size and convenient for measurements at low temperatures. As the coil size is reduced, higher magnetic fields are obtained. Examples of the attainable maximum fields with various sizes are as follows: 184 T for d, = 8 mm, I= 8 mm: 201 T ford,=6mm./=7mm;246Tfor~l,=4.1mm./=6 mm: and 263 T for d, = 2 mm, I = 2 mm. Observed waveforms of the current and field pulses are shown in fig. 4. The maximum of the magnetic field occurs earlier than that of the current because of deformation of the coil. The change of field current ratio. B/I, is shown as a function of time in fig. 4. It should be noted that the curves for R and I are fairly smooth without any kinks or breaks related to the destruction of the coil. Also. no noise related to the coil destruction appears in any of the solid state measurements. While the field increases roughly linearly as a function of the coil diameter. the maximum field is limited to 263 T which is obtained for d, = 2 mm. For smaller coils. the peak field is even lower. When we used tantalum as a coil material. slightly better results were obtained. The highest field we obtained with this system was 280 T using a Ta coil with d, = 3 mm and I = 4.1 mm. The field homogeneity of the standard coil (d, = 10 mm) is reasonably good. At maximum field. the field strength falls to 98.6% and to 93.8% of the optimal value at the centre if off-set in the axial direction by 1 and 2 mm, respectively. For low temperature experiments, samples are mounted in a sample holder made of double thin glass or phenolic tubes placed in a vacuum space in the field. By flowing a large amount of liquid helium through the gap between the tubes, the sample temperature can be lowered to about 4 K. Such low temperature experiments can be performed with the coil diameter down to d, = 6 mm by which fields up to 200 T can be obtained. A remarkable advantage of this method is that the samples are not destroyed in each experiment if they are protected properly. because the explosion of the coil occurs only in the outward direction. The sample holders and pick-up coils for the field measurement are not destroyed either. Therefore we can repeat measurements on one sample many times. Furthermore. measurements can be done twice in one pulse in both rising and decreasing fields. These characteristics ensure the reliability of the data. As compared with the electromagnetic flux compression method. the system is more compact. so that we can carry out experiments fairly easily and quickly. although the available field is limited to a few megagauss.
4. Non-destructive
sub-megagauss fields
For pursuing solid state experiments in megagauss fields, subsidiary measurements in the non-destructive field range are often required. To this end. non-destructive magnetic fields up to 45 T are generated using conventional pulse magnets with a long pulse duration. The design of the magnet is illustrated in fig. 6. Copper wire containing Cr is wound on a FRP bobbin and it ~j impregnated with epoxy resin. The winding is mounted in :I stainless steel cylinder. and placed inside :I cryostat with an operating temperature of 77 K. Samples can be cooled to as low as 1.6 K with this syatcm. Two condenser banks with a stored energy of 200 and 1 I2 k.1 are installed for supplying current to the magnets. The 200 kJ bank can bc utilized as either ;I IO kV. 4 mF bank or as a 5 kV, 16 mF bank by switching the scrics/parallel mode of operation [Y]. The pulse duration and the maximum field can be varied depending on the capacitance of the condenser bank. as well as the magnet sire which determines the inductance. Typical waveforms of the fields are shown in fig. 6 (curves :I and b). For a pulse width of 3 IIIS. a maximum field 01 45 7 can bc obtained, whereas for 20 ms. 37 T cm bc obtained. The 112 kJ bank. on the other hand. can bc utilized to generate a flat-top pulse as shown by curve c in fig. 6. The condensers
are divided
is connected
to an
inductor
into
five units;
to form
each
of these
a pulse-forming-
By adjusting the capacitance and inductance of each unit and also the load inductance, the flat top
network. part
of
the
pulse
can
be
made
fairly
long
(I.4
tm
allowing 2% change). This arrangement is useful to obtain a constant field for some period or a rapid field decay
in comparison
Time
0
1
to the whole
field
duration.
(ms)
2
3
4
Time
(ms)
I
5
6
-T
Fig. 6. Various waveforms of non-destructive pulse fields generated by conventional copper wire magnet\. A crash-\cctional \iew of the magnet is shown in the inset.
