Explain experimental evidence to demonstrate the existence of ferromagnetic domains.

Temperature increases, thermal motion, or entropy, competes with ferromagnetic tendency for dipoles to align. When temperature rises beyond a certain point, known as Curie temperature, there is a second-order phase transition and system can no longer maintain a spontaneous magnetization, though it still responds paramagnetically to an external field. Below that temperature, there is a spontaneous symmetry breaking and random domains form (in absence of an external field). Curie temperature itself is a critical point, where magnetic susceptibility is theoretically infinite and, though there is no net magnetization, domain-like spin correlations fluctuate at all length scales.