Van der Waals (VdW) materials have opened new directions in the study of low dimensional magnetism. A largely unexplored arena is the intrinsic tuning of VdW magnets toward new ground states. Chromium trihalides provided the first such example with a change of interlayer magnetic coupling emerging upon exfoliation. Here, we take a different approach to engineer previously unknown ground states, not by exfoliation, but by tuning the spin-orbit coupling (SOC) of the nonmagnetic ligand atoms (Cl, Br, I). We synthesize a three-halide series, CrCl_{3 − x − y}Br_xI_y, and map their magnetic properties as a function of Cl, Br, and I content. The resulting triangular phase diagrams unveil a frustrated regime near CrCl₃. First-principles calculations confirm that the frustration is driven by a competition between the chromium and halide SOCs. Furthermore, we reveal a field-induced change of interlayer coupling in the bulk of CrCl_{3 − x − y}Br_xI_y crystals at the same field as in the exfoliation experiments.

Searching for a Kitaev spin liquid phase motivated intense research on the honeycomb iridate materials. However, access to a spin liquid ground state has been hindered by magnetic ordering. ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ is a new honeycomb iridate without thermodynamic signatures of a long-range order. Here, we use muon spin relaxation to uncover the magnetic ground state of ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$. We find a two-component depolarization with slow- and fast-relaxation rates corresponding to distinct regions with dynamic and static magnetism coexisting in the material. X-ray absorption spectroscopy and first-principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of ${\mathrm{Cu}}^{2+}$ ions nucleate regions of static magnetism, whereas the majority of ${\mathrm{Cu}}^{+}/{\mathrm{Ir}}^{4+}$ on the honeycomb lattice give rise to a Kitaev spin liquid.

The family of binary lanthanum monopnictides, LaBi and LaSb, has attracted a great deal of attention as it displays an unusual extreme magnetoresistance (XMR) that is not well understood. Two classes of explanations have been offered for this: the presence of nontrivial topology and the compensation between electron and hole densities. Here, by synthesizing a new member of the family, LaAs, and performing transport measurements, angle-resolved photoemission spectroscopy, and density functional theory calculations, we show that (a) LaAs retains all qualitative features characteristic of the XMR effect but with a significant reduction in magnitude compared to LaSb and LaBi, (b) the absence of a band inversion or a Dirac cone in LaAs indicates that topology is insignificant to XMR, (c) the equal number of electron and hole carriers indicates that compensation is necessary for XMR but does not explain its magnitude, and (d) the ratio of electron and hole mobilities is much different in LaAs compared to LaSb and LaBi. We argue that the compensation is responsible for the XMR profile and the mobility mismatch constrains the magnitude of XMR.