N. M~uru et al. / Megaguuss
5. Application of megagauss fields to magnetism Extremely high magnetic fields as described in previous sections open up various new possibilities in solid state physics. Since electronic states in solids are greatly altered by ultra-high fields, we may expect many new phenomena to occur including various types of phase transitions. For example, a magnetic field of 500 T makes the spin Zeeman splitting gp,H as large as 670 K for g = 2. It may exceed various internal fields acting in solids. Technically, there are some limitations in performing solid state measurements due to the short pulse duration and the small space available. In addition, the sample heating due to the adiabatic change in the magnetization must be taken into account [14]. This is not as critical as eddy current heating in conducting materials because it is a bulk effect and it is reversible. However. the variety of possible experiments is more than one would first imagine. Optical measurements provide the most convenient techniques. They avoid the problem of large field induced voltages which occur in the sample wires of electrical and magnetic measurements. By using streak spectrometers, we can obtain a whole optical spectrum in the visible range in one pulse [15]. The magnetization processes in magnetic materials can be studied by Faraday rotation [16]. In conducting substances, we can obtain information on the transport properties by measuring magneto-absorption or magnetoreflection using lasers in the far infrared [S]. Although it is much more difficult to perform magnetic and electrical measurements, ac susceptibility [17] or ac conductivity [18] measurements are possible in the megagauss range if frequencies in the 30-150 MHz range are used. Examples of such measurements are given in references cited above. In this paper, recent results of the Faraday rotation in various iron garnets using the singleturn coil system will he shown. Iron garnets are ferrimagnetic suhstances, with molecular fields of the order of magagauss. Consequently, if we apply a sufficiently high field, these materials undergo magnetic phase transitions. In YIG, for example, two transitions take place. first from the ferrimagnetic to the spin canted phase, and then from the spin canted to the fully aligned spin phase successively. Iron garnets are transparent in the near infrared range. so we can observe such phase transitions by measuring the Faraday rotation. Fig. 7 shows the Faraday rotation in several iron garnets in fields up to 200 T. In YIG, the Faraday rotation is almost a linear function of magnetic field, which is consistent with previous data [19]. Although the line has a considerable gradient, it shows no kinks up to 200 T because the material stays in the ferrimagnetic phase at T = 170 K. In other substances. anomalies are observed at fields shown by arrows. Most of the anomalies correspond to the transition from the ferrimagnetic to the spin canted
1413
magnetic jtelds
Magnetic
Field(T)
-500
Fig. 7. The Faraday rotation angle as a function of magnetic field for various iron garnet crystals. The magnetic field direction with respect to the crystal axes and temperature are shown for each curve. Measurement was performed at 1.15 pm wavelength using a He-Ne laser.
phase. In ErIG, when a magnetic field is applied parallel to the (111) axis, a new type of transition was found as a discontinuous jump of the Faraday rotation. This jump corresponds to a magnetization jump, and is in sharp contrast to a ferrimagnetic-spin canted phase transition which only gives rise to a change of the slope. This phenomenon is believed to arise from the highly anisotropic exchange interaction of Er ions [20]. In DyIG, another kind of anomaly is observed at a higher field in addition to the usual spin-canted phase transition. Details of these results will be reported elsewhere. In the non-destructive submegagauss field range, a variety of accurate measurements can be performed. As an example of such measurements, fig. 8 shows the magnetization and the magnetoresistance in a semimagnetic semiconductor Hg, _,_,Cd,rMn.VTe. In both measurements. anomalies are observed at an identical magnetic field shown by the arrows. In this substance, Mn” ions are responsible for the dilute magnetism. The Mn ions with spin S = 5/2 are distributed randomly, and some of them form clusters occupying nearestneighbour sites with each other [21,22]. By an antiferromagnetic exchange interaction between a pair of nearest neighbour ions, the spin state takes the ground state S,,,, = 0. and this causes a reduction of the apparent saturation moment. At high magnetic fields, level crossing occurs between higher lying Zeeman split levels S,,,, = 1. 2,. . ,5, and this causes a step-wise increase of the magnetization at each crossing point. The observed anomaly in fig. 8 is actually the first step of this kind at a field where the S,,,,, = 0 state crosses with the SPalr = 1; Si = - 1 state. From this transition, the exchange
H%,_,Cd,MnyTe
6
-
III I’. Kapitza. Proc. Roy. Sot. A105 (1924) 691.. All5 (1927) 65X. Ill Very recently, a field up to 30.7 T was achieved at Tohohu University. Sendai. Japan with a hybrid magnet. 131 S. Foner and H.H. Kolm. Rev. Sci. Instr. 2X (1957) 799. (41 A. Yamaglshi and M. Date. Hugh Field Magnetism. ed. M. Date (North-Holland. Amsterdam. 1983) p. 2X9. 151 C.M. Fouler, W.B. Cram and R.S. Caird. J. 4ppl. Phy\. ?I
( 1960)58X. [61 N.
.~~
0
IO Magnettc
20 Field
30 (T)
.-J0 40
Fig.
