The GENERATION, MEASURING TECHNIQUE AND APPLICATION OF PULSED FIELDS. R.Grössinger

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1 High Magnetic Fields The GENERATION, MEASURING TECHNIQUE AND APPLICATION OF PULSED FIELDS R.Grössinger Coworker: M. Küpferling, H.Sassik, R.Sato, E.Wagner, O.Mayerhofer, M.Taraba ƒ1

2 Content CONTENT Generation of static magnetic fields Pulsed field systems Quasistatic systems Short pulse systems High field magnets Magnetisation measurements Pick-up systems Methods to increase the sensitivity Vibrating sample technique Surface coils Microtechnology for pulsed magnetic fields Application of pulsed high magnetic fields Magnetization measurements Anisotropy ƒ2

3 Introduction 1) Introduction High magnetic fields are important for: Semiconductor physics (and structure, fermi surface) Superconductivity (high Tc super- conductors) Magnetism (magnetic phase diagrams) Material characterisation Exchange coupling Onset of magnetism ƒ3

4 Static fields Generation of static magnetic fields Air gap of an electromagnet - fields limited by saturation magnetization up to 2.5T ƒ4

5 Static fields Hysteresograph with electromagnet from Magnet-Physik EP-2 Max. Field strength: Maximum pole diameter: Air gap: Load: Weight: 1600 ka/m (2 T, 20 kg) 92 mm 0-75 mm 3 kw 180 kg ƒ5

6 Static fields Higher static fields: superconducting coil. Fields up to 8T with Nb-Ti Fields up to 15T with Nb3Sn Fields up to 20T with hybrid. Advantage: high fields, low power comsumption Disadvantage: needs liquid helium, low dh/dt rate! ƒ6

7 Static fields Superconducting 20T coil system from cryomagnetics (USA) ƒ7

8 Static fields Superconducting coils: ƒ8

9 Hybrid magnet of NHMFL ƒ9

10 Hybrid magnet of NHMFL ƒ10

11 NHMFL at Los Alamos Pulsed field systems: Quasistatic systems 1.4 GW Generator Los Alamos ƒ11

12 NHMFL at Los Alamos High field laboratory Los Alamos ƒ12

13 AUSTROMAG AUSTROMAG - T.U.Vienna: 10 MW-1s power supply. Regulated DC current over 1s. Maximum available power is limited. Primary power: 16 MVA - transformer 10 kv to 2 840V; Can be switched: series, parallel or antiparallel. ac-current - rectified - bridges 6 thyristors I(t). Maximum dc-power: 10 MW/1 s or 5 MW/2 s or 1 MW/10s Current time profile chosale - 20 free points. Delivers field plateau s or linear dh/dt. ƒ13

14 AUSTROMAG Switching of the thyristor bridges- three types of pulses: i) Parallel switching: 2 x I max = 13600A, U max = 840V: highest current field pulse - one polarity. ii) Serial switching: I max = 6800A, 2 x U max = 1680V: highest voltage field pulse - one polarity 40T. iii) Antiparallel switching: I max = ± 6800A, U max =± 840V. Bipolar pulse - real hysteresis measurements. ƒ14

15 AUSTROMAG Electric circuit ƒ15

16 AUSTROMAG Block diagram of the Austromag high field installation. Low temperature system: max. 40 T in 25 mm bore; temperature between 1.5 K and 300K High Temperature system: maximum 35T in 40 mm; 300 K < T < 800K power transformer 16 MVA 2 x 570V/6800A parallel or seriel thyristor rectifier 12-pulses 2 antiparallel bridges 2 x 600V, 8300A/1s passive smoothing LC circuit high-temperature system 300K < T < 800K low-temperature system 1.5K < T < 300K m0h max = m0h max = 35 T 40 T 10 kv 2 x 1700 kva 50 Hz regulating electronic input H(t)! SECURITY! measuring electronics temp. controller test system modulation magnetostriction m0h max = 20 T ƒ16

17 Short pulse systems Short pulse systems Typical pulse duration 1-50 ms. Energy source: condensator battery - kj. Available power can be varied by the time constant of the system. Consist of: Energy source - C.U 2 /2 charging unit - reproducibility Pulse magnet - diameter, homogeneity measuring device pick-up coils + electronic Data storage + PC ƒ17

18 Short pulse system PULSED FIELD SYSTEM Block diagram of the condensator driven pulse field facility CHARGING UNIT CONDENSATOR BATTERY C=8mF/24mF U=2500V HIGH FIELD MAGNET PREAMPLIFIER GAIN: /4 LAYERS Pick- Up System TRANSIENT RECORDER PC ƒ18

19 Magnets High field magnets Optimized with respect to the available power, the heating of the magnet and the stresses. Heating of the magnet: j(t)...current density j 2 () t dt = T T 1 2 DcT. ( ) ρ( T) dt D...density of the conductor, c(t)...specific heat ρ(t)...specific resistivity of the conductor. ƒ19

20 Magnet Stresses in the magnet Stress in a high field magnet scales with B 2 ƒ20

21 Leuven Magnet ƒ21

22 AUSTROMAG AUSTROMAG 40T magnet Field versus time profile of a magnet driven by a maximum power of 5.5 Mw (I = 10kA); the temperature rises up to 150 K. ƒ22

23 AUSTROMAG STRESS in the AUSTROMAG magnet Axial, radial an tangential stresses at 38 T in the middle plane of the magnet. ƒ23

24 Magnetization measurements Magnetisation measurements Pick-up systems a) N N r Axial system: simple vibrational sensitive b) N/2 N N/2 r N/2-N-N/2 system: long Dipol: R 12 N 1 = R 22 N 2 c) N N 2 1 r 1 r2 u () t [ 2 N K R ] dh = µ π + dt [ 2 u t N K R ] 2() = µ π dh dt dm dt ƒ24

