In DC magnetization, susceptibility = M/H. However, in case of AC magnetization measurements we have both dc and ac magnetic fields and ac susceptibility = dM/dH. We get M' and M'' from AC measurements from PPMS.How do we calculate real and imaginary parts of AC susceptibility. Is it like real part of susceptibility = M'/H_ac field? or do we need to differentiate these?
Hmm, the document you enclosed actually gives a nice account of what are the real and imaginary part of the dynamic susceptibility (see equations on page 2). So you should indicate more clearly what specifically you do not understand (otherwise we would start guessing around).
This said, let me make some minor remarks:
in what I regard as the conventinal understanding, the term susceptibility normally refers to the slope of M(H) at zero field. Only when M(H) is linear, this is the same as M/H.
As described in the document, AC measurements can be made with or without a bias field. At zero bias, I would call that AC susceptibility. At finite field, I'd rather say it's the differential susceptibillity (assuming that on the range of the amplitude of the field around the bias the M(H) curve is nearly linear). If you measure the ac response in presence of significant nonlinearity, then you will also obtain a higher harmonic response (I guess that sometimes, this is actually desired and specifically analyzed.)
If your AC response is a linear one, then you need to analyze it as a superposition of two terms of the same frequency as your driving field. One is in phase (real part), the other one has a phase lag of 90° (imaginary part).
From your question it seems that the measurement device already produces these quantities. The response of a magnetometer is always the moment of the sample, not its magnetization. So, the same rules apply here as when going from a measured DC moment to magnetization (or susceptibility). Depending on the units in which you want to express the susceptibiiy, you have to supply the required information (weight, number of moles, number of atoms... of the magnetically active sample).
Hope this helps.
but, what is the meaning of real part? Because the imaginary part is relationated about absorption.
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