X. Magnetization M and magnetoresistance Ap/p,, in Hg,m, tCd, Mn,Te (x = 0.027. .L’= 0.009) as a function of magnetic field. The measurement was performed in a non-destructive pulse field. At the field indicated by arrows, both M and Ap/p,, show anomalies. In the top trace, the vertical scale for M is magnified by a factor of 10 to clarify the details of the anomaly.
constant can he estimated. In other samples the second step was also observed at higher fields. It is of considerable interest to determine the nature of the mechanism that is responsible for the anomalous conductivity change at the cross-over. In addition, further study is required to determine the band gap dependence of the antiferromagnetic exchange interaction and also to clarify the clustering properties of Mn ions, by observing each successive step at higher fields. To summarize, we have constructed an ultrahigh magentic field laboratory where several different types of megagauss magnets are routinely available for basic solid state physics research. Using novel extremely high magentic field generation techniques, we have found many new and interesting phenomena. Development of even higher fields is in progress. This new facility promises to provide a rich source of new data on the fundamental properties of condensed matter. interaction
Mlura, G. Kldo, M. Akihiro and S. Chikazuml. J. Magn. Magn. Mat. 11 (1979) 275. 171 b..C. C‘nare. J. Appl. Phys. 37 (1966) 3X12. [Xl N. Miura. G. Kido and S. Chikazumi. Application of tllgh Magnetic Fields in Semiconductor Physich. ed. G. Landhrhr (Springer-Verlag. Berlin. 1983) p. 505. K. Nakao and S. PI N. Miura. G. Kido, H. MiyaJima. C‘hikarumi. Physics in High Magnetic Fields. eds. S. C‘hikazumi and N. Miura. (Springer-Verlag, Berlin. 19X1) p. 64. [lOI N. Miura and F. Herlach, Strong and Ultrastrong Magnetlc Fields. ed. F. Herlach (Springer-Verlag. Berlin, 19X5) p. 247. 1111 H.P. Furth. M.A. Levinr and R.W. Waniek. Rev. SCI. Instr. 28 (1957)949. (121 F. Herlach and R. McBroom. J. Phys. t 6 (1973) 652. [I31 K. Nakao. F. Herlach, T. Goto. S. Takeyama. -1. Sakakibara and N. Mlura. J. Phys. E (to be published. 19X5). (141 T. Sakakibara et al.. High Field Magnetism. cd. M. Date (North-Holland. Amsterdam. 19X?) p. 167. J. llSl <;. Kido. N. Miura. H. Katayama and S. (‘hikarumi. Phys. E 14 (1981) 349. 1161 M. Suekane, G. Kido. N. Miura. and S. C‘hlkaruml. .I. Magn. Magn. Mat. 31-34 (1983) 773. 1171 S. Chikar.umi. N. Miura. G. Kido and M. Akihiro. lEEI_, Tranh Mag. MAG-14 (1978) 577. [IX1 N. Miura. T. Osada and T. Goto. Proc. 17th Intern. C‘onf. Physics of Semiconductors. eds. J.D. (‘hadi and &‘.A. Harrison (Springer-Verlag. Berlin 19X5) p. 973. Cl91 N. Miura. G. Kido. I. Oguro. K. Kawauchi. S. C‘hika/uml. J.F. Dillon. Jr. and L.C. Van Uitert; Physica H6&XXB (1977) 1219. l2Ol K. Nakao, T. Goto and N. Miura. J. Magn. Magn. Mat. 54-57 (19X6). Pll Y. Shapira. S. Foner. D.H. Ridgley, K. Dulght and A Weld. Phyh. Rec. 830 (19X4) 4021. R.L. Aggarwal. S.N. Jasperson, Y. Shapira. S. Foner. ‘1 WI Skakibara. T. Goto. N. Miura. K. Dwight and A. Weld. f’roc. 17th Intern. Conf. Physic\ of Semiconductors. vol. I.. cds. J.D. Chadi and W.A. Harrison (Springer-Vertag. Berlin. 19X5) p. 1419.
of Magnetism
and Magnetic
MEGAGAUSS N. MIURA,
Materials
MAGNETIC
T. GOTO,
Imtriute for Solid State Physm,
54-57
(19%)
1409
1409-1414
FIELDS - GENER4TION
K. NAKAO.
S. TAKEYAMA,
C’niwrsif~ of Tokw.
RoIpq/.
