# What is Curie-Weiss temperature?

What is Curie-Weiss temperature? What is the difference between Curie-Weiss temperature and Curie temperature?

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The Curie temperature or Curie point is the temperature at which a ferromagnetic or a ferri-magnetic material becomes paramagnetic when heated. The effect is reversible.

On the other hand,the Curie-Weiss temperature is the temperature at which a plot of the reciprocal molar magnetic susceptibility against the absolute temperature T intersects the T-axis. The Curie-Weiss temperature can adopt positive as well as negative values.

I hope,now you will get the difference.

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Naively, both temperatures are equal and they're the constant temperature $T_c$ entering the Curie-Weiss Law: $$\chi = \frac{C}{T-T_c}.$$ However, the behavior is often more complicated and the formula above doesn't describe the susceptibility $\chi$ well for all temperatures. When it's so, the Curie temperature $T_c$ is the temperature at which the susceptibility actually blows up, so $\chi=C/(T-T_c)$ holds for $T\sim T_c$ while the Curie-Weiss temperature is the temperature for which the law $\chi=C/(T-T_0)$ holds for $T\gg T_0$, i.e. one reconstructed from the "shape of the hyperbola far away".

The temperatures are close $T_0\sim T_c$ for materials for which the transition is first-order; the temperatures are very different if the transition is second-order.

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