Trupti S. Khaire

Observation of spin-triplet superconductivity in co-based josephson junctions

We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting-ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.

Critical current behavior in Josephson junctions with the weak ferromagnet PdNi

When a superconducting (S) metal is placed in contact with a normal (N) metal, the properties of both metals are modified near the S/N interface. The resulting superconducting proximity effect was widely studied in the 1960’s, 1 and again in the 1990’s. 2,3 When the normal metal is replaced by a ferromagnetic (F) metal, the resulting physics is extremely rich, due to the very different order parameters in the two metals. There has been sustained interest in S/F systems over at least the past decade. 4,5 Many new phenomena have been observed, including but not limited to: oscillations in the critical temperature of S/F bilayers as a function of F layer thickness, 6 variations in the critical temperature of F/S/F trilayers as a function of the relative magnetization direction of the two F layers, 7 and oscillations in the critical current of S/F/S Josephson junctions. 8 The oscillatory behaviors are a direct result of the exchange splitting of the spin-up and spin-down bands in the ferromagnet, which produce a momentum shift between the up- and down-spin electrons of a Cooper pair that “leaks” from S into F. 9 In the clean limit, the spatial period of the oscillations is governed by the exchange length, ξF = ~vF /2Eex, where vF and Eex are the Fermi velocity and exchange energy of the F material, respectively. Unlike the S/N proximity effect, which extends to distances of order 1 �m at sufficiently low temperature, the novel phenomena associated with the S/F proximity effect persist only over very short distances – limited either by ξF, by the mean free path, le, or by their geometric mean, the dirty-limit exchange length ξ � F =

Spin-triplet supercurrent in Co-based Josephson junctions

In the past year several groups have reported experimental evidence for spin-triplet supercurrents in Josephson junctions containing strong ferromagnetic materials. In this paper we present several new experimental results that follow up on our previous work. We study Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor (Nb), N is a normal metal, SAF is a synthetic antiferromagnet of the form Co/Ru/Co and X is an ferromagnetic layer necessary to induce spin-triplet correlations in the structure. Our work is distinguished by the fact that the generation of spin-triplet correlations is tuned by the type and thickness of the X layers. The most important new result reported here is the discovery that a conventional, strong ferromagnetic material, Ni, performs well as the X layer, if it is sufficiently thin. This discovery rules out our earlier hypothesis that out-of-plane magnetocrystalline anisotropy is an important attribute of the X layers. These results suggest that the spin-triplet correlations are most likely induced by noncollinear magnetization between the X layers and adjacent Co layers.

Measurement of spin memory lengths in PdNi and PdFe ferromagnetic alloys

length at 4.2K in sputtered Pd0.88Ni0.12 and Pd0.987Fe0.013 alloys using methods based on currentperpendicular-to-plane giant magnetoresistance. The alloys are incorporated into hybrid spin valves of various types, and the spin memory length is determined by fits of the Valet-Fert spin-transport equations to data of magnetoresistance vs. alloy thickness. For the case of PdNi alloy, the resulting values of the spin memory length are l P dNi sf = 2.8 ± 0.5nm and 5.4 ± 0.6nm, depending on whether or not the PdNi is exchange biased by an adjacent Permalloy layer. For PdFe, the spin memory length is somewhat longer, l P dF e sf = 9.6 ± 2nm, consistent with earlier measurements indicating lower spin-orbit scattering in that material. Unfortunately, even the longer spin memory length in PdFe may not be long enough to facilitate observation of spin-triplet superconducting correlations predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of magnetic inhomogeneity.