AND APPLICATION T. SAKAKIBARA
TO MAGNETISM
and F. HERLACH
*
MUUIIO-I&, Tokvo. Jopun
A review is presented on the recently constructed facilities for generating ultra-high magnetic fieldh in the megagauss range. These facilities include electromagnetic flux compression, direct discharge to a single turn coil and conventional non-deatructive pulse magnets. Applications of the high fields to studies of magnetism are discussed.
single-turn coil. The single-turn coil technique has the advantage that the system is so compact that experiments can be carried out very easily, but the available field range is limited to about 200 T. Moreover, the samples are not destroyed. Our new laboratory has also a non-destructive long duration pulse field facility using conventional copper wire-wound-magnets. Fields up to 45 T can be generated non-destructively. These are useful to perform precision measurements as subsidiary or preliminary measurements for megagauss experiments. The purpose of this paper is to present these facilities. techniques and some examples of their use for investigating the magnetic properties of solids.
1. Introduction High magnetic fields are a powerful means for investigating magnetic properties of solids. Much effort has been devoted to the generation of very high magnetic fields to extend the available field range since the pioneering work by Kapitza more than half a century ago [I]. So far, steady fields up to about 30 T are available with hybrid magnets [2] and pulsed fields close to 100 T with helical pulse magnets [3.4]. However, the generation of higher fields in the megagauss range (B > 100 T) is still difficult because of the coil destruction by the large electromagnetic forces. Different techniques have been developed for generating megagauss fields. Fowler et al. first reported the generation of fields up to 1400 T (14 MC) by explosive-driven flux compression technique [5]. A magnetic field in excess of 100 T will invariably destroy the structures in which it is generated and confined. although the degree of destruction may vary from one method to another. As a consequence, the duration of the field pulse is limited to a few microseconds or even much less. Moreover. the field volume is limited by the amount of energy available from a fast pulsed power source. For these and other reasons, it has not been easy to develop experiments with megagauss fields in solid state physics. However, by carefully designing proper measuring systems. many kinds of interesting solid state experiments can be performed with excellent accuracy in megagauss fields [6]. Recently. we have constructed a new magnet laboratory where megagauss fields are conveniently obtained for solid state physics research. Two different methods are employed for generating the megagauss fields. One is the electromagnetic flux compression technique which we have developed from our old system [6]. This technique is suitable for producing extremely high fields. Ry installing a large scale condenser bank (5 MJ). we are aiming to produce fields up to 500-1000 T. Our second technique is a direct fast current discharge into a
2. Electromagnetic flux compression Cnare was the first who reported megagauss field generation by electromagnetic flux compression [7]. Further development of this technique has been made at ISSP for practical application to solid state physics [6,8]. Fig. 1 shows the principle of electromagnetic flux compression. A large pulsed primary current is supplied from the main condenser bank. A secondary current is induced in the liner placed inside the primary coil, and
* Permanent
and present address: Katholieke Univeraiteit Leuven, Lahoratorium voor Lage Temperaturen en HogeVeldenfysika, Celestijnenlaan 200 D, B-3030 Leuven, Belgium.
0304-X853/86/$03.50
ej Elsevier Science
Publishers
Fig. 1. Schematic diagram compression coil system.
B.V.
showing
the electromagnetic
flux
it is squeezed
inward
by a large
repulsive
electromag-
This motion of the liner compresses the magnetic flux initially injected by an outer pair of injection coils. and the magnetic field at the centre becomes very high when the diameter of the liner becomes sufficiently small. In this method, the electric energy stored in the main condenser bank is converted to kinetic energy. compressing the liner. and then to magnetic field energy successively. Because of the limited conversion efficiency. a large condenser bank is required. A fairly large subcondenser bank is also necessary to produce the seed field in the large Initial volume. We have succeeded in producing fields up to 2X0 T using the main condenser bank with a stored energy of 2X5 kJ and a subcondenser bank of 72 kJ [6]. In our new laboratory, the main condenser bank of 5 MJ and a subcondenser bank of 1.5 MJ were installed. Fig. 2 illustrates the new condenser bank and coil systems [9]. The 5 MJ bank consists of 5 units: each of these stores 1 MJ of energy. when charged to 40 kV. Two coil systems were constructed for experiments with different energy requirements. One of these (coil system I) is connected to a 1 MJ unit of the bank. the other 4 MJ is usually connected to another coil system (II); this can he increased to 5 MJ by connecting all units together. The I .5 MJ subcondenser bank can be employed for both coil systems by switching connections. The system is operated by using two independent control panels. A maximum current of up to 6 MA can he obtained from the main bank. A total of 120 pressurized air gap switches are used for starting the primary current. Another 120 air gap switches are used to clamp the current to absorb the unused energy after the current maximum. The primary coil is connected to the condenser bank by means of 240 coaxial cables. The suhnetic
force.
