Nanotube Encapsulation of Single-Molecule and 1D van der Waals Magnets: Confinement-Tuned Magnetism.

Single-molecule magnets (SMMs) and low-dimensional van der Waals (vdW) magnets are highly promising for quantum technologies, spintronics, and data storage, yet their integration into practical devices is frequently impeded by chemical reactivity, aggregation, or loss of magnetic function during processing. Encapsulation within carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) has emerged as a versatile strategy to protect, isolate, align, and electronically address these fragile magnetic species at the atomic scale. This review provides a comprehensive overview of encapsulated magnetic materials, focusing on two interconnected themes: molecular magnets and one-dimensional (1D vdW magnetic heterostructures. We examine the fundamental principles governing nanotube encapsulation, contrasting the distinct host-guest interactions in metallic/semiconducting CNTs versus insulating BNNTs. Foundational studies demonstrate that SMMs from Mn12 acetate and TbPc2 to endohedral metallofullerenes and organometallic DyCp3 retain their magnetic bistability upon encapsulation, with confinement sometimes enhancing relaxation times and suppressing quantum tunneling. Parallel advances have enabled the synthesis of truly 1D 1D vdW magnetic materials inaccessible by conventional means, including transition metal trihalide chains (CrX3, VX3), dihalide chains (CrX2), and complex tellurides (VxTey), whose magnetic ground states such as ferromagnetic, antiferromagnetic, or spin glass are tunable through nanotube diameter and charge transfer. Spectroscopic signatures of host-guest interactions, magnetic properties, and emerging applications in spintronics, quantum information, and biomedical imaging are critically assessed. We conclude by identifying key challenges such as synthetic control, scalability, and device integration that must be overcome to realize the technological potential of these remarkable magnetic heterostructures.