Giant magnetoresistance of magnetic multilayer point contacts

We have studied the giant magnetoresistance (GMR) in magnetic multilayer po int contacts of three different types. The first generation contacts were m ade by deposition with molecular-beam epitaxy (MBE) of an uncoupled Co/Cu m ultilayer on a pre-etched hole in a thin membrane. These devices exhibited a GMR, but its ratio was low and, as deduced from finite element calculatio ns, in many cases was dominated by the resistance of the multilayer electro de. When corrected for this, the maximum point-contact GMR was 3%. The mult ilayer structure at some depth in the constriction was disrupted, as observ ed by transmission electron microscopy. This was identified as a cause of t he low GMR, together with contamination and an oxide layer in the constrict ion, resulting from ex situ sample rotation. The second generation was fabr icated by sputtering of a coupled Co/Cu multilayer before etching of the na nohole, giving a proper multilayer at the constriction. Further, the GMR si gnal from the electrode was shorted by a thick Cu cap. This did not bring t he expected increase of the GMR (ratio less than or equal to 5%), indicatin g that the so-called dead layers and the quality of the interface between t he GMR system and the contacting metal were limiting. This interface qualit y was strongly improved for the third generation of contacts by using in si tu rotation, while the question of multilayer quality was avoided by shifti ng to granular Co/Au. Granular Co/Au in the constriction was obtained by gr owing a discontinuous Co layer by MBE. The maximum GMR ratio of the granula r contacts was 14%, an improvement of a factor 3. These contacts displayed small jumps in the GMR, two-level fluctuations in the resistance time trace and ballistic transport, the latter being evident from phonon peaks in the point-contact spectrum of a high resistance contact.