Tunneling electron spin-polarization in clean magnetic tunnel junctions

The tunnel current in metal - insulator - metal junctions has a preferred spin contribution when at least one electrode is also ferromagnetic. The origin for this tunneling electron spin-polarization is twofold. Firstly, the supply of electrons, i.e., the density of itinerant electron states near the Fermi energy (where most tunnel current originates), differs for majority and minority spin electrons. The cause is the exchange splitting in the magnetic 3d elements, also resulting in a magnetic moment. Secondly, the tunneling probability depends sensitively on the matching of the (spin-dependent) wave functions of the ferromagnet to those of the insulator. It follows that the polarization depends on both the intrinsic properties of the ferromagnet as the ferromagnet - insulator combination. In the present study the polarizations of tunneling electrons of Ni, Fe, Co, and a range of NiFe and CoFe alloys have been obtained using the BCS-character of a thin superconducting Al layer as a spin-sensitive detector. The junctions were prepared fully in-situ using a glow discharge barrier formation process. It was found that the polarizations have been greatly improved over previous values as a result of cleaner interfaces. For the new results, the previously proposed relation between polarization and magnetic moment will be addressed. Further, the influence of the tunnel barrier will be discussed by comparing various insulating materials. Finally, the results will be related to the recently found large junction magnetoresistance at room temperature in ferromagnet - insulator - ferromagnet junctions.