Rare-earth iron garnets (RIGs), R3Fe5O12 (with R= Y, Tm, Er…) are of high interest in many research fields such as spintronics (small damping parameters, chiral spin textures…), magnonic and photonics (Faraday rotation, optical isolators). Here we want to assess the element specific magnetic properties of epitaxial TmIG and ErIG thin films and to determine the impact of rare-earth substitution on the local magnetic exchange couplings in these iron garnets.