Crossover between short- and long-range proximity effects in superconductor/ferromagnet/superconductor junctions with Ni-based ferromagnets

O M Kapran,T Golod,A A Golubov,A Iovan,A S Sidorenko,V M Krasnov

doi:10.1103/physrevb.103.094509

Abstract

We study Superconductor/Ferromagnet/Superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in Pt1-xNix with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.

Highlights

There are, reports about a longrange proximity effect (LRPE) [19,25,30,33,35], which is often ascribed to the spin-triplet order parameter that should be immune to the ferromagnetic order
Using in situ absolute Josephson fluxometry (AJF), we demonstrate that Ni interlayers in our junctions exhibit full saturation magnetization as in bulk Ni, which precludes the presence of extended dead magnetic layers
Analysis of Jc(T ) dependencies revealed that this counterintuitive result is caused by the dirtiness of disordered Ni alloys, leading to a short coherence lengths ξF1 ∼ 1 nm

Summary

INTRODUCTION

A competition between superconductivity and ferromagnetism leads to an unconventional proximity effect, studied both theoretically [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] and experimentally [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. The clue to understanding of our results is obtained from the analysis of evolution of temperature dependencies, Jc(T ), in Nb/Pt1−xNix/Nb JJ’s with increasing Ni concentration It shows that in diluted Ni alloys, x 0.5, the proximity effect is short range, despite a small Eex, due to an extremely short m.f.p. is such atomically disordered alloys. Pure Ni remains clean, facilitating ballistic Cooper pair transport and LRPE similar in scale to that in the normal metal Pt. Our results demonstrate that the proximity effect in ferromagnets depends on composition and Eex and essentially on the internal structure. Our results demonstrate that the proximity effect in ferromagnets depends on composition and Eex and essentially on the internal structure This may help to resolve some of the controversies around LRPE. In the appendices we provide additional information about: Appendix A—film structure, Appendix B—junction characteristics, Appendix C—properties of Nb/Pt1−xNix/Nb junctions, Appendix D— interface resistances in Nb/Pt1−xNix/Nb junctions, and Appendix E—extraction of magnetization curves from AJF analysis

SAMPLES AND EXPERIMENTAL

RESULTS AND DISCUSSION

Temperature dependencies of critical currents

CONCLUSIONS