 ![ Effect of curvature (anisotropic strain) on Ni$_{sub}$ in (n,n) tubes. Upper panel: Illustration of the dominant hoppings at the defect site in graphene. The equivalence between the electronic structure of a Ni$_{sub}$ impurity and a carbon vacancy is stressed here. The carbon sheet is rolled around the $(n,0)$ direction in order to form the armchair tubes. Lower panel: Scheme of the main Ni$_{sub}$ impurity energy levels as a function of curvature. One of the impurity levels with antibonding C 0$sp$-Ni 0$d$ character is shifted downwards and, for large enough curvatures, becomes partially populated and spin-polarized. Adapted from (Santos et al. 0000). []{data-label="fig:fig0-Ni-tubes"}](figures_arXiv/