Magnetic measurements (Pt. III) - AC

Ruslan Prozorov

October 2018

Physics 590B

lock-in amplifier

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 2

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lock-in summary

with integrator

phase-sensitive detector (PSD)

integrate out

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 6

Basic principles of AC susceptometry

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 7

MULTIPLIER

PHASE SHIFT to MAX the

signal – find the phase

REF

SIGNAL

Low-pass filter and integrator DC out

AC magnetic susceptibility

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 8

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 9

typical AC susceptometer

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 10

AC measurements

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 11

solenoid in external fields

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 12

now we place it in field

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resulting current

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collective behavior (spin glass, vortices, superparamagnetic particles)

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 15

superconductors – Tc, weak links, irreversibility line etc

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local AC response

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even simpler device – measure B(x)

superconducting sampleHac

small Hall probe

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 18

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the wave forms

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AC harmonics

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and the actual data

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further reading

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example of frequency dependence

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influence of vortex creepB

J

FL

Jc

F 0L

=

Activation

energy behavior

Pinning

•Vacancies, voids,

inhomogeneities, where

superconductivity is weak

•Pinning decreases energy

losses caused by flux

creep

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influence of vortex creep

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and how it affects the wave - forms

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local vs. global AC susceptibility

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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

sample

Hac

Idc

Vac to lock-in

sample

Hdc

Iac

Vac to lock-in

AC

DC

Hall probe

Hall probe

Hall H DC DCV R I H=

Hall H AC DCV R I H=

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different types and designs – the amplitude domain

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QD AC coil set for PPMS

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October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov

Analog vs frequency-domain measurements

zero-crossing detector

time

am

plit

ud

e

10-7 sec

time

am

plit

ud

e

time

am

plit

ud

e measure frequency

advantages of the frequency domain• arbitrary wave form • bandpass filtering• mixing• aggressive amplification• extremely stable standards

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resonant techniques

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov

measure resonant frequency SHIFT!

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October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov

driven vs. self-resonating circuit

frequency

am

plit

ude

frequency

am

plit

ud

eproblems: phase noise and finite Q - factor

frequency

am

plit

ude

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)

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mw cavity set-up

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microwave cavity-perturbation technique

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example of Cu cavity

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MW cavity perturbation measurements

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