Amelia Bengtson

Impact of substitutional and interstitial carbon defects on lattice parameters in MgB2

Carbon (C)-doping has been found to increase the upper critical field HC2 in superconducting MgB2 thin-film and bulk samples. However, the C effects on both HC2 and lattice parameters are very different between thin films and bulk, suggesting C may incorporate differently in the two cases. This paper combines ab initio calculations and thin-film lattice parameter measurements to explore the connection between substitutional and interstitial C in MgB2 and experimental bulk and thin-film lattice parameters.

Origin of the dependence of magnetoresistance on the composition of Co100−xFex electrodes in magnetic tunnel junctions

The tunneling magnetoresistance value of a Co100−xFex (4nm)∕AlOx 1.7nm∕Co100−xFex (4nm) magnetic tunnel junction has been demonstrated to depend on the composition of the Co100−xFex electrodes. The interface roughness, crystal structure, and tunneling spin polarization versus the composition of the Co100−xFex electrode were studied to address the origin of this compositional dependence. Ab initio calculations of s-like electron spin polarization predict a composition dependence similar to that observed experimentally. The combined experimental and computational results show that the trends in Co100−xFex tunneling magnetoresistance are modified slightly by the interface roughness but mainly determined by the s-like electron spin polarization values associated with different compositions and crystal structures.