Tunnel junctions consisting of a three layered stack of Co, Al 2 O 3 and Ni 80 Fe 20 (Permalloy) have been prepared by evaporation, oxidation and subsequent e-beam lithography using a technique known from the production of single electron transistors. Contacts to the tunnel junctions were provided by conventionally patterned Au conducting lines. With this technique, active areas of electron tunneling with dimensions down to 150nm x 200nm were achieved. The devices show consequently a pronounced coulomb blockade behavior at low temperature, indicating the dominance of single electron transport across the tunneling barrier. Measurements of the tunneling current as a function of an external magnetic field reveal an additional dependence of the tunneling resistance related to spin dependent transport of the electrons. At a temperature below about 100K, the resistance changes by up to 15% in a narrow range of the applied magnetic field around 5000e. This behavior can be attributed to the switching of the magnetization of one of the ferromagnetic tunneling contacts, i.e. to an antiparallel alignment of the magnetization directions. The properties of such devices are therefore promising for the application in magnetic memories (MRAM's) and spin-transistors.