In previous decades significant efforts have been devoted to increasing the magneto-optical efficiency of iron garnet materials for the miniaturization of nonreciprocal devices such as isolators and circulators. Elemental substitutions or proper nano-structuring to benefit from optical resonances have been pursued. However, all these approaches still require film thicknesses of at least several tens of microns to deliver useful device applications, and suffer from narrow bandwidths in the case of optical resonance effects. This Letter reports on a newly discovered enhancement of the Faraday Effect observed experimentally in nanoscale bismuth-substituted iron garnet films. It is shown here that this enhancement is not due to elemental substitution or compositional variations. Nor is it due to photon trapping or resonance effects. Comprehensive experimental and theoretical analysis of the Faraday rotation reveals a dramatic seven-fold amplification in the magneto-optic gyrotropy within only 2 nm of the air-surface interface, corresponding to just a couple of atomic monolayers as a result of symmetry-breaking at the airfilm interface. This finding opens up an avenue to the application of monolayer magnetic garnets for the control of light.Rare-earth iron garnet films are technologically important since they are among the main materials used in magneto-photonics, ultrafast magnetism and spintronics [1,2]. In particular, they are used extensively in optical circuits for the fabrication of nonreciprocal optical devices. Isolators and circulators, operating as optical filters rely on the non-reciprocal functionality of iron garnets to protect laser sources from reflected downstream light that might otherwise enter the cavity and destabilize the signal [2-4].Magneto-optical light modulators and magnetic sensors utilize iron garnet films for efficient magneto-optical response and outstanding performance [3][4][5]. On the other hand, iron garnet thin

No abstract