Question

what is the Ishing model? what are it's merits and demerits?. in what way Ishing model ids different from Landau phase transition model or theory?

Answer 1

Ising model is a model for ferromagnetism and is one of the simpliest, non-trivial model of systems with interacting degrees of freedom.

Merits:

The Ising model is appropriate for spin systems where the energy is invariant to reflecting every spin to its opposite orientation.

Demerits:

The Ising model is not useful for studying universal properties of phase transitions in spin systems with a certain symmetry, like those systems where the energy of a configuration of spins is invariant to rotating every spin in the same way around the unit sphere.

Comparison with Landau model:

Ising model is just an attempt to simulate the structure of a ferromagnetic substance. On the other hand, Landau model is a phenomenological analysis of the phase transitions that involve a change of symmetry.