Tunable ferromagnetic spin ordering in boron nitride nanotubes with topological fluorine adsorption

We find through first-principles calculations that fluorine atoms topologically adsorbed on boron nitride nanotubes induce long-ranged ferromagnetic spin ordering along the tube, offering strong spin polarization around the Fermi level. The spin polarization and magnetic moment increase significantly with decreasing tube radius, even giving rise to half-metal when the tube diameter is reduced to 3.3 A, while in a flat boron nitride sheet with the same topological fluorine arrangement the magnetic moment nearly disappears. This radius-dependent behavior is then developed into a local curvature modulation procedure to efficiently enhance or quench the ferromagnetic ordering, which enables the F-BNNTs to function as piezomagnetic nanotubes. These findings suggest a new route to facilitate the design of tunable spin devices.