Ultrathin layers of the versatile spinel oxide NiFe2O4 can be conductive or insulating depending on the growth conditions. Conductive NiFe2O4 (NFO) electrodes are inserted into conventional magnetic tunnel junctions containing La2/3Sr1/3MnO3/SrTiO3 (LMSO/STO) bilayers, and insulating and ferrimagnetic NiFe2O4 layers are used as barriers to define a spin filter (see figure). The ability of such heterostructures to highly spin-polarize electrons injected from a non-magnetic electrode is demonstrated.