Electrodynamics of Josephson junctions containing strong ferromagnets

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Electrodynamics of Josephson junctions containing strong ferromagnets

D. Massarotti, N. Banerjee, R. Caruso, G. Rotoli, M. G. Blamire, and F. Tafuri

Phys. Rev. B 98, 144516 – Published 22 October 2018

Abstract

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization survive longer barrier thicknesses when compared with singlet supercurrents. The distinctive feature of triplet supercurrents is the scaling of the characteristic voltage of the junction with increasing ferromagnetic barrier thickness - an algebraic decay in contrast to an exponential decay for singlet supercurrents. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show that new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics.

Received 1 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.144516

Physics Subject Headings (PhySH)

Josephson effect Spintronics

Ferromagnets Josephson junctions Magnetic multilayers Superconductors

Resistivity measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

D. Massarotti^1,2,*, N. Banerjee^3,†, R. Caruso^4,2, G. Rotoli⁵, M. G. Blamire⁶, and F. Tafuri^4,2

¹Dipartimento di Ingegneria Elettrica e delle Tecnologie dell'Informazione, Università di Napoli Federico II, via Claudio-80125-Napoli, Italy
²CNR-SPIN, c/o complesso di Monte S. Angelo, via Cinthia-80126-Napoli, Italy
³Department of Physics, Loughborough University, Loughborough LE11 3TU, United Kingdom
⁴Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, via Cinthia-80126-Napoli, Italy
⁵Dipartimento di Ingegneria Industriale e dell'Informazione, Università della Campania Luigi Vanvitelli, via Roma-81031-Aversa (CE), Italy
⁶Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom

^*Corresponding author: davide.massarotti@unina.it
^†Corresponding author: N.Banerjee@lboro.ac.uk

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Vol. 98, Iss. 14 — 1 October 2018

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Images

Figure 1
(a) Sketch of a typical trilayer junction with the corresponding layer thicknesses. The single and bilayer junctions have similar Nb, Cu, Co and Ho thicknesses. (b) Resistance vs temperature behavior for single layer S2 (black curve) and bilayer B2 JJs (blue curve), respectively. The inset shows a magnified view of S2 JJ curve, below 7 K, which highlights the superconducting transition at about 4 K. (c) IV characteristics for single layer S2 JJ (black line, left and down axis) and for trilayer T2 JJ (red curve, right and top axis), respectively. Both IV curves have been measured at 0.3 K. Definition of $I_{c}, I_{R}$ , and $V_{sw}$ are indicated on the IV curve of the trilayer JJ. Note that the two axes have different current and voltage scales. (d) $I_{c} R_{N}$ product as a function of $V_{sw}$ (left axis) for single layer (black dots), bilayer (blue triangles), and trilayer (red squares) JJs, respectively. The hysteresis $H_{y}$ for the same junctions is shown by the corresponding open symbols (right axis).
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Figure 2
(a) Temperature dependence of the critical current $I_{c}$ of single layer S1 (black dots), bilayer B1 (blue triangles) and trilayer T1 (red squares) JJs, respectively. $I_{c}$ is normalized to the value measured at 0.3 K, while $T$ is normalized to $T_{c}$ . Once normalized, the curves for bilayer and trilayer JJs overlap and show a characteristic exponential behavior for $T >$ 0.3 $T_{c}$ , while a linear trend at high temperatures has been observed in single layer JJs. In panel (b) the $I_{c}$ vs $T$ measurements are reported for B1 JJ (blue triangles), without normalization. The orange curve is the Usadel fit for $T \geq$ 1.5 K and provides an estimation of the Thouless energy of about $25 μ eV$ (see the text). The retrapping current $I_{R}$ is shown by the green circles. For $T \geq$ 2 K, $I_{R} = I_{c}$ and no hysteresis is present in the IV characteristics.
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Figure 3
(a) Dynamics of a phase particle in a tilted washboard potential for $i$ slightly less than 1. Thermal activation (TA) above the barrier $Δ U (i)$ (green dotted lines), retrapping processes in the phase diffusion (PD) regime and running motion along the potential are qualitatively sketched by the red arrow, orange dashed line, and gray dashed line, respectively. The inset shows the IV curve of the bilayer B1 JJ, with the threshold voltage (red dashed line) of about $20 μ V$ for the switching measurements. (b) Measurements of SCDs as a function of the temperature for junction B1. The insets show the zoom of the SCDs measured at 0.3 K (red dots, asymmetric distribution in the TA regime) and at 1.6 K (blue dots, symmetric distribution in the PD regime). The lines are guides for the eye. (c) Temperature behavior of the standard deviation $σ$ (black dots), extracted from the SCDs reported in panel (b). The blue line is the fit obtained by Monte Carlo simulation of the phase dynamics, with a quality factor Q = 1.46 at 0.3 K. At $T^{*}$ = 1.4 K, Q = 1.12 and at 1.9 K, Q = 0.87. Above 2 K the IV curves are nonhysteretic; see Fig. 2.
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