What new features does the Heisenberg Model have compared to the Ising Model? Both the Ising and the Heisenberg Models describe spin lattices with interaction on first neighbors. The Hamiltonian in each case is quite similar, despite the fact of treating de spins as Ising variables (1 or -1) or as quantum operators. In the Ising case it looks like
$$H_\textrm{Ising} = -~J \sum_{\langle i\ j\rangle} s^z_{i}\ s^z_{j}$$
where J is the coupling constant ($J>0$ for ferromagnet and $J<0$ for anti-ferromagnet), $\langle i\ j\rangle$ represents sum over first neighbors and $s^z$ is the spin in z direction. On the other hand, the Heisenberg model is
$$H_\textrm{Heisenberg} = -~J \sum_{\langle i\ j\rangle} \hat{S}_{i} \cdot \hat{S}_{j}$$
where the only difference lies in the spins being operators. (In both cases I took away the interaction with an external field for simplicity)
My Question is: What new phenomena does treating the spins as operators brings? I can see that  $\hat{S}_{i}\ .\ \hat{S}_{j}$ takes account of the spin in every direction and not just z, but I can't see the physical implication of that.
 A: One of the main differences is that the Ising model lies on a discrete symmetry (the $Z_2$ symmetry) while the Heisenberg model lies on a continuous one (rotational symmetry). It will affect the phase transitions that these models undergo.
In particular, because of the Mermin-Wagner theorem, there can be no finite-temperature phase transition of the Heisenberg model in $d=2$ (leaving apart the very special case of the BKT transition).
This is not the case of the Ising model that undergoes a phase transition at finite temperature from a high-$T$ disordered state to a low-$T$ ordered state in $d=2$ (the exact solution has even been calculated by Onsager and later by others).
There's probably much more to it than this particular case, feel free to edit my answer if you feel like adding something.
A: As this is a list-like question, let me list a few things (without much discussion -- feel free to ask specific questions about individual points). Each item mentions what the Heisenberg model (HM) has as opposed to the Ising model (IM). 


*

*continuous symmetry vs. discrete symmetry

*as a consequence: gapless excitations whenever the symmetry is broken (i.e. in all cases except the 1D antiferromagnet -- in that case, however, there are gapless modes due to the Lieb-Schultz-Mattis theorem)

*as a consequence thereof: no spontaneous symmetry breaking in one and two dimensions at finite temperature (as opposed to the 2D HAFM), this is the Mermin-Wagner theorem

*non-commuting terms: The eigenstates will typically not have a simple form (as opposed to the IM, which has commuting terms)

*the IM Hamiltonian has integer eigenvalues (times $J$), while we cannot easily characterize the eigenvalues of the HM
Some caveats, however:


*

*some of these properties hold because of continuous vs. discrete symmetry, rather than classical vs. quantum

*some of the properties hold because of commuting vs. non-commuting rather than discrete vs. continuous symmetry

*some of these properties only hold for (infinite) lattices, others already on the level of just a few spins
