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Josephson coupling and proximity effects through helimagnetic rare earth interfaces



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Börcsök, Bence 


This thesis presents research on the effect of superconducting-helimagnetic heterostructures. First, the electric and magnetic properties of epitaxial Nb(20 nm)/HoDy(40 nm) bilayers are studied. The saturation magnetisation values are consistent with previously reported values for Ho and Dy. The magnetic spiral is observed up to 43% Dy content although the Dy content changes the stability of the helimagnetic state. The transition temperature (Tc) of the bilayers is obtained from resistance vs temperature curves for different external magnetic field values. Tc lowered as the HoDy alloy transitioned from the helimagnetic state to the ferromagnetic state. The change in Tc decreases with the Dy content.

Highly oriented Nb/Ho/Nb Josephson junctions are also studied in this work. The electron mean-free-path in the normal state is longer in the textured junctions than in the polycrystalline junctions reported below but still below the thermal coherence length implying that the transport processes are diffusive. The decay length of the critical current (Ic) in the helimagnetic state is over 100 nm and consistent with the non-magnetic coherence length in the dirty limit.

The external magnetic field (H) dependence of Ic deviates from the expected Fraunhofer pattern and shows split peaks with extra minima and concave central peak. A theoretical model was constructed to explain the Ic(H) curves. The model assumes that the magnetisation increases faster along the edges of the junction and the highly magnetised regions propagate from the outside to the centre of the Ho barrier. The model successfully reproduces the characteristic features observed in the junctions and implies that in certain junctions, regions with 0 and π phase change coexist next to each other.





Robinson, Jason


superconductivity, magnetism, Josephson effect, proximity effect, helimagnetism, holmium


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge