<P> Where M s (\ displaystyle M_ (s)) is the saturation magnetization at 0K . </P> <P> Later, the quantum theory made it possible to understand the microscopic origin of the Weiss field . The exchange interaction between localized spins favored a parallel (in ferromagnets) or an anti-parallel (in anti-ferromagnets) state of neighboring magnetic moments </P> <P> The reason a piece of magnetic material such as iron spontaneously divides into separate domains, rather than exist in a state with magnetization in the same direction throughout the material, is to minimize its internal energy . A large region of ferromagnetic material with a constant magnetization throughout will create a large magnetic field extending into the space outside itself (diagram a, right). This requires a lot of magnetostatic energy stored in the field . To reduce this energy, the sample can split into two domains, with the magnetization in opposite directions in each domain (diagram b right). The magnetic field lines pass in loops in opposite directions through each domain, reducing the field outside the material . To reduce the field energy further, each of these domains can split also, resulting in smaller parallel domains with magnetization in alternating directions, with smaller amounts of field outside the material . </P> <P> The domain structure of actual magnetic materials does not usually form by the process of large domains splitting into smaller ones as described here . When a sample is cooled below the Curie temperature, for example, the equilibrium domain configuration simply appears . But domains can split, and the description of domains splitting is often used to reveal the energy tradeoffs in domain formation . </P>

A decrease in magnetic field strength is link to what