Composition-Dependent Antiferromagnetism in Low-Dimensional Hybrid Organic Cobalt-Halide Semiconductors

Low-dimensional magnets exhibit complex magnetic phases for studying fundamental spin ordering phenomena, while semiconducting variants of these materials are explored for all-optical control and readout of magnetic states. Thus, the discovery of systems with compositional tuning of optical and magnetic properties unlocks emerging functionalities and fundamental investigations. Here, we report the synthesis of low-dimensional (canted-)antiferromagnets (R)-3YMBA 2 CoX 4 (Y, X = halide; MBA = methylbenzylamine) and study their magnetic and optical properties. Structural characterization confirms distorted Co-halide tetrahedra separated by organic groups, forming chiral crystal structures. Magnetic measurements reveal composition-dependent (canted) antiferromagnetism at high cryogenic temperatures. While (R)-3ClMBA 2 CoBr 4 was identified as an easy-plane-type antiferromagnet, (R)-3BrMBA 2 CoI 4 was found to exhibit canted antiferromagnetism showing a 50 times higher spontaneous magnetic moment than that of reported hybrid metal halides. Using optical measurements, we find an optically active metal–ligand charge-transfer state involving cobalt atoms that participate in the magnetic ordering. Using photoexcitation of the metal–ligand transfer state for optical spin-alignment in ultrafast magneto-optical measurements, we find a spin dephasing time of 60 ps in our hybrid cobalt compounds, exceeding spin lifetimes of related low-dimensional hybrid metal halides. Our report introduces a promising material platform for symmetry-mediated spin control, paving the way for hybrid quantum materials with optical spin manipulation capabilities.