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If an ferromagnetic object is heated and reaches Tc the magnetization gradually drops as we get closer to Tc or it's a instant drop? Can I assume as I heat the object, the magnetization is weakening gradually? Likewas as it cools?

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Gradual drop. And its zero above the curie-temperature.

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  • $\begingroup$ Ah, so M gets weaker as the temperature generally increases? $\endgroup$ – AxtII Oct 29 '13 at 2:08
  • $\begingroup$ Yes, well, |M| does $\endgroup$ – user27799 Oct 29 '13 at 2:09
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Answer by Allah Abdulala Gustavo ElFakir is mostly correct although steep jump can be also observed. Here is an example. As this is first order transition there is some thermal hysteresis. On heating temperature of transition is slightly higher than on cooling. Gradual drop is dramatically more common though. And in this case it occurs at the same temperature for cooling and heating.

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For the simplest systems exhibiting continuous phase transitions, the magnetization is saturated at zero-temperature and drops continuously until it reaches a vanishing value. This is point is called the critical temperature. Roughly, the Curie temperature (or more precisely, the Curie-Weiss temperature) is an estimate of the critical temperature at which the magnetization vanishes. Intuitively, the magnetization at very high temperatures is zero since the spins, that are the elementary sources of magnetic dipolar fields which gives finite macroscopic magnetization in a given material, fluctuate strongly and on average, do not point in any specific direction, thus giving an overall zero magnetization. Conversely, at zero-temperature (again in the simplest systems), spins that are subject to ferromagnetic interactions want to align to minimize the overall energy of the system and on average give an overall finite magnetization.

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