Interface structures and magnetoresistance in magnetic tunnel junctions

The interface structures of magnetic tunnel junctions were studied using x- ray photoelectron spectroscopy (XPS). The structures were correlated with m agnetoresistance (MR) characteristics. For MR measurements, Fe(50 nm)/AlOx/ CoFe(30 nm) junctions with an in situ naturally oxidized Al tunnel barrier were fabricated. The thickness of the Al layer, an important parameter in M R characteristics, was varied from 0 to 5 nm. MR curves showed that the lar gest MR ratio occurred when the Al layers were 2-3 nm in thickness. XPS ana lysis showed that an Al layer greater than 1 nm thick covers the entire sur face of the Fe underlayer. However, if the Al layer is more than 1 nm thick , the unoxidized Al remaining after the oxidation process increases as the thickness is increased. For Al layers that are greater than 3 nm thick, the MR ratio is strongly affected by unoxidized Al, probably due to the decrea se in spin polarization at the surface of an Fe/Al electrode. On the other hand, the hysteresis loops indicate that the difference in coercive force b etween Fe and CoFe layers reduces with decreasing Al thickness for Al layer s less than 2.5 nm thick. This means that the antiparallel direction of mag netization in the two layers becomes incomplete due to the gradual increase of the ferromagnetic coupling between them. As a result, the MR ratio decr eases, although a 1-nm-thick Al layer seems to be enough to cover the Fe su rface. (C) 1999 American Institute of Physics. [S0021-8979(99)49908-6].