Carbon nanotube confinement reshapes exchange topology in a quasi-one-dimensional VOCl ribbon.

Abstract

Carbon nanotube confinement can provide more than structural support for one-dimensional guest materials: it can reshape the exchange network that selects their magnetic state. Using first-principles calculations, we show that a relaxed quasi-one-dimensional vanadium oxychloride (VOCl) ribbon weakly favors ferromagnetism among the tested collinear states but remains energetically unfavorable relative to bulk VOCl. Encapsulation inside an armchair (11,11) carbon nanotube binds and preserves the ribbon while strengthening its ferromagnetic preference relative to the free ribbon. Comparative exchange mapping separates the effects of dimensional reduction, structural deformation, and the nanotube environment: deformation into the confined geometry alone stabilizes an alternating antiferromagnetic state, whereas restoring the nanotube reverses the sign of the transverse exchange interaction and strengthens the ferromagnetic diagonal exchange network. The FM preference persists for Ueff = 3-5 eV and in static magnetic calculations performed on the final snapshot of a 5 ps NVT ab initio molecular dynamics simulation at 300 K. Constrained finite-displacement calculations with the CNT host retained show no imaginary modes within the VOCl subspace. Electronic-structure and spin-density analyses identify V 3d states as the primary magnetic degrees of freedom, while the exchange comparison demonstrates that the weakly coupled nanotube host materially alters the magnetic hierarchy. Spin-orbit calculations place the easy magnetization direction along the tube axis. Under equivalent confinement, FeOCl remains strongly antiferromagnetic, confirming that the exchange response is chemistry dependent. These results demonstrate that nanotube confinement can selectively reorganize exchange interactions in metastable quasi-one-dimensional magnetic motifs.

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