AC magnetic measurements etc

Transcription

1 physics 590 ruslan prozorov AC magnetic measurements etc

2 lock-in amplifier

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6 lock-in summary with integrator integrate out phase-sensitive detector (PSD)

7 AC magnetic susceptibility

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9 typical AC susceptometer

10 AC measurements

11 solenoid in external fields

12 now we place it in field

13 resulting current

14 collective behavior (spin glass, vortices, superparamagnetic particles)

15 superconductors Tc, weak links, irreversibility line etc

16 local AC response

17 even simpler device measure B(x) small Hall probe H ac superconducting sample

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26 creep B J F L Activation energy behavior Φ c 0 FL = J Pinning Vacancies, voids, inhomogeneities, where superconductivity is weak Pinning decreases energy losses caused by flux creep

27 influence of vortex creep

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30 local vs. global AC susceptibility

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32 AC susceptometers A true AC susceptometer must have an AC component of the applied field The use of lock-in amplifier does not guarantee that the device is an AC susceptometer AC H ac Hall probe sample V ac to lock-in VHall = RH IDCHDC I dc DC H dc Hall probe V ac to lock-in sample VHall = RH IACHDC I ac

33 classifications Local measure magnetic induction B(t) MO, Hall-probe, GMR Global measure total magnetic moment SQUID based, VSM, torque, Faraday balance Amplitude measure the amplitude Frequency measure frequency shift MW cavity perturbation, TDR, resonant (see below) Resonant taps to a resonance in the SAMPLE NMR, FMR, EPR Non-resonant measure non-resonant response (may still use resonant circuit) Microscopic scattering Mossbauer phase contrast etc

34 different types and designs the amplitude domain

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36 QD AC coil set for PPMS

37 Analog vs frequency-domain measurements amplitude measure frequency time 10-7 sec zero-crossing detector advantages of the frequency domain arbitrary wave form bandpass filtering mixing aggressive amplification extremely stable standards

38 resonant techniques measure resonant frequency SHIFT!

39 driven vs. self-resonating circuit amplitude amplitude frequency frequency problems: phase noise and finite Q - factor self-resonating circuit is equivalent to an infinite - Q resonator. phase noise is the only issue (can be dealt with with ultra-high stability clocks)

40 mw cavity set-up

41 microwave cavity-perturbation technique

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45 what is measured and the calibration

46 tunnel-diode: negative differential resistance A tunnel diode or Esaki diode is named after Leo Esaki (Nobel Prize in Physics 1973). Heavily doped narrow (~100 Å wide) p-n junction. Doping results in a conduction band on the electronic n-side to overlap with the (hole states) valence band on the p-side. Tunneling current decreases with bias, because band overlap decreases 18 November 2009 Physics AC Magnetic Measurements

47 self-resonating circuit 4.2 K LT K I (µa) R~1 kω V (mv) 2π f = 1 LC Tesla 1 LC

48 actual hardware new student breadboard toy research grade resonator TDR 3 He cryostat (0.3 K) dilution refrigerator (0.01 K)

49 measurements of dynamic magnetic susceptibility 2π f = 1/ LC f L V f f 2L 2V 3 fmax / sample coil 4πχ 4 λ ( µ ) tanh R πχ µ R λ µ 1 ( ) z H y H x H direct spin contribution Superconducting gaps: 0.02<Δ<20 mev Corresponding frequencies: 1 GHz <Δ<10 THz Tunnel diode energy: E = 50 nev = 0.5 mk superconducting penetration depth λ R. Prozorov et al., PRB 62, 115 (2000); APL 77, 4202 (2000); PRL 85, 3700 (2000). 18 November 2009 Physics AC Magnetic Measurements

50 AC measurements - conclusions very useful when frequency is important (collective behavior, resonances, hysteresis etc) sensitivity is enhanced due to use of lock-in amplifiers or frequency-domain measurements disadvantage perturbs the sample, usually significantly variety of possible artifacts, stringent requirements for electronics etc

51 comparison with conventional techniques technique standard best real lab induction (extraction) coil torque (torsion) /H-dependent/ VSM Faraday balance SQUID magnetometer two-coil AC susceptibility microwave cavity perturb Tunnel-diode resonator x10-12