condenser bank uses ignitron switches, 5 to start and 20 for a crowbar circuit. The coil systems are enclosed in steel chambers to protect the measuring apparatus from the scattered liner fragment\. to shield electrical measuring instruments form excessive noise and to confine the sound from the liner explosion. For a 1 MJ experiment, the primary coil is made of an iron plate with a hole at the center and slits for guiding the current [6]. This type of primary coil structure is very strong and convenient to clamp from every side to avoid deformation. which would otherwise be caused by the current pulse. A typical example of the field pulse ix shown in fig. 3. A copper liner 80 mm in outer diameter, 25 mm in width and 2 mm in thickness wax employed. The magnetic field reached 220 T at its maximum and started to decrease until the probe was broken by the impact of the liner at the point shown by the arrow. This phenomenon is called the turn-around effect. It takes place if the liner movement is slowed down due to the high magnetic pressure inside. The turn-around of the field is actually caused by the field diffusion into the liner in relation to the particle speed of shock wave generated within the liner [IO]. Therefore, the maximum field is mainly determined by the liner velocity. The liner velocity can he measured by taking framing pictures of the liner motion with :I high speed camera. In fig. 3 the ratio of the tine1 radius I’ to its initial value (1 is plotted. From such measurements. the liner velocity is estimated to be 1.0 km/s. When thinner liners are used. the velocity iticreases. Fig. 3 also shows the experimental trace ol Faraday rotation in GaP. While the magnetic field is
^^ 4c
Magnetlc t
0
Primary
IO
25
30 Time
Fig. 2. Block-diagram of the condenser bank and the coil hyhtemh for the electromagnetic flux compression field generation techmque using a 5 MJ main bank and a 1.5 MJ subbank.
F!eld
Current
40
50
60
( ps)
Fig. 3. Waveforms of the primary current and the magnetic field generated by electromagnetic flux compression with the I MJ bank. The optical signal of the Faraday rotation m GaP i\ also shown. It was measured at 632.8 nm *avelength for a 0.287 mm thick sample. The open circles denote the ratio of the liner radius r to its initial value a. measured by high speed photography. The time scale i5 changes at I = 25 ps. The arrn\h indicates the break point where the pick-up prohe wa\ broken by the impact of the liner.
N. MIWU et al. / Megugauss
usually measured by integrating the voltage induced in a pick-up coil, Faraday rotation is a useful secondary means to calibrate the generated field, because the rotation angle is proportional to the magnetic field. The magnetic field measured by a pick-up probe and the Faraday rotation are in good agreement with each other with an accuracy better than l%, as is shown in fig. 3. The reproducibility of the field generation is excellent. For 4 MJ or 5 MJ experiments, the use of a primary coil made of an iron plate as used in the 1 MJ experiment is no longer adequate, because the primary current is so large (4-6 MA) that the clamping system for the primary coil cannot endure the strong electromagnetic force. The clamping system itself will be destroyed every time. So we are using a primary coil with a shape as shown in fig. 1. The idea of the coil design is essentially the same as that of the single-turn coil system described in the next section. We do not clamp the coil except for the wide flat part at the electrodes, and we let it expand freely during the shot. The electrodes are connected to the collector plates with a contact pressure supplied by a hydraulic press. The explosion of the primary coil is stopped by a massive steel box which encloses the coil. With such a device, we can safely supply a current up to 4.4 MA to the primary coil. The energy conversion efficiency using this type of primary coil was found to be much better than using one made of iron plates because of the more concentrated current path. With this system the liner velocity can reach 2.5 km/s. Experiments are in progress to achieve the highest possible field using this system.
magnetic fields
0
1411
4
2 Time
6
8
(fis)
Fig. 4. An example of current I and magnetic field B waveforms generated by the single turn coil technique. The value of B/I is also shown as a function of time. The inset shows schematically the principle of the single-turn coil technique.
position by a hydraulic press. The clamping system provides a straight current path from the collector to the coil. To catch the fragments of the coils and to protect
Insulating
3. Single-turn coil system In addition to the electromagnetic flux compression, another technique is employed for generating megagauss fields. That is direct discharge of a fast current to a single-turn coil, as schematically shown in the inset in fig. 4. This technique relies on a fast current discharge which allows the megagauss field generation before the coil destruction. The single-turn coil technique was first attempted by Furth et al. in 1957 [ll]. Later, Herlach et al. built a laboratory instrument for practical use in various experiments, using small thin-walled coils [12]. We built an improved and larger megagauss generator of this type for general use in solid state experiments u31. For this method, a very fast condenser bank is required to provide a fast discharge. A 40 kV, 100 kJ condenser bank with 20 pressurized air gap switches was installed. The internal inductance of the bank is 18 nH, and the condenser bank delivers 2.78 MA with a rise time of 2.42 ~LS(quarter period) when short-circuited at the terminal where the coils are clamped. The design of the coil system is shown in fig. 5. The single-turn coil is made by cutting and bending copper sheet of 3 mm thickness. The coil is firmly clamped in
Fragment catchlyg
box
:or
Fig. 5. Drawing of the coil and its supporting system for the single-turn coil system. A perspective view of the coil is shown in the inset.
