We investigated the spin filter effect in tunnel junctions with a Ni1 + xFe2 - xO4 ferrimagnetic tunnel barrier. For higher T-c above room temperature, Ni1 + xFe2 - xO4 film should be thicker than 4.0 nm and grown above 400 degrees C. The spin filter junctions (SFJs) of Fe3O4(001)/Ni1 + xFe2 - xO4(001)/Al2O3/Fe/Au grown on MgO(001) substrates exhibited an inverse magnetoresistance effect at room temperature, which is consistent with the band calculation of NiFe2O4. (C) 2011 Elsevier B.V. All rights reserved.