Spin accumulation and transport studied in double magnetic tunnel junctions

Magnetic CoFe nanoparticles have been produced by gas-aggregation and incorporated into sputtered MgO tunnel junction structures. Scanning tunnelling microscopy (STM) has been developed as a technique for examining spin accumulation and transport in these nanoscale junctions.
The particles were initially characterised for their magnetic properties; x-ray magnetic circular dichroism on 11-14~nm diameter clusters was performed. The orbital-to-spin moment ratio was found to be enhanced over the bulk value and to decrease with increasing average diameter, which complements previous studies on smaller particles. The size dependence of the combined data is found not to follow predicted trends based on reduced orbital moment quenching in the outer shell. In particular for these large particles, the quenching is far more rapid than expected. Magnetometry studies on random arrays of nanoparticles at percolation show interesting effects attributed to complex magnetic dipolar interactions. This includes very broad range anisotropy and large blocking temperatures.
For transport measurements, cryogenic STM is used to address individual islands and forms the top electrode of a double magnetic tunnel junction. Single electron charging effects are observed in these confined structures and the charging energy correlates to the size of the particle. New theory was developed to simulate these structures, giving an analytical solution to the current numerical orthodox theory. These solutions showed that TMR measurements, a current major barrier to studying nanospintronics using STM, were unnecessary. We are able characterise the tunnel junction parameters, including spin polarisation and accumulation, in a single I-V sweep of high information density. The spin polarisations of the opposing electrodes are found to be aligned anti-parallel despite a parallel magnetisation axis. Finally the spin lifetime on the island was calculated and found to exceed 1 us, longer than measured in previous studies.