Zero toπtransition in superconductor-ferromagnet-superconductor junctions

Abstract

We report a systematic study of Nb/ferromagnet ͑FM͒/Nb trilayer structures in which the FM layer is one of the strong ferromagnets Co, Fe, Ni, and Ni 80 Fe 20 ͑Py͒. Accurate control of the FM layer thickness has enabled detailed studies of the magnetic and transport properties in the superconducting state. In all cases, we estimate the thickness of the magnetic dead layer and the exchange energies of the ferromagnetic layers; in doing so, we demonstrate inconsistencies between the exchange energies derived elsewhere from superconductor ͑S͒/FM bilayer experiments and from S/FM/S junction measurements compared to their bulk Curie temperatures, which may hint at further complexity in the underlying physics. We show results in support of a recent publication ͓J. W. A Robinson et al., Phys. Rev. Lett. 97, 177003 ͑2006͔͒, focus in detail on a single 0-phase transition, and show evidence for the appearance of a second harmonic in the current-phase relation at the minimum of the critical current.Thin films of Nb/ Co/ Nb, Nb/Py/Nb, Nb/ Ni/ Nb, and Nb/ Fe/ Nb were prepared on thermally oxidized ͑100͒ Si substrates with a 250 nm oxide layer on the surface in an PHYSICAL REVIEW B 76, 094522 ͑2007͒

Pure second harmonic current-phase relation in spin-filter Josephson junctions

“…The NbN/GdN/NbN films were grown on oxidized Si substrates precoated with a 10-nm-thick layer of MgO; the MgO layer protects the oxidized Si during fabrication by acting as an etch stop layer while etching NbN with CF 4 , and serves no other purpose in device function. The thicknesses of the top and bottom NbN layers were 90 and 100 nm respectively.…”

“…While there have been experimental reports of half-integer Shapiro steps near to 0-p transitions in S/F/S JJs with both weak 3 and strong 4 diffusive F layers, evidence of a second harmonic in CPR has not been reported in I c (H) patterns, although the periodicity of I c (H) is the most unambiguous probe of CPR.…”

“…More generally, the CPR can be expressed as I ¼ P n!1 I cn sin nj ð Þ 1,2 and under circumstances in which the first harmonic is suppressed (for example at a 0-p transition), the second harmonic may become detectable 3,4 . Recently it has been predicted that, in JJs with diffusive ferromagnetic barriers and asymmetric spin-active interfaces, the CPR should be inherently dominated by the second harmonic as a consequence of the coherent transport of two triplet pairs 5,6 , but no theoretical predictions of a similar nature exists for the tunnelling limit.…”

Nanopillar junctions

“…Because the film multilayer structure is not constrained by the patterning process, either by resist thickness, which would limit stack thickness during lift-off, or by mask erosion, which would limit stack thickness during milling, arbitrarily large thicknesses can be processed using this technique: Leung et al [33] used this process to measure the perpendicular-to-plane spin diffusion length of Cu thin films ( figure 4). Recently, the process has been demonstrated as an effective method of patterning nanopillar devices for studies of Josephson junctions containing various magnetic barriers [34][35][36][37][38] and for CPP magnetoresistance and STT studies [39][40][41]. The primary limitation of the process is the potential for resputtering of material onto the device side walls, a more severe problem than for conventionally ion-milled mesas because of the depth to which material needs to be removed and hence the volume of material that may be resputtered.…”

Flux-closure pattern in a two-dimensional NbN–Fe superconductor-ferromagnet nanocomposite: Anisotropy of the angular magnetoresistance

“…[10][11][12][13] Single JJs with magnetic barriers have been studied in recent years and effects such as oscillating critical current 14,15 and long range proximity effect suggesting a singlet-to-triplet conversion at the junction interface, 16 have been observed. An important factor which affects the response of an FM-SC heterostructure is the behavior of magnetic domain walls ͑DW͒ in the FM layer.…”