Magnetic single wall CrI3 nanotubes encapsulated within multiwall Carbon Nanotubes

CrI3 is a layered ferromagnetic insulator that has recently attracted enormous interest as it was the first example of a stand-alone monolayer ferromagnet, paving the way towards the study of two-dimensional magnetic materials and their use as building blocks of hybrid van der Waals layered heterostructures. Here we go one step down in the dimensionality ladder and report the synthesis and characterization of a tubular one-dimensional van der Waals heterostructure where CrI3 nanotubes are encapsulated within multiwall carbon nanotubes, integrating a magnetic insulator and a conductor. By means of the capillary filling of multi-wall carbon nanotubes (MWCNT), we obtained single-wall CrI3 nanotubes with diameters ranging between 2 nm and 10 nm, with an average of 5.3 nm. Using aberration corrected electron microscopy in combination with spectroscopic techniques we confirm the structure and chemical composition of the nanotubes. SQUID measurements, combined with element-specific X-ray magnetic circular dichroism (XMCD) indicate unequivocally that the Cr atoms in encapsulated CrI3 nanotubes are magnetic with a collective state compatible with a radial magnetization state predicted both by first-principles calculations and a model Hamiltonian. Our results represent a step forward in establishing 1D van der Waals heterostructures as a playground for the exploration of non-collinear magnetic states arising from the interplay between magnetic anisotropy and curvature in tubular geometries.