Presentation on theme: "Design of ac susceptometer using closed cycle helium cryostat"— Presentation transcript:
1 Design of ac susceptometer using closed cycle helium cryostat N. AlzayedDepartment of Physics and AstronomyKing Saud UniversityRiyadh, Saudi ArabiaParts of this presentation were written by Sue KilcoyneDepartment of Physics and Astronomy,University of Leeds, Leeds LS2 9JT
2 Why use AC susceptibility ? ac susceptibility was originally developed for thermometry at sub-K temperaturesIt 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 techniqueSusceptibility is measured directlyThe applied field is very small, so it can be easily establishedDynamic magnetic and superconducting phenomena can be measured directlyusually over a frequency range of10Hz to 100kHz…..and it is cheap!
3 Non-linear complex AC susceptibility Flux density(Tesla)Magnetic fieldA.m-1Volume magnetisationA.m-1where is the susceptibilityHowever in a sinusoidally modulated magnetic excitation field H(t) the volume magnetisation Mv will also be time dependentSo, 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 susceptibilitygiving
4 Non-linear complex AC susceptibility Here n=1 denotes the fundamental susceptibility, while n=2,3,4...etc are the higher order harmonics associated with non-linear terms inThe real and imaginary components of the susceptibility and are determined directly from Mv(t) through the relationshipsis the fundamental ‘real’ component associated with the dispersive magnetic responseis the fundamental ‘imaginary’ component associated with absorptive or irreversible components which arise from energy dissipation within the sample.
5 Measuring the AC susceptibility AC susceptibility is most conveniently measured using the mutual inductance principleprimary coildual phase lock-in amplifierreferencesecondary coilss1s2A sample is subject to a small alternating field ( A.m-1) produced by the primary coilThe resulting emf induced in a secondary coil, s1, wound around the sample is detected and analysedsampleAny background signal is nulled by an identical secondary, s2, connected in series opposition.
9 suceptometer Sample tube Wilson seal Quartz tube Gate valve Vacuum pumpVacuum SealingSecondaryPrimarysampleSecondary coilDouble walled sample wellTo the Cold Head and vac-uum syst-em19 mm32 mmFig. 2 Schematic diagram of ac susceptometer inside the sample well of cryocooler.He gasSample holdersuceptometer
12 Design AdvantagesNo Eddy Current Higher Freq. Limit Metal can disturbe Uniformity of Field
13 Space Limit Heat can increase Temp. Applied Field Limitation Design DisadvantagesSpace Limit Heat can increase Temp. Applied Field Limitation
14 ConclusionWe 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