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Magnetic measurements: basic aspects

Magnetic measurements: basic aspects . G. Hilscher TU Vienna. Introduction Magnetic units Force methods Induction methods SQUID Magnetometer. Magnetic Characterisation. e. T. T c. Low Temperatures & High Fields . µ B H/k B T. Magnets. SC Magnets 9 – 22 T .

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Magnetic measurements: basic aspects

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  1. Magnetic measurements: basic aspects G. Hilscher TU Vienna Introduction Magnetic units Force methods Induction methods SQUID Magnetometer

  2. Magnetic Characterisation e T Tc

  3. Low Temperatures & High Fields µBH/kBT

  4. Magnets SC Magnets 9 – 22 T Electromagnets 1.5 -2.5 T 5cm Nb3Sn filaments

  5. Pulsed Fields 40 – 60 T 15 mF 440 kJ 10 kV 10 -20 ms pulse duration access to the mains 10 MW 1 s

  6. 1.5 K Pot 3He liquid 300mK

  7. 3He/ 4He Fridge 1.5 K 99% 3He 3He <3He/ 4He

  8. Magnetic Units SI System:

  9. Frequently used unit in magnetism: emu/g g

  10. CGS System:

  11. Force Methods Faraday-Balance Pendulm-Balance

  12. Force or Torque Measurement with a cantilever dHz /dz sample

  13. Torque Magnetometer piezorestistivecantilever

  14. Induction Methods B(H) B = µ0 (H+M) M(H)

  15. N...number of windings • A...winding area • C...coupling factor N1 A1 – N2 A2  10-3

  16. Extraction magnetometer H T,H t Sensitivity: 10-3 – 10-4 emu Movement: 3cm with 0.5 - 1Hz Sample mass 0.1 -10g Field 0 - 15T Measurement @ H = const. Temperature 2K - 300K He M (T,H)

  17. Vibrating Sample Magnetometer, Foner Magnetometer Sensitivity 10-4 - 10-8 emu Field 0- 17T Sample movement 1mm, 82Hz Temperature 2 - 400K (800 K) Sample mass: 0.05 - 0.5g Laudspeaker 82Hz Vibration 82 Hz Oscillator Lock-In Amplifier M Lock-In: links the 82Hz sample movement with the P.U. coil signal (82Hz),

  18. AC or initial susceptibility PSD Oscillator

  19. SQUID Magnetometer SC SC wire;

  20. SQUID Magnetometer Flux-Response SC SC wire; 2nd order Gradiometer coil

  21. Superconductivity SQUID Superconducting Quantum Interference Device 2) Meissner - Ochsenfeld effect : Field expulsion Bintern = 0 n  0 3) Flux quantisation:  = BF = n 0 0 = h/2e = 2,07.10-15 Vs But only in multiply connected SC!

  22. Flux in the SC ring:  int = ext + LI S External flux is compensated by the flux expulsion LIS until IS = IC ; we set LIC =  0 / 2 int /  0 IS ext /  0 IC IS  ext /  0 1 2

  23. V DC-SQUID: 2 weak links V I

  24. V DC-SQUID: 2 weak links B V V  ext /  0 I 0 1 2 Ibias

  25. V DC-SQUID: 2 weak links V V  ext /  0 0 1 2 Ibias = I1+I2 = Ic1 sin 1 + Ic2 sin 2 Simplification: Ic1 = Ic2 =Ic and point contacts n 0

  26. DC SQUIDwith integrated flux transformer

  27. Magnetic Signal Levels

  28. RF und DC SQUID Elektronik Flux Locked Mode: Additional to the external flux, Flux with opposite sign is coupled via the modulation Coil into the SQUID that the total flux is kept constant The deviation is measured with a PSD amplified with an Integrator coupled back to the system Flux Locked Mode: Zusätzlich zum externen Fluß wird über die Modulationsspule Fluß mit entgegengesetztem Vorzeichen in das Squid eigekoppelt, daß der Gesamtfluß im Squid immer konstant bleibt. Die Abweichung wird mit einem PSD gemessen über einen Integrator verstärkt und rückgekoppelt.

  29. Demagnetising factor Hext Hintern - N.M Hext

  30. Sphere Or

  31. First Nb RF SQUID Einkopplungsspule, RF Spule Punktkontakt mit Nb Schrauben

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