Focused Electron Beam Induced Deposition (FEBID) and Focused Ion Beam Induced Deposition (FIBID) are ideal tools for rapid prototyping of 3D nanostructures. In this project, in search for novel functionalities, FEBID and FIBID were used to create junctions between a ferromagnet and a superconductor. Cobalt was used for the ferromagnet and tungsten for the superconductor. When such a junction is biased, Andreev reflection occurs, which can be used to determine the spin polarisation of the ferromagnet. In this way the magnetic properties of a device can locally be probed and measured.We measured composition and electrical conductivity of the various deposits. Two point electrical measurements were done in order to determine the material's resistivity and composition was analysed in-situ by performing Electron Dispersive X-Ray Spectroscopy. The resistivity of the materials was a few orders higher than the bulk value which could be attributed to the materials not being pure.Superconducting properties of the tungsten deposits were investigated. For this four point measurements were performed ex-situ at low temperatures using a liquid helium flow cryostat. In this way the superconductivity of the tungsten was varied and investigated. A dependence of the critical temperature with the applied current was found, with a critical temperature at zero current of 5.73 ± 0.10 K.In order to determine spin polarisation of FEBID cobalt, a well-suited tool is the Point Contact Andreev Reflection method. To perform these measurements, ferromagnet-superconductor nano point contacts were created. Low temperature current-voltage measurements were done as well as direct measurement of the conductance using a lock-in amplifier. Measurements with both the techniques were compared where measurements from the latter showed less noise. From measurements on the nano contacts an increase of conductance at low bias voltage was observed, corresponding with the presence of Andreev reflection.Some additional minima at slightly higher bias voltage were also observed. These minima were temperature dependent and occurred when a current ran through the sample corresponding to the critical current as determined before. In the light of other literature reporting on these minima we concluded that they are caused by a transition of the superconducting material to the normal state because the current through the junction exceeds the critical current.In some of the measurements, an additional oscillation on top of the conductance curve was measurable. Since the oscillation did not appear on constant intervals, we speculated that it is caused by the presence of different conduction paths. Because one of these paths might be dirtier than the other, their conductance curves had different shape. The measured conductance is then a summation of several of these curves. If the conduction of a single channel is isolated, the method can be used to probe spin polarisation locally, and at the nanoscale.
Hybrid ferromagnet-superconductor nano devices created by FEBID and FIBID: towards spin polarisation measurements
Vugts, P. C. W. G. (Author). 31 Jul 2013
Student thesis: Master