Effects of spin–orbit interactions in condensed matter are an important and rapidly evolving topic. Strong competition between spin–orbit, on-site Coulomb and crystalline electric field interactions in iridates drives exotic quantum states that are unique to this group of materials. In particular, the ‘ J _eff = ½’ Mott state served as an early signal that the combined effect of strong spin–orbit and Coulomb interactions in iridates has unique, intriguing consequences. In this Key Issues Review , we survey some current experimental studies of iridates. In essence, these materials tend to defy conventional wisdom: absence of conventional correlations between magnetic and insulating states, avoidance of metallization at high pressures, ‘S-shaped’ I – V characteristic, emergence of an odd-parity hidden order, etc. It is particularly intriguing that there exist conspicuous discrepancies between current experimental results and theoretical proposals that address superconducting, topological and quantum spin liquid phases. This class of materials, in which the lattice degrees of freedom play a critical role seldom seen in other materials, evidently presents some profound intellectual challenges that call for more investigations both experimentally and theoretically. Physical properties unique to these materials may help unlock a world of possibilities for functional materials and devices. We emphasize that, given the rapidly developing nature of this field, this Key Issues Review is by no means an exhaustive report of the current state of experimental studies of iridates.

Colossal magnetoresistance is of great fundamental and technological significance and exists mostly in the manganites and a few other materials. Here we report colossal magnetoresistance that is starkly different from that in all other materials. The stoichiometric ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ is an insulator featuring a ferrimagnetic transition at 78 K. The resistivity drops by seven orders of magnitude with an applied magnetic field above 9 T, leading to an insulator-metal transition at up to 130 K. However, the colossal magnetoresistance occurs only when the magnetic field is applied along the magnetic hard axis and is surprisingly absent when the magnetic field is applied along the magnetic easy axis where magnetization is fully saturated. The anisotropy field separating the easy and hard axes is 13 T, unexpected for the Mn ions with nominally negligible orbital momentum and spin-orbit interactions. Double exchange and Jahn-Teller distortions that drive the hole-doped manganites do not exist in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. The phenomena fit no existing models, suggesting a unique, intriguing type of electrical transport.

We report an unusual magnetic ground state in single-crystal, double-perovskite $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ and Sr-doped $\mathrm{B}{\mathrm{a}}_2\mathrm{YIr}{\mathrm{O}}_6$ with $\mathrm{I}{\mathrm{r}}^{5+}\left(5d^4\right)$ ions. Long-range magnetic order below 1.7 K is confirmed by dc magnetization, ac magnetic susceptibility, and heat-capacity measurements. The observed magnetic order is extraordinarily delicate and cannot be explained in terms of either a low-spin $S=1$ state, or a singlet $J_{\mathrm{eff}}=0$ state imposed by the spin-orbit interactions (SOI). Alternatively, the magnetic ground state appears consistent with a SOI that competes with comparable Hund's rule coupling and inherently large electron hopping, which cannot stabilize the singlet $J_{\mathrm{eff}}=0$ ground state. However, this picture is controversial, and conflicting magnetic behavior for these materials is reported in both experimental and theoretical studies, which highlights the intricate interplay of interactions that determine the ground state of materials with strong SOI.

Electrical control of structural and physical properties is a long-sought, but elusive goal of contemporary science and technology. We demonstrate that a combination of strong spin-orbit interactions (SOI) and a canted antiferromagnetic Mott state is sufficient to attain that goal. The antiferromagnetic insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ provides a model system in which strong SOI lock canted Ir magnetic moments to ${\mathrm{IrO}}_6$ octahedra, causing them to rigidly rotate together. A novel coupling between an applied electrical current and the canting angle reduces the Néel temperature and drives a large, nonlinear lattice expansion that closely tracks the magnetization, increases the electron mobility, and precipitates a unique resistive switching effect. Our observations open new avenues for understanding fundamental physics driven by strong SOI in condensed matter, and provide a new paradigm for functional materials and devices.

We report an optimized chemical vapor transport method to grow single crystals of (Mn_1−xNi_x)₂P₂S₆ where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature T_N is non-monotonous; with increasing Ni²⁺ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn₂P₂S₆ (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni₂P₂S₆ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn_1−xNi_x)₂P₂S₆ series. Transport studies under hydrostatic pressure on the parent compound Mn₂P₂S₆ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa.