Control of light by an external magnetic field is one of the important methods for modulation of its intensity and polarisation [1]. Magneto-optical effects at the nanoscale are usually observed in magnetophotonic crystals [2,3], nanostructured hybrid materials [4][5][6] or magnetoplasmonic crystals [7][8][9][10][11]. An indirect action of an external magnetic field (e.g. through the Faraday effect) is explained by the fact that natural materials exhibit negligible magnetism at optical frequencies. However, the concept of metamaterials overcome this limitation imposed by nature by designing artificial subwavelength meta-atoms that support a strong magnetic response, usually termed as optical magnetism, even when they are made of nonmagnetic materials [12]. The fundamental question is what would be the effect of the interaction between an external magnetic field and an optically-induced magnetic response of metamaterial structures. Here we make the first step toward answering this fundamental question and demonstrate the multifold enhancement of the magneto-optical response of nanoantenna lattices due to the optical magnetism.Nanophotonics is often associated with plasmonic structures made of metals such as gold or silver. However, it is known that plasmonic structures suffer from high losses of metals, heating, and incompatibility with CMOS fabrication processes. Recent developments in the nanoscale optical physics gave birth to a new branch of nanophotonics aiming at the manipulation of opticallyinduced Mie-type resonances in dielectric nanoparticles made of materials with high refractive indices [12][13][14][15]. It has been shown recently that resonant dielectric structures offer unique opportunities for reduced dissipative losses and large resonant enhancement of both electric and magnetic fields. High-index dielectric structures can be employed as new building blocks to obtain unique functionalities such as magnetic Fano resonances [16,17], highly transmittable metasurfaces [18,19], and novel metadevices [20,21]. Here we extend the concept of high-index resonant nanophotonics to the case of magnetically active materials and study the magneto-optical (MO) response of a dielectric metasurface covered with a thin magnetic film, as shown schematically in Fig. 1. * Corresponding author: fedyanin@nanolab.phys.msu.ruWe emphasize that at the microscopic level the optical Figure 1. Schematic illustration of the enhancement of magneto-optical effects in a nanoscale structure. Linearly polarized light is focused on a dielectric metasurface composed of silicon nanoparticles supporting magnetic Mietype resonances, the metasurface is covered by a thin nickel film. The sample is subjected to the action of an external magnetic field oriented perpendicular to the wave vector of the incident light. Inset shows an SEM image of the sample.response is driven by electric dipoles, but high-index dielectric nanoparticles with this microscopic response generate effectively magnetic multipoles. Despite its geometrical simplicity, a ...