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Introduction
Publications
Publications (143)
Josephson junctions containing magnetic materials may be useful in superconducting electronics and memory. One design for a memory cell uses a junction containing an S/F/F'/S spin valve where the two ferromagnetic layers (F, F') have different switching fields. Such a spin valve can be made more robust if the hard layer is pinned by exchange bias w...
Studies of ferromagnet-superconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings. The electromagnetic (EM) proximity effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet. In this work, we use a Nb superconducting layer and Ni...
Josephson junctions containing ferromagnetic layers exhibit fascinating physics and the potential for promising applications in superconducting logic and memory. The junctions in a demonstrated superconducting memory prototype contain a magnetic spin-valve structure with Ni as the fixed layer and NiFe (Permalloy) as the free layer. However, NiFe ex...
Studies of ferromagnet-superconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings. The electromagnetic (EM) proximity effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet. In this work, we use a Nb superconducting layer and Ni...
Josephson junctions containing ferromagnetic materials exhibit interesting physics and show promise as circuit elements for superconducting logic and memory. For memory applications, the properties of the junction should be controllable by changing the magnetic configuration inside the junction. To achieve good magnetic switching properties, one sh...
Josephson junctions containing ferromagnetic materials exhibit interesting physics and show promise as circuit elements for superconducting logic and memory. For memory applications, the properties of the junction should be controllable by changing the magnetic configuration inside the junction. To achieve good magnetic switching properties, one sh...
Josephson junctions containing ferromagnetic materials are generating interest for use in superconducting electronics and cryogenic memory. Optimizing the performance of such junctions is an ongoing effort, requiring exploration of a broad range of magnetic systems. Here, we study supercurrent transmission through Ni/Ru/Ni synthetic antiferromagnet...
Josephson junctions containing ferromagnetic materials are generating interest for use in superconducting electronics and cryogenic memory. Optimizing the performance of such junctions is an ongoing effort, requiring exploration of a broad range of magnetic systems. Here we study supercurrent transmission through Ni/Ru/Ni synthetic antiferromagnets...
Superconducting (S) thin film superlattices composed of Nb and a normal metal spacer (N ) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of S/N superlattices us...
Josephson junctions containing three ferromagnetic layers with noncollinear magnetizations between adjacent layers carry spin-triplet supercurrent under certain conditions. The signature of the spin-triplet supercurrent is a relatively slow decay of the maximum supercurrent as a function of the thickness of the middle ferromagnetic layer. In this w...
Josephson junctions containing three ferromagnetic layers with non-collinear magnetizations between adjacent layers carry spin-triplet supercurrent under certain conditions. The signature of the spin-triplet supercurrent is a relatively slow decay of the maximum supercurrent as a function of the thickness of the middle ferromagnetic layer. In this...
Superconducting (S) thin film superlattices composed of Nb and a normal metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb thin films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence length of S/N superlattices...
It has been suggested by theoretical works that equal spin-triplet Cooper pairs can be generated in Josephson junctions containing both a ferromagnet and a source of spin-orbit coupling. Our recent experimental work suggested that spin-triplet Cooper pairs were not generated by a Pt spin-orbit coupling layer when the ferromagnetic weak link had ent...
It has been suggested by theoretical works that equal spin-triplet Cooper pairs can be generated in Josephson junctions containing both a ferromagnet and a source of spin-orbit coupling. Our recent experimental work suggested that spin-triplet Cooper pairs were not generated by a Pt spin-orbit coupling layer when the ferromagnetic weak link had ent...
Josephson junctions containing ferromagnetic layers have generated interest for application in cryogenic memory. In a junction containing both a magnetically hard fixed layer and soft free layer with carefully chosen thicknesses, the ground-state phase difference of the junction can be controllably switched between 0 and π by changing the relative...
Josephson junctions containing ferromagnetic materials have attracted intense interest both because of their unusual physical properties and because they have potential application for cryogenic memory. There are two ways to store information in such a junction: either in the amplitude of the critical current or in the ground-state phase difference...
Josephson junctions containing ferromagnetic layers have generated interest for application in cryogenic memory. In a junction containing both a magnetically hard fixed layer and soft free layer with carefully chosen thicknesses, the ground-state phase difference of the junction can be controllably switched between 0 and {\pi} by changing the relat...
The proximity effect between a superconducting material and a non-superconducting normal metal can extend over distances of the order of micrometres at sufficiently low temperatures. If the normal metal is replaced by a ferromagnetic material, the spatial extent of the proximity effect drops precipitously due to the exchange splitting between the m...
The lengthscale over which supercurrent from conventional BCS, $s$-wave, superconductors ($S$) can penetrate an adjacent ferromagnetic ($F$) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction, and can thus penetrate an $...
