We study the ground state of a
d-dimensional Ising model with both long-range (dipole-like) and nearest-neighbor ferromagnetic (FM) interactions. The long-range interaction is equal to ${r}^{\ensuremath{-}p}$,
p>d, while the FM interaction has strength
J. If
p>d+1 and
J is large enough the ground state is FM, while if $d<p\ensuremath{\leqslant}d+1$ the FM state is not the ground state for any choice of
J. In
d=1 we show that for any
p>1 the ground state has a series of transitions from an antiferromagnetic state of period 2 to
2h-periodic states of blocks of sizes
h with alternating sign, the size
h growing when the FM interaction strength
J is increased (a generalization of this result to the case $0<p\ensuremath{\leqslant}1$ is also discussed). In $d\ensuremath{\geqslant}2$ we prove, for $d<p\ensuremath{\leqslant}d+1$, that the dominant asymptotic behavior of the ground-state energy agrees for large
J with that obtained from a periodic striped state conjectured to be the true ground state. The geometry of contours in the ground state is discussed.
Using the coupled-cluster method for high orders of approximation and Lanczos exact diagonalization we study the ground-state phase diagram of a quantum spin-
21 J1-
J2 model on the square lattice with plaquette structure. We consider antiferromagnetic (
J1>0) as well as ferromagnetic (
J1<0) nearest-neighbor interactions together with frustrating antiferromagnetic next-nearest-neighbor interaction
J2>0. The strength of interplaquette interaction $\ensuremath{\lambda}$ varies between $\ensuremath{\lambda}=1$ (which corresponds to the uniform
J1-
J2 model) and $\ensuremath{\lambda}=0$ (which corresponds to isolated frustrated 4-spin plaquettes). While on the classical level ($s\ensuremath{\rightarrow}\ensuremath{\infty}$) both versions of models (i.e., with ferro- and antiferromagnetic
J1) exhibit the same ground-state behavior, the ground-state phase diagram differs basically for the quantum case
s=1/2. For the antiferromagnetic case (
J1>0) N\'eel antiferromagnetic long-range order at small
J2/J1 and $\ensuremath{\lambda}\ensuremath{\gtrsim}0.47$ as well as collinear striped antiferromagnetic long-range order at large
J2/J1 and $\ensuremath{\lambda}\ensuremath{\gtrsim}0.30$ appear which correspond to their classical counterparts. Both semiclassical magnetic phases are separated by a nonmagnetic quantum paramagnetic phase. The parameter region, where this nonmagnetic phase exists, increases with decreasing $\ensuremath{\lambda}$. For the ferromagnetic case (
J1<0) we have the trivial ferromagnetic ground state at small
J2/∣J1∣. By increasing
J2 this classical phase gives way for a semiclassical plaquette phase, where the plaquette block spins of length
s=2 are antiferromagnetically long-range ordered. Further increasing of
J2 then yields collinear striped antiferromagnetic long-range order for $\ensuremath{\lambda}\ensuremath{\gtrsim}0.38$, but a nonmagnetic quantum paramagnetic phase $\ensuremath{\lambda}\ensuremath{\lesssim}0.38$.