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[17] The jump in magnetization Delta M is essentially independent of temperature at low T. Since Delta M is the integral of the susceptibility, one would naively expect that area below the peak in chi' (Fig. 2) to be also independent of T. But this is true only for the dc susceptibility, or, more accurately, for chi' measured at frequencies lower than the inverse of the longest relaxation time. The fact that we can measure an out of phase response Delta chi'' reveals we are actually measuring dynamic response, i.e. that our frequencies are high and some relaxation processes do not contribute to Delta chi'. Since relaxation times grow on lowering the temperature, the area loss observed in this figures is quite natural.
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