  Ultrafast non-thermal manipulation of magnetization by light relies on either
indirect coupling of the electric field component of the light with spins via
spin-orbit interaction or direct coupling between the magnetic field component
and spins. Here we propose a novel scenario for coupling between the electric
field of light and spins via optical modification of the exchange interaction,
one of the strongest quantum effects, the strength of which can reach 1000
Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be
called the inverse magneto-refraction, is allowed in a material of any
symmetry. Its existence is corroborated by the experimental observation of THz
emission by magnetic-dipole active spin resonances optically excited in a broad
class of iron oxides with a canted spin configuration. From its strength we
estimate that a sub-picosecond laser pulse with a moderate fluence of ~ 1
mJ/cm^2 acts as a pulsed effective magnetic field of 0.01 Tesla, arising from
the optically perturbed balance between the exchange parameters. Our findings
are supported by a low-energy theory for the microscopic magnetic interactions
between non-equilibrium electrons subjected to an optical field which suggests
a possibility to modify the exchange interactions by light over 1 %.
