  We present a method for inducing and controlling van der Waals torques
between two parallel slabs using a constant magnetic field. The torque is
calculated using the Barash theory of dispersive torques. In III-IV
semiconductors such as $InSb$, the effect of an external magnetic field is to
induce an optical anisotropy, in an otherwise isotropic material, that will in
turn induce a torque.
  The calculations of the torque are done in the Voigt configuration, with the
magnetic field parallel to the surface of the slabs. As a case study we
consider a slab made of calcite and a second slab made of $InSb$. In the
absence of magnetic field there is no torque. As the magnetic field increases,
the optical anisotropy of $InSb$ increases and the torque becomes different
from zero, increasing with the magnetic field. The resulting torque is of the
same order of magnitude as that calculated using permanent anisotropic
materials when the magnetic fields is close to 1 T.