It is now well established that a Josephson junction made from conventional spin-singlet superconductors containing ferromagnetic layers can carry spin-triplet supercurrent under certain conditions. The first experimental signature of that fact is the propagation of such supercurrent over long distances through strong ferromagnetic materials. Surpr...
We present measurements of Josephson junctions containing three magnetic layers with noncolinear magnetizations. The junctions are of the form $S/F^{\prime}/N/F/N/F^{\prime \prime}/S$, where $S$ is superconducting Nb, $F^\prime$ is either a thin Ni or Permalloy layer with in-plane magnetization, $N$ is the normal metal Cu, $F$ is a synthetic antife...
Josephson junctions containing two ferromagnetic layers are being considered for use in cryogenic memory. Our group recently demonstrated that the ground-state phase difference across such a junction with carefully chosen layer thicknesses could be controllably toggled between zero and $\pi$ by switching the relative magnetization directions of the...
Josephson junctions with ferromagnetic layers are vital elements in a new class of cryogenic memory devices. One style of memory device contains a spin valve with one “hard” magnetic layer and one “soft” layer. To achieve low switching fields, it is advantageous for the soft layer to have low magnetization and low magnetocrystalline anisotropy. A c...
Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a phase shift of $\pi$ for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale elliptically-shaped Josephson junctions containi...
Josephson junctions made with conventional s-wave superconductors and
containing multiple layers of ferromagnetic materials can carry spin-triplet
supercurrent in the presence of certain types of magnetic inhomogeneity. In
junctions containing three ferromagnetic layers, the triplet supercurrent is
predicted to be maximal when the magnetizations of...
Superconductivity and ferromagnetism are antagonistic forms of order, and
rarely coexist. Many interesting new phenomena occur, however, in hybrid
superconducting/ferromagnetic systems. For example, a Josephson junction
containing a ferromagnetic material can exhibit an intrinsic phase shift of pi
in its ground state for certain thicknesses of the...
Josephson junctions containing ferromagnetic materials are being considered for applications in cryogenic random access memory. The road to such applications requires thorough characterization of junction properties, including critical current and ground-state phase shift, as a function of the thickness of a single ferromagnetic layer. We carried o...
One aspect of the present invention includes a Josephson magnetic memory system. The system includes a superconducting electrode that conducts a read current. The system also includes a hysteretic magnetic Josephson junction (HMJJ). The HMJJ can store a binary value and convert superconducting pairs associated with the read current flowing through...
Josephson junctions containing ferromagnetic (F) materials are under consideration for applications in cryogenic memory by several groups. The approach we are pursuing is loosely based on commercial magnetic random access memory (MRAM), but with the individual memory element being a ferromagnetic Josephson junction. The junction contains two or mor...
The coexistence of superconductivity and ferromagnetism in bulk materials occurs rarely in nature, but the interplay between them in artificially layered structures leads to very rich and interesting physics. One fascinating phenomenon that can be observed in such hybrid systems is the crossover from a standard Josephson junction (0-junction) to a...
Josephson junctions containing ferromagnetic layers are under consideration as the basic elements for cryogenic random access memory. For memory applications, either the amplitude of the critical current or the phase shift across the junction must be controllable by changing the direction of magnetization of one or more of the ferromagnetic layers...
We present recent work on S/F'/F/F''/S Josephson Junctions with the
magnetic multilayer Ni/[Co/Ni]n as the central F layer [1].
This multilayer possesses strong intrinsic perpendicular (out-of-plane)
anisotropy at Co and Ni thicknesses of a few monolayers. If a hard
ferromagnet is used for F', and a softer ferromagnet is used for F'',
both with mag...
The current-phase relationship of ferromagnetic spin-triplet Josephson
junctions was predicted to be tuned by the magnetization orientations of
different magnetic layers [1]. Given the random domain structure in
large-area ferromagnetic junctions, the possibility of a random
distribution of 0 or π sub-junctions across the area could lead to a
globa...
The spin-triplet supercurrent in ferromagnetic Josephson junctions is
obtained by surrounding the central ferromagnet with noncollinear
ferromagnetic layers, F' [1]. In metallic ferromagnets, the long-range
nature of the spin-triplet supercurrent has only been tested to lengths
of a few tens of nm [2]. In this work, we are fabricating and measuring...
We have measured spin-triplet supercurrent in Josephson junctions of the form
S/F'/F/F'/S, where S is superconducting Nb, F' is a thin Ni layer with in-plane
magnetization, and F is a Ni/[Co/Ni]n multilayer with out-of-plane
magnetization. The supercurrent in these junctions decays very slowly with
F-layer thickness, and is much larger than in simi...
Josephson junctions containing multiple ferromagnetic layers can carry
spin-triplet supercurrent under certain conditions. Large-area junctions
containing multiple domains are expected to have a random distribution of 0 or
pi coupling across the junction surface, whereas magnetized samples should have
uniquely pi coupling everywhere. We have measur...
