  NMR experiments on superfluid 3He in a nematic aerogel have shown that the
stability of the half-quantum vortex (HQV) realized in the polar phase depends
on the relative direction between the magnetic field and the anisotropy axis
brought by the aerogel. The vortex energy in the polar phase is examined in
terms of the Ginzburg-Landau free energy incorporating the dipole energy, and
the reason why the HQV in the polar phase in nematic aerogels became unstable
in a transverse magnetic field upon a field-cooling is explained.
