Rare-earth element Gd substituted Fe3O4 films were fabricated by reactive sputtering. The magnetic properties together with theoretical calculations indicate that Gd3+ ions prefer to occupy the tetrahedral sites. The structural analyses and magnetic properties show that phase Gd3Fe5O12 exists in the Gd-rich Fe3 -xGdxO4 films. Moreover, theoretical calculations suggest that Fe2.75Gd0.25O4 is half metal with 100% spin polarization. The carrier transport across Fe2.8Gd0.2O4/Nb:SrTiO3 interface was studied to estimate the spin polarization. Current-voltage curves of Fe2.8Gd0.2O4/Nb:SrTiO3 junction show rectifying behavior and the spin polarization of the transport electrons from Fe2.8Gd0.2O4 into Nb:SrTiO3 is determined to be 0.26 at 300 K. (C) 2015 Elsevier B.V. All rights reserved.