Ferromagnetic contacts for single-molecule devices

• Main Author: Brooke, Richard J.
• Published: University of Bristol 2015
• Subjects: 621.381
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.702499

Recently techniques have been developed which enable the conductance of individual molecules sandwiched between metal electrodes to be measured. This has revealed important fundamental science and has the potential to deliver unparalleled miniaturisation of electronic devices. The use of ferromagnetic metal electrodes is expected to lead to devices with interesting spin transport behaviour, however experiments are typically performed under ambient conditions so that Au or another non-reactive metal must be used to prevent sample contamination. Here a scanning tunnelling microscope break junction technique was developed to measure the conductance of single-molecule junctions with clean ferromagnetic electrodes under electrochemical conditions. The use of electrochemical control protects the metal contacts from oxidation and enables electrochemical gating behaviour to be explored. An electromagnet was constructed to provide an in-situ magnetising field during the experiments and a procedure for the automatic selection of data exhibiting features consistent with the formation of molecular junctions was also developed. Initial measurements were performed using Au contacts. The conductance was found to depend strongly on the chemical structure of the molecules and was in good agreement with previously reported values. 4,4'-Bipyridine and closely related molecules measured using Ni contacts exhibited larger conductance and stronger gating behaviour compared to Au-based junctions, whilst Co contacted molecules produced even larger conductance values. Ni 14,4' vinylenedipyridine 1 Ni junctions were also influenced by changes in pH. The differences between Ni and Au contacts were explained in terms of the strong interaction between the molecule and the Ni d-band, which unlike Au has a large density of states at the Fermi level. Magnetoresistance measurements were carried out for Ni 14,4'-bipyridine 1 Ni single-molecule junctions by comparing the conductance obtained with a variety of different magnetic fields applied. A giant magnetoresistance of 48±6% was measured, however this result was not reproducible due to inadequate control of the magnetic configuration. Anisotropic magnetoresistance experiments did not reveal any magnetoresistance effect.