the other parts of the device, two nose pieces on the clamping device and a steel box with a wood lining are set around the coil. The entire coil system including all these devices is mounted in a sturdy steel compartment, A maximum field of 150 T is obtained when we use a coil with 10 mm inside diameter d, and 10 mm axial length 1. This is our standard coil size and convenient for measurements at low temperatures. As the coil size is reduced, higher magnetic fields are obtained. Examples of the attainable maximum fields with various sizes are as follows: 184 T for d, = 8 mm, I= 8 mm: 201 T ford,=6mm./=7mm;246Tfor~l,=4.1mm./=6 mm: and 263 T for d, = 2 mm, I = 2 mm. Observed waveforms of the current and field pulses are shown in fig. 4. The maximum of the magnetic field occurs earlier than that of the current because of deformation of the coil. The change of field current ratio. B/I, is shown as a function of time in fig. 4. It should be noted that the curves for R and I are fairly smooth without any kinks or breaks related to the destruction of the coil. Also. no noise related to the coil destruction appears in any of the solid state measurements. While the field increases roughly linearly as a function of the coil diameter. the maximum field is limited to 263 T which is obtained for d, = 2 mm. For smaller coils. the peak field is even lower. When we used tantalum as a coil material. slightly better results were obtained. The highest field we obtained with this system was 280 T using a Ta coil with d, = 3 mm and I = 4.1 mm. The field homogeneity of the standard coil (d, = 10 mm) is reasonably good. At maximum field. the field strength falls to 98.6% and to 93.8% of the optimal value at the centre if off-set in the axial direction by 1 and 2 mm, respectively. For low temperature experiments, samples are mounted in a sample holder made of double thin glass or phenolic tubes placed in a vacuum space in the field. By flowing a large amount of liquid helium through the gap between the tubes, the sample temperature can be lowered to about 4 K. Such low temperature experiments can be performed with the coil diameter down to d, = 6 mm by which fields up to 200 T can be obtained. A remarkable advantage of this method is that the samples are not destroyed in each experiment if they are protected properly. because the explosion of the coil occurs only in the outward direction. The sample holders and pick-up coils for the field measurement are not destroyed either. Therefore we can repeat measurements on one sample many times. Furthermore. measurements can be done twice in one pulse in both rising and decreasing fields. These characteristics ensure the reliability of the data. As compared with the electromagnetic flux compression method. the system is more compact. so that we can carry out experiments fairly easily and quickly. although the available field is limited to a few megagauss.
4. Non-destructive
sub-megagauss fields
For pursuing solid state experiments in megagauss fields, subsidiary measurements in the non-destructive field range are often required. To this end. non-destructive magnetic fields up to 45 T are generated using conventional pulse magnets with a long pulse duration. The design of the magnet is illustrated in fig. 6. Copper wire containing Cr is wound on a FRP bobbin and it ~j impregnated with epoxy resin. The winding is mounted in :I stainless steel cylinder. and placed inside :I cryostat with an operating temperature of 77 K. Samples can be cooled to as low as 1.6 K with this syatcm. Two condenser banks with a stored energy of 200 and 1 I2 k.1 are installed for supplying current to the magnets. The 200 kJ bank can bc utilized as either ;I IO kV. 4 mF bank or as a 5 kV, 16 mF bank by switching the scrics/parallel mode of operation [Y]. The pulse duration and the maximum field can be varied depending on the capacitance of the condenser bank. as well as the magnet sire which determines the inductance. Typical waveforms of the fields are shown in fig. 6 (curves :I and b). For a pulse width of 3 IIIS. a maximum field 01 45 7 can bc obtained, whereas for 20 ms. 37 T cm bc obtained. The 112 kJ bank. on the other hand. can bc utilized to generate a flat-top pulse as shown by curve c in fig. 6. The condensers
are divided
is connected
to an
inductor
into
five units;
to form
each
of these
a pulse-forming-
By adjusting the capacitance and inductance of each unit and also the load inductance, the flat top
network. part
of
the
pulse
can
be
made
fairly
long
(I.4
tm
allowing 2% change). This arrangement is useful to obtain a constant field for some period or a rapid field decay
in comparison
Time
0
1
to the whole
field
duration.
