Abstract
The heterogeneous properties around the interface between ferromagnet (F) and conventional s-wave superconductor (S) hybrid structures exhibit several remarkable phenomena. The superconductivity and ferromagnetism are two competing orders. The ferromagnetic order forces the spins to align in parallel, and Cooper pairs in singlet superconductivity prefer an antiparallel spin orientation with total spin zero. Due to the incompatible nature of S and F, singlet superconductivity and ferromagnetic order should not coexist in bulk materials. Thus, fabricating S/F heterostructure is the immediate way to study the proximity phenomena in the competition of the two order parameters. However, the S/F proximity effect has attracted much attention over the past years. Aspect of the S/F proximity effect, the “inverse proximity” effect which is the ferromagnetic order-parameter penetrates into the superconductor, has gained renewed interest only in the last decade. Following the literatures, the characterizations of the S/F lateral junction have been studied rear, especially the inverse proximity. Therefore, it gives us the motivations to study the inverse proximity effect in S/F lateral junction. In this dissertation, we systematically investigate the lateral S/F junctions by electron transport measurement. We fabricated two types of S/F lateral junction. One is the junctions between two superconductor Nb leads were connected by two bars of the ferromagnetic NiFe, another is lateral Josephson junction with a weak link by fabricating a ferromagnetic NiFe strip across a superconducting Nb strip. In the lateral two NiFe bars junctions, the separation distance l between NiFe bars was varied from 150 to 700 nm to study the spin relaxation effect in Nb. In small l (150 and 250 nm) samples, a spin-valve like behavior related to the inverse proximity effect was observed, which manifest itself as charge accumulation due to spin imbalance near the interface. However, for samples with large l (500 and 700 nm), the spin-valve behavior vanishes and the anisotropic magnetoresistance effect dominates. The spin-valve and anisotropic magnetoresistance (AMR) behaviors were also observed on multiple bars samples. From the two different behaviors observation, the spin relaxation length of Nb lateral device can be estimated. The transition temperature enhancement of Nb was observed on the spin-valve behavior samples. The transition temperature enhancement of the l = 250 and 300 nm samples are 35.5 and 58.3 mK, respectively. In addition, there are some striking resistance peaks observed in the R-T measurements. It is attributed to the interaction between S and F. In the lateral Josephson junctions with a weak link by fabricating a ferromagnetic NiFe strip across superconducting Nb strips. We argue that the effective weak link of the Josephson junction was induced by the inverse proximity effect around the Nb interface in contact with the NiFe. The argument has been proved by the observation of the dc and ac Josephson effect. We also estimated the strength of the Josephson junctions by Thouless energy analysis. In our observations, the supercurrents can go through such long distance up to 2.1μm of the junctions, implying that a spin triplet component possibly exists as the theoretical prediction, the spin triplet component induced by inverse proximity effect. In addition, we also discussed the correlation of the vortex dynamics and the critical current modulations, and ruled out the possibility of the vortex dynamics in our data.