Negative refraction in natural ferromagnetic metals

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.