We have observed long-range spin-triplet supercurrents in Josephson junctions containing ferromagnetic (F) materials, which are generated by noncollinear magnetizations between a central Co/Ru/Co synthetic antiferromagnet and two outer thin F layers. Here we show that the spin-triplet supercurrent is enhanced up to 20 times after our samples are su...
In 2010, several experimental groups obtained compelling evidence for spin-triplet supercurrent in Josephson junctions containing strong ferromagnetic materials. Our own best results were obtained from large-area junctions containing a thick central Co/Ru/Co "synthetic antiferromagnet" and two thin outer layers made of Ni or PdNi alloy. Because the...
We have measured the evolution of the tunneling density of states (DOS) in
superconductor/ferromagnet (S/F) bilayers with increasing F-layer thickness,
where F in our experiment is the strong ferromagnet Ni. As a function of
increasing Ni thickness, we detect multiple oscillations in the DOS at the
Fermi energy from differential conductance measure...
We have determined the spin-memory-loss parameter, $\delta_{Co/Ru}$, by
measuring the transmission of spin-triplet and spin-singlet Cooper pairs across
Co/Ru interfaces in Josephson junctions and by Current-Perpendicular-to-Plane
Giant Magnetoresistance (CPP-GMR) techniques. The probability of spin-memory
loss at the Co/Ru interface is $(1-exp(-\de...
We report magneto-transport measurements in epitaxially grown thin films of Sb2Te3. Sb2Te3 is a topological insulator candidate expected to possess a single Dirac cone on its surface. ootnotetextH. Zhang et al., Nat. Phys. 5, 438 (2009). Both semiconducting and metallic samples were measured, identified by the temperature response of their resistiv...
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)...
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, wh...
The conventional proximity effect in superconducting/ferromagnetic (S/F) hybrid systems decays over a short length scale in the ferromagnet, as the two electrons from the spin-singlet Cooper pair enter different spin bands and rapidly lose phase coherence. Spin-triplet Cooper pairs are not subject to this constraint, hence a spin-triplet proximity...
The discovery that a few nm of PdNi alloy near a Nb superconductor induces spin-triplet superconducting correlations [1] begs the question: what other materials produce a similar effect? In this talk we will discuss several other materials we have tried in place of PdNi. So far, only Cu0.48Ni0.52 alloy has been successful, producing a factor of 20...
A magnetic field can create tiny currents that flow continuously in an ordinary metal ring, despite its electrical resistance.
Weakly ferromagnetic alloys are being used by several groups in the study of
superconducting/ferromagnetic hybrid systems. Because spin-flip and spin-orbit
scattering in such alloys disrupt the penetration of pair correlations into the
ferromagnetic material, it is desirable to have a direct measurement of the
spin memory length in such alloys. We...
We report results on superconducting tunneling spectroscopy of a carbon
nanotube quantum dot. Using a three-probe technique that includes a
superconducting tunnel probe, we map out changes in conductance due to band
structure, excited states, and end-to-end bias. The superconducting probe
allows us to observe enhanced spectroscopic features, such a...
We report measurements of the critical current vs. Co thickness in
Nb/Cu/Co/Ru/Co/Cu/Nb Josephson junctions, where the inner Co/Ru/Co trilayer is
a "synthetic antiferromagnet" with the magnetizations of the two Co layers
coupled antiparallel to each other via the 0.6 nm-thick Ru layer. Due to the
antiparallel magnetization alignment, the net intrin...
Fluctuations of the current through a tunnel junction are measured using a Josephson junction. The current noise adds to the bias current of the Josephson junction and affects its switching out of the supercurrent branch. The experiment is carried out in a regime where switching is determined by thermal activation. The variance of the noise results...
We have studied the variation of critical current in Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a function of ferromagnet thickness (dF) using a weakly ferromagnetic alloy, Pd82Ni12. The critical current density oscillates and decays over five orders of magnitude as dF is increased from 32 to 100 nm. These oscillations...
We report measurements of the nonequilibrium electron energy distribution in carbon nanotubes. Using tunneling spectroscopy via a superconducting probe, we study the shape of the local electron distribution functions, and hence energy relaxation rates, in nanotubes that have bias voltages applied between their ends. At low temperatures, electrons i...
We have studied the variation of critical current in
Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a
function of ferromagnet thickness using a weakly-ferromagnetic alloy Pd82Ni12.
Measurements were performed for the thickness range 32 to 100 nm, over which
the critical current density decreases by five orders of magnitude...
We report the results of several nonequilibrium experiments performed on superconducting/normal/superconducting (S/N/S) Josephson junctions containing either one or two extra terminals that connect to normal reservoirs. Currents injected into the junctions from the normal reservoirs induce changes in the electron energy distribution function, which...
