Chemical Design of\nSpin Frustration to Realize Topological\nSpin Glasses

Patterning spins to generate collective behavior is at\nthe core\nof condensed matter physics. Physicists develop techniques, including\nthe fabrication of magnetic nanostructures and precision layering\nof materials specifically to engender frustrated lattices. As chemists,\nwe can access such exotic materials through targeted chemical synthesis\nand create new lattice types by chemical design. Here, we introduce\na new approach to induce magnetic frustration on a modified honeycomb\nlattice through a competition of alternating antiferromagnetic (AFM)\nand ferromagnetic (FM) nearest-neighbor interactions. By subtly modulating\nthese two types of interactions through facile synthetic modifications,\nwe created two systems: (1) a topological spin glass and (2) a frustrated\nspin-canted magnet with low-temperature exchange bias. To design this\nunconventional magnetic lattice, we used a metal–organic framework\n(MOF) platform, Ni<sub>3</sub>(pymca)<sub>3</sub>X<sub>3</sub> (NipymcaX\nwhere pymca = pyrimidine-2-carboxylato and X = Cl, Br). We isolated\ntwo MOFs, NipymcaCl and NipymcaBr, featuring canted Ni<sup>2+</sup>-based moments. Despite this similarity, differences in the single-ion\nanisotropies of the Ni<sup>2+</sup> spins result in distinct magnetic\nproperties for each material. NipymcaCl is a topological spin glass,\nwhile NipymcaBr is a rare frustrated magnet with low-temperature exchange\nbias. Density functional theory calculations and Monte Carlo simulations\non the NipymcaX lattice support the presence of magnetic frustration\nas a result of alternating AFM and FM interactions. Our calculations\nenabled us to determine the ground-state spin configuration and the\ndistribution of spin–spin correlations relative to paradigmatic\nkagomé and triangular lattices. This modified honeycomb lattice\nis similar to the electronic Kekulé-O phase in graphene and\nprovides a highly tunable platform to realize unconventional spin\nphysics.