(ms)
2
3
4
Time
(ms)
I
5
6
-T
Fig. 6. Various waveforms of non-destructive pulse fields generated by conventional copper wire magnet\. A crash-\cctional \iew of the magnet is shown in the inset.
N. M~uru et al. / Megaguuss
5. Application of megagauss fields to magnetism Extremely high magnetic fields as described in previous sections open up various new possibilities in solid state physics. Since electronic states in solids are greatly altered by ultra-high fields, we may expect many new phenomena to occur including various types of phase transitions. For example, a magnetic field of 500 T makes the spin Zeeman splitting gp,H as large as 670 K for g = 2. It may exceed various internal fields acting in solids. Technically, there are some limitations in performing solid state measurements due to the short pulse duration and the small space available. In addition, the sample heating due to the adiabatic change in the magnetization must be taken into account [14]. This is not as critical as eddy current heating in conducting materials because it is a bulk effect and it is reversible. However. the variety of possible experiments is more than one would first imagine. Optical measurements provide the most convenient techniques. They avoid the problem of large field induced voltages which occur in the sample wires of electrical and magnetic measurements. By using streak spectrometers, we can obtain a whole optical spectrum in the visible range in one pulse [15]. The magnetization processes in magnetic materials can be studied by Faraday rotation [16]. In conducting substances, we can obtain information on the transport properties by measuring magneto-absorption or magnetoreflection using lasers in the far infrared [S]. Although it is much more difficult to perform magnetic and electrical measurements, ac susceptibility [17] or ac conductivity [18] measurements are possible in the megagauss range if frequencies in the 30-150 MHz range are used. Examples of such measurements are given in references cited above. In this paper, recent results of the Faraday rotation in various iron garnets using the singleturn coil system will he shown. Iron garnets are ferrimagnetic suhstances, with molecular fields of the order of magagauss. Consequently, if we apply a sufficiently high field, these materials undergo magnetic phase transitions. In YIG, for example, two transitions take place. first from the ferrimagnetic to the spin canted phase, and then from the spin canted to the fully aligned spin phase successively. Iron garnets are transparent in the near infrared range. so we can observe such phase transitions by measuring the Faraday rotation. Fig. 7 shows the Faraday rotation in several iron garnets in fields up to 200 T. In YIG, the Faraday rotation is almost a linear function of magnetic field, which is consistent with previous data [19]. Although the line has a considerable gradient, it shows no kinks up to 200 T because the material stays in the ferrimagnetic phase at T = 170 K. In other substances. anomalies are observed at fields shown by arrows. Most of the anomalies correspond to the transition from the ferrimagnetic to the spin canted
1413
magnetic jtelds
Magnetic
Field(T)
-500
Fig. 7. The Faraday rotation angle as a function of magnetic field for various iron garnet crystals. The magnetic field direction with respect to the crystal axes and temperature are shown for each curve. Measurement was performed at 1.15 pm wavelength using a He-Ne laser.
phase. In ErIG, when a magnetic field is applied parallel to the (111) axis, a new type of transition was found as a discontinuous jump of the Faraday rotation. This jump corresponds to a magnetization jump, and is in sharp contrast to a ferrimagnetic-spin canted phase transition which only gives rise to a change of the slope. This phenomenon is believed to arise from the highly anisotropic exchange interaction of Er ions [20]. In DyIG, another kind of anomaly is observed at a higher field in addition to the usual spin-canted phase transition. Details of these results will be reported elsewhere. In the non-destructive submegagauss field range, a variety of accurate measurements can be performed. As an example of such measurements, fig. 8 shows the magnetization and the magnetoresistance in a semimagnetic semiconductor Hg, _,_,Cd,rMn.VTe. In both measurements. anomalies are observed at an identical magnetic field shown by the arrows. In this substance, Mn” ions are responsible for the dilute magnetism. The Mn ions with spin S = 5/2 are distributed randomly, and some of them form clusters occupying nearestneighbour sites with each other [21,22]. By an antiferromagnetic exchange interaction between a pair of nearest neighbour ions, the spin state takes the ground state S,,,, = 0. and this causes a reduction of the apparent saturation moment. At high magnetic fields, level crossing occurs between higher lying Zeeman split levels S,,,, = 1. 2,. . ,5, and this causes a step-wise increase of the magnetization at each crossing point. The observed anomaly in fig. 8 is actually the first step of this kind at a field where the S,,,,, = 0 state crosses with the SPalr = 1; Si = - 1 state. From this transition, the exchange
H%,_,Cd,MnyTe
6
-
III I’. Kapitza. Proc. Roy. Sot. A105 (1924) 691.. All5 (1927) 65X. Ill Very recently, a field up to 30.7 T was achieved at Tohohu University. Sendai. Japan with a hybrid magnet. 131 S. Foner and H.H. Kolm. Rev. Sci. Instr. 2X (1957) 799. (41 A. Yamaglshi and M. Date. Hugh Field Magnetism. ed. M. Date (North-Holland. Amsterdam. 1983) p. 2X9. 151 C.M. Fouler, W.B. Cram and R.S. Caird. J. 4ppl. Phy\. ?I
( 1960)58X. [61 N.