Superconducting correlations cannot penetrate into a ferromagnet over a large distance due to the pair breaking effect of the exchange field. Ferromagnet/Superconductor (F/S) systems are often studied using weak ferromagnetic alloys with smaller exchange energy and correspondingly larger penetration depth. We are studying S/F/S junctions using the...
Non-Gaussian fluctuations of the electrical current can be detected with a Josephson junction placed on-chip with the noise source. We present preliminary measurements with an NIS junction as a noise source, and a Josephson junction in the thermal escape regime as a noise detector. It is shown that the Josephson junction detects not only the averag...
Non‐Gaussian fluctuations of the electrical current can be detected with a Josephson junction placed on‐chip with the noise source. We present preliminary measurements with an NIS junction as a noise source, and a Josephson junction in the thermal escape regime as a noise detector. It is shown that the Josephson junction detects not only the averag...
Ferromagnet/Superconductor (F/S) hybrid systems have received considerable interest owing to the interplay between their competing types of order. A number of open questions remain about these systems. In particular, several groups have observed ``superconducting spin switch'' behavior in F/S/F trilayers, occasionally with conflicting results [1,2]...
The authors report the spin diffusion length at 4.2 K in sputtered Ni of lsfNi = 21±2 nm, and spin-dependent scattering parameters in Ni and at Ni/Cu interfaces. They have employed current perpendicular to plane giant magnetoresistance in both a traditional and an alternative exchange-biased spin valve geometry that inserts a Ni “spoiler” layer int...
Carbon nanotubes are one-dimensional metallic or semiconducting wires that serve as good model systems to study Luttinger liquids, in which electron-electron interaction are essential to electronic transport. Luttinger behavior has previously been measured via transport through the ends of nanotubes. We have fabricated novel nanotube devices with t...
We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins...
We have measured the superconducting transition temperature T_c of F/S/F
trilayers using Permalloy (Py=Ni_84 Fe_16) as a strongly polarized
ferromagnetic material. For a parallel (P) or anti-parallel (AP) alignment of
the magnetization directions of the outer ferromagnets, we observe a T_c
difference as large as 20 mK, with a stronger suppression o...
We examined magnetoresistance (MR) effects of Fe and Cu co-doped ZnO epitaxial films synthesized by pulsed laser deposition. The film exhibits a large positive MR at low temperatures (similar to 32.5% at 4 T and 10 K), but negative MR takes over above 40 K. This crossover behavior is in contrast with the case of pure ZnO with only negative MR. The...
BiFe1-xMnxO3 thin films were grown by pulsed laser deposition on Nb-doped SrTiO3 substrates. Piezoresponse force microscopy was utilized to investigate the piezoelectric properties of these films. The piezoelectric hysteric behavior of the films was confirmed at room temperature. It is further shown that the piezoelectric coefficient of BiFe1-xMnxO...
Using tunneling spectroscopy, we have measured the local electron energy distribution function in the normal part of a superconductor-normal metal-superconductor (SNS) Josephson junction containing an extra lead to a normal reservoir. In the presence of simultaneous supercurrent and injected quasiparticle current, the distribution function exhibits...
Hybrid systems combining ferromagnetic (F) and superconducting (S) metals in contact exhibit a wide range of fascinating behaviors. Several experimental groups have used weak ferromagnetic alloys in F/S experiments to enhance the penetration of Cooper pairs into the ferromagnet. In an F/S/F trilayer structure, a difference in the critical temperatu...
We have measured the superconducting transition temperature Tc of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in occurs when the Nb thickness is just above the critical thickness at which superconductivity disappears completely. We have observed a difference in...
The spin-diffusion length of the electron represents a fundamental transport parameter and plays a central role in the development of spintronic devices. While several measurements of the spin-diffusion length in normal metals and in ferromagnetic alloys exist, measurements in elemental ferromagnets (Ni, Fe, Co) have been scarce. We present a novel...
We have measured the superconducting transition temperature $T_c$ of Ni/Nb/Ni
trilayers when the magnetizations of the two outer Ni layers are parallel (P)
and antiparallel (AP). The largest difference in $T_c$ occurs when the Nb
thickness is just above the critical thickness at which superconductivity
disappears completely. We have observed a diff...
In order to probe quantitatively the effect of Kondo impurities on energy exchange between electrons in metals, we have compared measurements on two silver wires with dilute magnetic impurities (manganese) introduced in one of them. The measurement of the temperature dependence of the electron phase coherence time on the wires provides an independe...
Injecting normal current into a superconductor/normal metal/superconductor (SNS) Josephson junction via another lead attached at the middle of the junction, creates a non- equilibrium situation in which a pi-junction can be created [1]. In this three-terminal geometry, one side of the SNS junction has both normal and supercurrent flowing in the sam...
