Design of ac susceptometer using closed cycle helium cryostat N. Alzayed Department of Physics and...
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Transcript of Design of ac susceptometer using closed cycle helium cryostat N. Alzayed Department of Physics and...
Design of ac susceptometer using closed cycle
helium cryostat
N. Alzayed
Department of Physics and AstronomyKing Saud UniversityRiyadh, Saudi Arabia
Parts of this presentation were written by Sue KilcoyneDepartment of Physics and Astronomy,
University of Leeds, Leeds LS2 9JT
Why use AC susceptibility?
…..and it is cheap!
ac susceptibility was originally developed for thermometry at sub-K temperatures
It is now used extensively to study spin glass phenomena, superconducting transitions, vortex dynamics and critical current densities and to map magnetic phase diagrams:
it is a very simple technique
Susceptibility is measured directly
The applied field is very small, so it can be easily established
Dynamic magnetic and superconducting phenomena can be measured directly
usually over a frequency range of 10Hz to 100kHz
Non-linear complex AC susceptibility)MH(B Vo
Flux density(Tesla)
Magnetic fieldA.m-1
Volume magnetisationA.m-1
)1(Ho
HMVwhere is the susceptibility
However in a sinusoidally modulated magnetic excitation field H(t) the volume magnetisation Mv will also be time dependent
So, if H(t) =HacIm(eiwt) = Hacsin(wt)
the time dependent volume magnetisation Mv(t) can be expanded as a Fourier series of the non-linear complex AC susceptibility
nnn igiving
)e(H)t(M tinn
1nacv
Im
1nnnac )tncos()tnsin(H
Non-linear complex AC susceptibility
The real and imaginary components of the susceptibility and are determined directly from Mv(t) through the relationships
n n
)t(d)tnsin()t(MH1 2
0v
acn
)t(d)tncos()t(MH1 2
0v
acn
1n is the fundamental ‘real’ component associated with the dispersive magnetic response
1n is the fundamental ‘imaginary’ component associated with absorptive or irreversible components which arise from energy dissipation within the sample.
1nnnacv )tncos()tnsin(H)t(M
Here n=1 denotes the fundamental susceptibility, while n=2,3,4...etc are the higher order harmonics associated with non-linear terms in
Measuring the AC susceptibilityAC susceptibility is most conveniently measured using the mutual inductance principle
dual phase lock-in amplifier
reference
prim
ary
coil
seco
ndar
y co
ils
s1
s2
sample
A sample is subject to a small alternating field (10-100 A.m-1) produced by the primary coil
The resulting emf induced in a secondary coil, s1, wound around the sample is detected and analysed
Any background signal is nulled by an identical secondary, s2, connected in series opposition.
Primary And Secondary Coiles
Primary & Secondary
Fig. 3 Schematic diagram of (1) primary and (2) secondary coils. Dimensions in mm
1 2
18
14 9
14
Sample Holder
Heater wire
Steal tube
Hylum tube
Sapphire plate
Thermocouple
Sample
Heater Element
Fig. 4 Sample holder
Sample tubeWilson seal
Quartz tube
Gate valve
Vacuum pump
Vacuum Sealing
Vacuum pump
Gate valve
Secondary
Primary
sample
Secondary coil
Double walled sample well
To the Cold Head and vac-uum syst-em
19 mm
32 mm
Fig. 2 Schematic diagram of ac susceptometer inside the sample well of cryocooler.
He gas
Sample holder
suceptometer
Photos
Photos
Design Advantages
•No Eddy CurrentHigher Freq. LimitMetal can disturbe Uniformity of Field
Design Disadvantages
•Space LimitHeat can increase Temp.Applied Field Limitation
Conclusion
• We have discussed the design aspect of the low cost ac susceptometer using closed cycle helium cryostat. The limitation of measurement of χ at higher frequency due to eddy current problem has been solved in placing the coils assembly inside the sample well. The main feature is that the coils are kept at 10 K and sample's temperature is varied
independently