Ok, that makes sense. But if it wasn't a spin-1/2 system, like in part (f) of question 5.3, then the Ising and Heisenberg models are not equivalent?
So, is it safe to always project spins on the z-axis in an exam when spins are not 1/2, saying this is due to assuming strong anisotropy?
Sorry for being a bit pedantic!
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