Editing Ferromagnetism (section)

===Curie temperature===
{{Main|Curie temperature}}
As the temperature of a material increases, thermal motion, or [[entropy]], competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the [[Curie temperature]], there is a second-order [[phase transition]] and the system can no longer maintain a spontaneous magnetization, so its ability to be magnetized or attracted to a magnet disappears, although it still responds [[Paramagnetism|paramagnetically]] to an external field. Below that temperature, there is a [[spontaneous symmetry breaking]] and magnetic moments become aligned with their neighbors. The Curie temperature itself is a [[critical point (thermodynamics)|critical point]], where the [[magnetic susceptibility]] is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all length scales.

The study of ferromagnetic phase transitions, especially via the simplified [[Ising model|Ising]] spin model, had an important impact on the development of [[statistical physics]]. There, it was first clearly shown that [[mean field theory]] approaches failed to predict the correct behavior at the critical point (which was found to fall under a ''universality class'' that includes many other systems, such as liquid-gas transitions), and had to be replaced by [[renormalization group]] theory.{{citation needed|date=December 2012}}