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Spin-triplet supercurrent in Josephson junctions containing a synthetic antiferromagnet with perpendicular magnetic anisotropy

Physical Review B

Click, J.A.; Edwards, S.E.; Korucu, D.K.; Aguilar, V.A.; Niedzielski, B.M.; Loloee, R.L.; Pratt, W.P.; Birge, N.O.; Kotula, Paul G.; Missert, Nancy A.

In this paper, we present measurements of Josephson junctions containing three magnetic layers with noncollinear magnetizations. The junctions are of the form

S/F'/N/F/N/F"/S

, where

S

is superconducting Nb,

F'

is either a thin Ni or Permalloy layer with in-plane magnetization,

N

is the normal metal Cu,

F

is a synthetic antiferromagnet with magnetization perpendicular to the plane, composed of Pd/Co multilayers on either side of a thin Ru spacer, and

F"

is a thin Ni layer with in-plane magnetization. The supercurrent in these junctions decays more slowly as a function of the

F

-layer thickness than for similar spin-singlet junctions not containing the

F'

and

F"

layers. The slower decay is the prime signature that the supercurrent in the central part of these junctions is carried by spin-triplet pairs. Finally, the junctions containing

F'=

Permalloy are suitable for future experiments where either the amplitude of the critical current or the ground-state phase difference across the junction is controlled by changing the relative orientations of the magnetizations of the

F'

and

F"

layers.

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TYPE Journal Article YEAR 2017
OSTIDOI

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

Journal of Applied Physics

Glick, Joseph A.; Khasawneh, Mazin A.; Niedzielski, Bethany M.; Loloee, Reza; Pratt, W.P.; Birge, Norman O.; Gingrich, E.C.; Kotula, Paul G.; Missert, Nancy A.

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thicknesses. We present studies of Nb based micron-scale elliptically shaped Josephson junctions containing ferromagnetic barriers of Ni81Fe19 or Ni65Fe15Co20. By applying an external magnetic field, the critical current of the junctions is found to follow characteristic Fraunhofer patterns and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0-π transitions based on existing theories.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Planar Ohmic Contacts to Al 0.45 Ga 0.55 N/Al 0.3 Ga 0.7 N High Electron Mobility Transistors

ECS Journal of Solid State Science and Technology

Klein, Brianna A.; Baca, A.G.; Armstrong, Andrew A.; Allerman, A.A.; Sanchez, Carlos A.; Douglas, Erica A.; Crawford, Mary H.; Miller, Mary A.; Kotula, Paul G.; Fortune, Torben R.; Abate, Vincent M.

Here, we present a low resistance, straightforward planar ohmic contact for Al0.45Ga0.55N/Al0.3Ga0.7N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950°C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900°C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900°C anneal) and a Schottky (850°C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρc = 2.5 × 10-5 Ω cm², sheet resistances of RSH = 3.9 kΩ/$\blacksquare$, and maximum current densities of 75 mA/mm (at VGATE of 2 V). Electrical measurements from -50 to 200°C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. These contacts enabled state-of-the-art performance of Al0.45Ga0.55N/Al0.3Ga0.7N HEMTs.

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TYPE Journal Article YEAR 2017
OSTIDOI

Analysis of multilayer devices for superconducting electronics by high-resolution scanning transmission electron microscopy and energy dispersive spectroscopy

IEEE Transactions on Applied Superconductivity

Missert, Nancy A.; Kotula, Paul G.; Rye, Michael J.; Rehm, Laura; Sluka, Volker; Kent, Andrew D.; Yohannes, Daniel; Kirichenko, Alex F.; Vernik, Igor V.; Mukhanov, Oleg A.; Bolkhovsky, Vladimir; Wynn, Alex; Johnson, Leonard; Gouker, Mark

A focused ion beam was used to obtain cross-sectional specimens from both magnetic multilayer and Nb/Al-AlOx/Nb Josephson junction devices for characterization by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX). Automated multivariate statistical analysis of the EDX spectral images produced chemically unique component images of individual layers within the multilayer structures. STEM imaging elucidated distinct variations in film morphology, interface quality, and/or etch artifacts that could be correlated to magnetic and/or electrical properties measured on the same devices.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Magnetically Recoverable Pd/Fe3O4 Core–Shell Nanowire Clusters with Increased Hydrogenation Activity

ChemPlusChem

Watt, John D.; Kotula, Paul G.; Huber, Dale L.

Core–shell nanostructures are promising candidates for the next generation of catalysts due to synergistic effects which can arise from having two active species in close contact, leading to increased activity. Likewise, catalysts displaying added functionality, such as a magnetic response, can have increased scientific and industrial potential. Here, Pd/Fe3O4 core–shell nanowire clusters are synthesized and applied as hydrogenation catalysts for an industrially important hydrogenation reaction: the conversion of acetophenone to 1-phenylethanol. During synthesis, the palladium nanowires self-assemble into clusters which act as a high-surface-area framework for the growth of a magnetic iron oxide shell. This material demonstrates excellent catalytic activity due to the presence of palladium while the strong magnetic properties provided by the iron oxide shell enable facile catalyst recovery.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI
Results 51–75 of 318
Results 51–75 of 318