  We present theoretical evidence, based on total-energy first-principles
calculations, of the existence of spin-polarized states well localized at and
extended along the edges of bare zigzag boron nitride nanoribbons. Our
calculations predict that all the magnetic configurations studied in this work
are thermally accessible at room temperature and present an energy gap. In
particular, we show that the high spin state, with a magnetic moment of 1
$\mu_B$ at each edge atom, presents a rich spectrum of electronic behaviors as
it can be controlled by applying an external electric field in order to obtain
metallic $\leftrightarrow$ semiconducting $\leftrightarrow$ half-metallic
transitions.
