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In this paper, we have proposed a magneto-optic (MO) surface plasmon polariton (SPP) modulator based on variations of refractive index. For description of modulator operation, we have analyzed the MO effects in the insulator–metal–insulator (IMI) SPP slab waveguides in a transversal configuration in which the applied magnetic field is parallel to the interfaces and normal to the wave propagation direction. We have derived an exact dispersion relation by considering MO effects for one of the side layers by the separation of variables method. The cut-off conditions have been studied for the SPP modes guided by IMI structures as a function of the variations of the dielectric constants of the side layers. We have shown that the SPP modes always propagate in a symmetric structure and the SPP odd modes do not have a cut-off dielectric constant in an asymmetric structure. Also, we have shown that in an asymmetric IMI configuration, the SPP even mode has a cut-off effective dielectric constant for all metal layer thicknesses. These configurations can be used to design active devices, such as switches and modulators to be used in photonic integrated circuits.