Critical current in ferromagnet/superconductor hybrid structures

Because of their property to only weakly break Cooper pairs, diluted ferromagnets allow the investigation of the interplay between superconductivity and ferromagnetism. In this thesis hybrid structures consisting of superconducing and weakly ferromagnetic layers were studied. A method was established to fabricate films of the diluted ferromagnet palladium iron with varying concentration ratios by thermal evaporation. The thin films were characterized electrically and magnetically at low temperatures utilizing the anomalous Hall effect and SQUID measurements. From these ferromagnetic films narrow stripes were fabricated and then covered by broader niobium stripes. The thereby obtained hybrid structures contained ferromagnet/superconductor bilayers surrounded by pure niobium layers. Both the palladium iron stripe and the niobium layer were contacted by several leads. The temperature dependent resistance showed the superconducting behavior of the niobium, the proximity effect in the palladium iron and also charge imbalance caused by injection of quasiparticles from the ferromagnet into the superconductor. In a sweeping magnetic field, pronounced magnetoresistance oscillations were observed, which were identified as a superposition of several oscillations with different periods. Further explorations of this effect by measuring the differential resistance while rising the magnetic field in fixed steps for each consecutive current sweep, produced rich patterns in a color coded plot. By varying the contact configurations at the superconductor and the ferromagnet, the pattern changed considerably. The link between the magnetoresistance oscillations and the differential resistance patterns was made by comparing their similar properties. It was also affirmed by a numerical simulation. The cause for this effect can be found in the establishment of several different superimposed current paths in the sample. This leads to quantum interference and thereby to the characteristic fluctuations of the electrical parameters. The exact localization of the paths still remains to be clarified