Question

why in domain theory of ferromagnrtiswm the domain wall wii shrink or expand ?

Answer 1

Growing is enthalpically favored, because the density of opposite-spin interaction is lowered. But it reduces the entropy, also. So how it works out generally depends on the temperature. At T=0 the stable state is one giant domain -- no walls. At T = infinity the stable state is a very large number of very small domains. Generally we observe a sharp 2nd order phase transition at what's called the Curie temperature, e.g at 770C for iron.