25 Pick-up Compensatable pick-up system used in pulsed fields up to 50T IFW Dresden. a b c 56 2k7 4k2 3k3 ƒ25

26 Pick-up Problems: Temperature dependent measurements - compensation depends sensitively on the position of the pick-up system. with respect to the high field magnet. With increasing field the noise increases due to vibrations. Metallic systems - the eddy currents also cause a measuring error. maximum achievable sensitivity aout 0.01 emu. Sensitivity depends directly coupling between sample and pick-up coil. Small samples - thin films - very difficult! ƒ26

27 Eddy currents How eddy currents are created ƒ27

28 Eddy currents Eddy Currents due to pulsed magnetic fields Cu-sample cylinder with increasing diameter. M (A/m) µ 0 H (T) Cu, cylinder, h=8mm d=2mm d=6mm d=8mm d=9,8mm d=4mm 8mF, T=9.1ms ƒ28

29 Eddy currents Cu-sample cylinder with increasing length Cu, cylinder, d=4mm h=2mm h=4mm h=6mm h=10mm h=8mm 8mF, T=9.1ms M (A/m) µ 0 H (T) ƒ29

30 Eddy currents EDDY CURRENTS ON MAGNETIC SAMPLES Ni M (A/m) Ni, cylindre, d=4mm, h=8mm ρ=8908kg/m 3 m=0,89183g annealed, 4h, 500 C 8mF, 2000V B (T) M (A/m) Ni, cylinder, 8mF eddy-current-corrected with specific resistance of 8µΩcm B (T) ƒ30

31 Measurement methods Methods to increase the sensitivity Only so-called DC-methods used. Integrating everything which comes. High noise - limits achievable sensitivity. Methods which enhance signal to noise ratio - lock-in technique + improve coupling between the sensor ( the pick-up coil) and sample. ƒ31

32 Measurement method Vibrating sample technique Static magnetometers - vibrating sample method. Lock-in technique - improves signal to noise ratio. Pick up coils - ac-signal - proportional to M. Pulsed field systems: i) Modulation method eddy current problems. ii) Piezo electrique actuator Effects of induction voltages can be suppressed. ƒ32

33 Measurement method Surface coils Thin coils are close to the sample positioned. Two coils in order to compensate the effect of H. Can be: Thin film coils - bad L/R ratio phase problems Wire wound coils The ultra-thin pick-up coils are manufactured in thin-film technique as indicated in the figure. ƒ33

34 Surface coils Thin film coils Magnetization curve taken on BaFeO-film. ƒ34

35 Surface coils Wire wound coils (Magnet Physik) Low impedance! Type PKS 5 PKS 3 Turn area, approx. 5 cm 2 3 cm 2 Coil thickness 1 mm 0,5 mm Coil width 5 mm Ø 5 mm Ø ƒ35

36 Toulouse - Cantilever Microtechnology for pulsed magnetic fields ƒ36

37 Cantilever TU-Vienna: U shaped cantilever Deflection of the cantilever is detected optically ƒ37

38 Cantilever Characteristic of the combination cantilever-optical readout. The inset shows how the displacement of the cantilever is measured. The diagram is scaled to unity. ƒ38

39 Results Application of pulsed high magnetic fields K, x = K, x = K, x = K, x = K, x = K, x = 2 M(Am 2 /kg) YCo 5-x Cu x perpendicular µ 0 H int ƒ39

40 Results Accurate hysteresis measurements on industrial magnets µ 0 M [T] 0,4 0,2 0, µ 0 H [ka/m] -0,2-0,4 PTB - Ferrite cylinder long pulse short pulse dynamic: J H C = 213 ka/m; B r = T PTB: J H C = 208 ka/m; B r = T ƒ40

41 Results Room temperature hysteresis loop of a two phase spherical and a cylindrical Nd-Fe-B sample. The curves were corrected for the demagnetizing factor. µ 0 M (T) 1,0 0,5 0, ,5-1,0 H in (MA/m) cylindre sphere corrected with demagnetizing factor ƒ41

42 Results Anisotropy of nanocrystalline mechanical alloyed Pr-Fe µ 0 H a (T) Pr 18 Fe 76 B 6 Pr 12 Fe 82 B 6 Pr 9 Fe 85 B Temperature (K) ƒ42

43 Results Magnetic viscosity dh/dt [(GA/m)/s)] 20 Cu annealed 1.5 SmCo Cu Cu annealed 5-x x Cu annealed 2.5 Cu annealed 3.0 Cu as-cast 2.0 Cu as-cast H [MA/m] c Coercive field as a function of the sweep rate dh/dt measured in as cast and annealed SmCo 5-x Cu x ƒ43

44 Results Magnetostriction Magnetostriktion [ppm] Bariumferrite HF 24/16 T=300K λ pc λ cc λ pp λ cp B [T] Magnetostriction measurements at room temperature on an anisotropic barium ferrite magnet made by Schramberg (HF24/16). ƒ44

45 Summary Summary of high magnetic fields High magetic fields are necessary for many aspects of solid state physics Pulsed fields allow also smaller laboratories access to medium up to high fields. Measuring technique still has to be improved. Many applications in the area of magnetism. Magnetization, anisotropy, magnetostriction... ƒ45

EDDY CURRENT EFFECTS IN A PULSED FIELD MAGNETOMETER

EDDY CURRENT EFFECTS IN A PULSED FIELD MAGNETOMETER R.Grössinger, M.Küpferling Institut. f. Festkörperphysik; Techn. Univ. Vienna; Austria 1) Introduction Two transient effects in magnetic materials: a)