I think existence of other weak interactions which have higher transition temperature than the observed prominent interaction(ferro/antiferro) can perhaps lead to such results, but I am confused when multiple slope changes are observed above Tc. (If it had been para-magnetic then it should have been a straight line.) Also can such interactions exist above temperatures like 600 degrees?
Short range correlations can persist certainly up to 2T_c. If those other interaction you have in mind don't give rise to long range order then they should not spoil the asymptotic Curie behavior. Have you considered the possibility of a diamagnetic background? In addition what did you actually measure? If you measured magnetization vs. T in the presence of an applied magnetic field then you may not be in the linear response regime when your applied field is too large. Try a smaller applied field and see if the deviation from the Curie law is reduced. Even better, try scaling with systematic variation of the applied magnetic field.
Thank you Dr.Christian for the inputs.Unfortunately, M vs T measurements have been done only at 1000 Oe. Is that too high a field?
Also, high - temperature VSM was done on the sample up to nearly 900K, and the fitting was attempted from 400 - 900 K. Would the short range correlations exist up to such high temperatures? When 1/Chi is plotted against T for 400 - 900K, there are multiple changes of slope for the linear graph. Why would this be?
The expected Curie constant for the compound should be around 1.5 (which is the contribution measured for the free ion which is contributing to the magnetism of the compound), whereas the value obtained after fitting is coming out to be around 4. What could the other reasons be, for this large deviation?
(A constant term has been added to account for the diamagnetic contribution as well as Pauli paramagnetism in the fitting already, but no improvement in the results)
Interconfiguration interaction (mixed-valent effect) can persist above 300 K, as well as crystal field effects, and 4f or 5d electron-phonon interactions can do such deviations from the Curie-Weiss low. Try to fit modified Curie-Weiss low adding interkonfiguration interaction. Further, try to apply electron-phonon interaction.
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