In this paper, we have demonstrated a detailed study of Electromagnetic interaction with a biased gyromagnetic medium. Our key contributions in this paper are

• For the first time, we categorized the topology classes of isofrequency surfaces and identified the connection between the evolution of the isofrequency surface topology with the gyrotropicity of the medium.

• We revealed the relation between photonic-spin and the isofrequency topology.

• The existence of cutoff-less waveguide modes is shown in the paper, owing to the hyperbolic isofrequency surface topology. This paves the way for extremely small guided structures.

• A new type of cut-off for the waveguide mode is revealed, which we call gyrotropy-induced cut-off. This cut-off manifests as a result of interaction between the material-induced and structure-induced photonic spins.

This paper got published online in Physical Review A on 25th February 2022.