  It is generally believed that Veselago's criterion for negative refraction
cannot be fulfilled in natural materials. However, considering imaginary parts
of the permittivity ({\epsilon}) and permeability ({\mu}) and for metals at not
too high frequencies the general condition for negative refraction becomes
extremely simple: Re({\mu}) < 0 --> Re(n) < 0. Here we demonstrate
experimentally that in such natural metals as pure Co and FeCo alloy the
negative values of the refractive index are achieved close to the frequency of
the ferromagnetic resonance. Large values of the negative refraction can be
obtained at room temperature and they can easily be tuned in moderate magnetic
fields.
