magnetic measurements (pt. iii) -...

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

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 3

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 4

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 5

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

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 13

resulting current

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 14

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

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 16

local AC response

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 17

even simpler device – measure B(x)

superconducting sampleHac

small Hall probe

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

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 19

the wave forms

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 20

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 21

AC harmonics

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 22

and the actual data

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 23

further reading

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 24

example of frequency dependence

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 25

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

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 26

influence of vortex creep

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 27

and how it affects the wave - forms

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 28

local vs. global AC susceptibility

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 29

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=

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 30

different types and designs – the amplitude domain

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 31

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 32

QD AC coil set for PPMS

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 33

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

34

resonant techniques

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

measure resonant frequency SHIFT!

35

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)

36

mw cavity set-up

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 37

microwave cavity-perturbation technique

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 38

example of Cu cavity

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 39

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 40

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 41

MW cavity perturbation measurements

October 2018 Basics of Magnetic Measurements. Part III (AC). Prof. Ruslan Prozorov 42