Magnetic Monopoles: Magnets come with a north and a south pole. Despite being predicted to exist, searches in astronomy and in high-energy particle physics experiments for magnetic monopoles (either north or south on their own) have defied observation. Theoretical work in condensed-matter systems has predicted that spin-ice structures may harbor such elusive particles (see the Perspective by Gingras ). Fennell et al. (p. 415 , published online 3 September) and Morris et al. (p. 411 , published online 3 September) used polarized neutron scattering to probe the spin structure forming in two spin-ice compounds—Ho ₂ Ti ₂ O ₇ and Dy ₂ Ti ₂ O ₇ —and present results in support of the presence of magnetic monopoles in both materials.

The magnetic structure of the quasi-two-dimensional antiferromagnet ${\mathrm{NiPS}}_3$ has been determined by magnetometry and a variety of neutron diffraction techniques. The experiments show that the samples must be carefully handled, as gluing influences the magnetometry measurements while preferred orientation complicates the interpretation of powder diffraction measurements. Our global set of consistent measurements show numerous departures from previously published results. We show that the compound adopts a k = $\left[010\right]$ antiferromagnetic structure with the moment directions mostly along the $a$ axis, and that the paramagnetic susceptibility is isotropic. The critical behavior was also investigated through the temperature dependence of the magnetic Bragg peaks below the Néel temperature.

The pyrochlore material ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been suggested to show “spin ice” behavior. We present neutron scattering and specific heat results that establish unambiguously that ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ exhibits spin ice correlations at low temperature. Diffuse magnetic neutron scattering is quite well described by a nearest neighbor spin ice model and very accurately described by a dipolar spin ice model. The heat capacity is well accounted for by the sum of a dipolar spin ice contribution and an expected nuclear spin contribution, known to exist in other ${\mathrm{Ho}}^{3+}$ salts. These results settle the question of the nature of the low temperature spin correlations in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ for which contradictory claims have been made.

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of ${\text{FePS}}_3$, an $S=2$ Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of ${\mathbf{k}}_M=\left[01\frac{1}{2}\right]$ and the moments are collinear with the normal to the $ab$ planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were $J_1=1.46, J_2=-0.04$, and $J_3=-0.96$ meV. The single-ion anisotropy is large, $\mathrm{\Delta }=2.66$ meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, $J^{\prime }=-0.0073$ meV, proving that ${\text{FePS}}_3$ is a very good approximation to a two-dimensional magnet.

${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been suggested to be a realization of the frustrated $〈111〉\mathrm{XY}$ pyrochlore lattice antiferromagnet, for which theory predicts fluctuation-induced symmetry breaking in a highly degenerate ground state manifold. We present a theoretical analysis of the classical model compared to neutron scattering experiments on the real material, both below and above $T_N=1.173\mathrm{}\left(2\right)\mathrm{}\mathrm{K}.$ The model correctly predicts the ordered magnetic structure, suggesting that the real system has order stabilized by zero-point quantum fluctuations that can be modeled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features.