# What is curie weiss law?

Curie Weiss law is the qualititative study of Ferromagnetism. Ferromagnetic materials has magnetic domains, each domain has magnetic moment and they are spontaneously magnetised. This spontaneous magnetization because of the existence of internal molecular field is known as Weiss field.

Weiss proposed an effective field approximation is which he considered a magnetic atom and replaced its interactions with the remaining atoms of the crystal by effective magnetic field (Heff).

Case I

Consider a solid consists of N identical atoms. They are arranged in a array of regular lattice. Each atom has net electronic spin (s) and magnetic moment. Each atom can interact with its neighbours. In the past we assumed spins were non-interacting. This is good as long as KBT >> the interaction energy (or) exchange interaction. In this case the localized spins should obey curie’s law $left( chi =frac{1}{T} right)$ the magnetization (M) should vanishes as H ® 0 (this is how a paramagnet behaves).

Case II

If the exchange interaction (i.e.) interaction energy between the spins >> KBT magnetic ordering may occur. This is due to an effective magnetic field produced at a given site by its neighbours.

Suppose a given electron spin is ­ if it produces a field at a neighbouring site parallel to itself. Then the neighbouring atom also tend to be magnetized as ­. We can expect all ­ spins (or) ¯ spins in the ground state. This shows ferromagnetic in nature and it has spontaneous magnetization even (Hext = 0)

On the other hand the field produced is Anti-parallel to the spins ­¯­¯. This shows Anti-ferromagnetic in nature and has zero-net magnetization.

All the forms of magnetic ordering disappear as the substance is heated. The typical temperature at which spontaneous magnetization disappears is known as curie temperature (TC) of Ferromagnetic materials. The ordering temperature of Anti-ferromagnetic materials is called Neel temperature (TN)