Negative refraction in natural ferromagnetic metals

TL;DR

This paper demonstrates that natural ferromagnetic metals like Co and FeCo can exhibit negative refraction near ferromagnetic resonance frequencies, challenging the belief that such phenomena require artificial metamaterials.

Contribution

It shows experimentally that natural ferromagnetic metals can achieve negative refractive indices at room temperature near ferromagnetic resonance, with tunability via magnetic fields.

Findings

Negative refraction observed in Co and FeCo alloys.

Negative refractive index achieved near ferromagnetic resonance.

Refractive index tunable with moderate magnetic fields.

Abstract

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.