.~~
0
IO Magnettc
20 Field
30 (T)
.-J0 40
Fig.
X. Magnetization M and magnetoresistance Ap/p,, in Hg,m, tCd, Mn,Te (x = 0.027. .L’= 0.009) as a function of magnetic field. The measurement was performed in a non-destructive pulse field. At the field indicated by arrows, both M and Ap/p,, show anomalies. In the top trace, the vertical scale for M is magnified by a factor of 10 to clarify the details of the anomaly.
constant can he estimated. In other samples the second step was also observed at higher fields. It is of considerable interest to determine the nature of the mechanism that is responsible for the anomalous conductivity change at the cross-over. In addition, further study is required to determine the band gap dependence of the antiferromagnetic exchange interaction and also to clarify the clustering properties of Mn ions, by observing each successive step at higher fields. To summarize, we have constructed an ultrahigh magentic field laboratory where several different types of megagauss magnets are routinely available for basic solid state physics research. Using novel extremely high magentic field generation techniques, we have found many new and interesting phenomena. Development of even higher fields is in progress. This new facility promises to provide a rich source of new data on the fundamental properties of condensed matter. interaction
Mlura, G. Kldo, M. Akihiro and S. Chikazuml. J. Magn. Magn. Mat. 11 (1979) 275. 171 b..C. C‘nare. J. Appl. Phys. 37 (1966) 3X12. [Xl N. Miura. G. Kido and S. Chikazumi. Application of tllgh Magnetic Fields in Semiconductor Physich. ed. G. Landhrhr (Springer-Verlag. Berlin. 1983) p. 505. K. Nakao and S. PI N. Miura. G. Kido, H. MiyaJima. C‘hikarumi. Physics in High Magnetic Fields. eds. S. C‘hikazumi and N. Miura. (Springer-Verlag, Berlin. 19X1) p. 64. [lOI N. Miura and F. Herlach, Strong and Ultrastrong Magnetlc Fields. ed. F. Herlach (Springer-Verlag. Berlin, 19X5) p. 247. 1111 H.P. Furth. M.A. Levinr and R.W. Waniek. Rev. SCI. Instr. 28 (1957)949. (121 F. Herlach and R. McBroom. J. Phys. t 6 (1973) 652. [I31 K. Nakao. F. Herlach, T. Goto. S. Takeyama. -1. Sakakibara and N. Mlura. J. Phys. E (to be published. 19X5). (141 T. Sakakibara et al.. High Field Magnetism. cd. M. Date (North-Holland. Amsterdam. 19X?) p. 167. J. llSl <;. Kido. N. Miura. H. Katayama and S. (‘hikarumi. Phys. E 14 (1981) 349. 1161 M. Suekane, G. Kido. N. Miura. and S. C‘hlkaruml. .I. Magn. Magn. Mat. 31-34 (1983) 773. 1171 S. Chikar.umi. N. Miura. G. Kido and M. Akihiro. lEEI_, Tranh Mag. MAG-14 (1978) 577. [IX1 N. Miura. T. Osada and T. Goto. Proc. 17th Intern. C‘onf. Physics of Semiconductors. eds. J.D. (‘hadi and &‘.A. Harrison (Springer-Verlag. Berlin 19X5) p. 973. Cl91 N. Miura. G. Kido. I. Oguro. K. Kawauchi. S. C‘hika/uml. J.F. Dillon. Jr. and L.C. Van Uitert; Physica H6&XXB (1977) 1219. l2Ol K. Nakao, T. Goto and N. Miura. J. Magn. Magn. Mat. 54-57 (19X6). Pll Y. Shapira. S. Foner. D.H. Ridgley, K. Dulght and A Weld. Phyh. Rec. 830 (19X4) 4021. R.L. Aggarwal. S.N. Jasperson, Y. Shapira. S. Foner. ‘1 WI Skakibara. T. Goto. N. Miura. K. Dwight and A. Weld. f’roc. 17th Intern. Conf. Physic\ of Semiconductors. vol. I.. cds. J.D. Chadi and W.A. Harrison (Springer-Vertag. Berlin. 19X5) p. 1419.