The effect of a controlled ion beam pre-treatment of (111)-oriented Gd3Ga5O12 substrates on the magneto-optical properties and surface morphology of the ultrathin bismuth-substituted yttrium-iron garnet films with a composition Bi2.8Y0.2Fe5O12 was studied. It has been shown that the observed sign inversion of magneto-optical effects (Faraday rotation and magnetic circular dichroism) observed in films that were deposited on the GGG substrate pre-treated by 1 keV and 4 keV Ar+ ion beams is a result of the substrate surface amorphization caused by the ion bombardment. (C) 2014 Elsevier Ltd. All rights reserved.