  Structured light provides unique opportunities to spatially tailor the
electromagnetic field of laser beams. This includes the possibility of a
sub-wavelength spatial separation of their electric and magnetic fields, which
would allow isolating interactions of matter with pure magnetic (or electric)
fields. This could be particularly interesting in molecular spectroscopy, as
excitations due to electric and -- usually very weak -- magnetic transition
dipole moments can be disentangled. In this work, we show that the use of
tailored metallic nanoantennas drastically enhances the strength of the
longitudinal magnetic field carried by an ultrafast azimuthally polarized beam
(by a factor of $\sim65$), which is spatially separated from the electric field
by the beam's symmetry. Such enhancement is due to favorable phase-matching of
the magnetic field induced by the electronic current loops created in the
antennas. Our particle-in-cell simulation results demonstrate that the
interaction of moderately intense ($\sim10^{11}$ W/cm$^2$) and ultrafast
azimuthally polarized laser beams with conical, parabolic, Gaussian, or
logarithmic metallic nanoantennas provide spatially isolated magnetic field
pulses of several tens of Tesla.
