Heisenberg ferromagnet
= Heisenberg ferromagnet
{c}
A <Heisenberg ferromagnet> has negative exchange coupling in $H=J\sum_{\langle ij\rangle}\mathbf S_i\mathbin\cdot\mathbf S_j$, so neighboring <spins> energetically prefer parallel alignment.
= Heisenberg ferromagnet
{c}
A <Heisenberg ferromagnet> has negative exchange coupling in $H=J\sum_{\langle ij\rangle}\mathbf S_i\mathbin\cdot\mathbf S_j$, so neighboring <spins> energetically prefer parallel alignment.