I already fitted my inverse Chi-T graph by Curie-Weiss law and I got Tc=3.72K. But when I tried to plot a calculated Chi-T graph using the Tc I got form inverse one, I found that my calculated line deviates from experiment data at 7.95K. So now I'm confuse. Which one is the real transition temperature? 3.72K or 7.95K?
You have a pretty complicated temperature dependence. What kind of material you are measuring? What is the exact composition of your sample?
Curie Temperature, TC divides the temperature axis into two regions: the one with paramagnetic phase (above TC) and ferromagnetic phase (below TC). Your situation is clearly not of this kind. On the other hand even for true paramagnets we will observe the deviations from 1/(T-TC) law at sufficiently low temperatures (the influence of non-isotropic crystal field). The deviation systematically increases with decreasing temperature. Therefore TC is approximated by taking high temperature observations. In other words: the (expected) straight line in "inverse" plot is only the asymptote of observed results. In summary: I vote for 7.95 K, if at all.
Unless, Marek, this sample exhibits unusually strong fluctuations around (both below and above) TC. In any case, I guess we need an exact composition (probably a poor ceramics) of this sample to give the author any clue.
Sergei, fluctuations near TC are normal thing. This is why we never observe susceptibility divergence at TC but also why we have to use only high temperature measurements to estimate TC. Yet I have to agree that knowing the composition we might be able to somehow explain non-monotonic behavior of susceptibility vs. temperature - as seen in the enclosed picture.
My sample is Prussian Blue with sodium ions occupied the center of cubic structure. The composition from XRD refinement is Na1.35[FeFe(CN)6]0.85
My current understanding is there are two possible sets of interaction in my sample leading to such Chi-T graph.
The first one is Fe(III)-Fe(III) interaction located at each edge of cubic
The second one is Fe(III)-Fe(III) interaction located at the opposite corner of cubic.
Most likely, Ray, you have an inhomogeneous (non-uniform) sample. It's origin may be extrinsic or intrinsic. Extrinsic means you have a granular sample (like ceramics) which produces two types of FM transitions: intragranular (with Tc1=7.95K) and intergranular (with Tc2=3.72K). The latter means weak FM due to spin exchange (or tunneling) between FM grains thru some barriers (could be due to extra Na).
Intrinsic inhomogeneity means that you have a rather good quality sample (single crystal or thin film) with rich FM clusters (responsible for Tc1=7.95K) separated by poor FM regions (responsible for Tc2=3.72K). Another possibility is anisotropy. Your sample may exhibit different transition temperatures in plane and out of plane. But to observe this difference you need a really good sample. In most cases, however, a broad transition (between Tc1 and Tc2) usually indicates a presence of a granular structure in the sample.
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