9 research outputs found
The unusual distribution of spin-triplet supercurrents in disk-shaped Josephson junctions
The phenomenon of s-wave spin triplet Cooper pairs induced in ferromagnetic
metals has been researched now for more than a decade, and its main aspects are
well understood. Crucial in converting s-wave singlet pairs in the
superconductor to s-wave triplets in the ferromagnet is the engineering of
well-defined magnetic inhomogeneity (the 'generator') at the interface with the
superconductor. Vertical layer stacks are typically used as such, where two
separate thin ferromagnetic layers with homogeneous but non-collinear
magnetizations, provide the inhomogeneity. Alternatively, magnetic textures,
like ferromagnetic domain walls and vortices, are possible triplet generators,
although they are far less studied. In this paper we review our experiments on
lateral disk-shaped Josephson junctions where a ferromagnetic bottom layer
provides a weak link with a vortex magnetization imposed by the shape of the
disk. We present three different junction configurations, exhibiting their own
generator mechanism. In the first, we utilize the non-collinearity with a
second ferromagnetic layer to produce the triplet correlations. The second
configuration consists of only the bottom ferromagnet and the superconducting
contacts; it relies on the vortex magnetization itself to generate the
spin-polarized supercurrents. In the third case we exploit an intrinsic
generator by combining a conventional superconductor (NbTi) and a half-metallic
ferromagnetic oxide (LaSrMnO). We find strong supercurrents
in all cases. A particularly interesting finding is that the supercurrents are
strongly confined at the rims of the device, independent of the generating
mechanism, but directly related to their triplet nature. What causes these rim
currents remains an open question
Little-Parks oscillations with half-quantum fluxoid features in Sr2RuO4 micro rings
In a micro ring of a superconductor with a spin-triplet equal-spin pairing
state, a fluxoid, a combined object of magnetic flux and circulating
supercurrent, can penetrate as half-integer multiples of the flux quantum. A
candidate material to investigate such half-quantum fluxoids is
SrRuO. We fabricated SrRuO
micro rings using single crystals and measured their resistance behavior under
magnetic fields controlled with a three-axes vector magnet. Proper Little-Parks
oscillations in the magnetovoltage as a function of an axially applied field,
associated with fluxoid quantization are clearly observed, for the first time
using bulk single crystalline superconductors. We then performed magnetovoltage
measurements with additional in-plane magnetic fields. By carefully analyzing
both the voltages () measured at positive (negative) current, we
find that, above an in-plane threshold field of about 10 mT, the magnetovoltage
maxima convert to minima. We interpret this behavior as the peak splitting
expected for the half-quantum fluxoid states.Comment: 16 pages, 15 figure
Mesoscopic superconducting memory based on bistable magnetic textures
With the ever-increasing energy need to process big data, the realization of
low-power computing technologies, such as superconducting logic and memories,
has become a pressing issue. Developing fast and non-volatile superconducting
memory elements, however, remains a challenge. Superconductor-ferromagnet
hybrid devices offer a promising solution, as they combine ultra-fast
manipulation of spins with dissipationless readout. Here, we present a new type
of non-volatile Josephson junction memory that utilizes the bistable magnetic
texture of a single mesoscopic ferromagnet. We use micromagnetic simulations to
design an ellipse-shaped planar junction structured from a Nb/Co bilayer. The
ellipse can be prepared as uniformly magnetized or as a pair of vortices at
zero applied field. The two states yield considerably different critical
currents, enabling reliable electrical readout of the element. We describe the
mechanism used to control the critical current by applying numerical
calculations to quantify the local stray field from the ferromagnet, which
shifts the superconducting interference pattern. By combining micromagnetic
modeling with bistable spin-textured junctions, our approach presents a novel
route towards realizing superconducting memory applications
High-Quality CrO
Superconductor-ferromagnet (S-F) hybrids based on half-metallic ferromagnets, such as CrO_{2}, are ideal candidates for superconducting spintronic applications. This is primarily due to the fully spin-polarized nature of CrO_{2}, which produces enhanced long-range triplet proximity effects. However, reliable production of CrO_{2}-based Josephson junctions (JJs) has proved to be extremely challenging because of a poorly controlled interface transparency and an incomplete knowledge of the local magnetization of the CrO_{2} films. To address these issues, we use a bottom-up approach to grow CrO_{2} nanowires on prepatterned substrates via chemical-vapor deposition. A comprehensive study of the growth mechanism enables us to reliably synthesize faceted, homogeneous CrO_{2} wires with a well-defined magnetization state. Combining these high-quality wires with a superconductor produces JJs with a high interface transparency, leading to exceptionally large 100% spin-polarized supercurrents, with critical current densities exceeding 10^{9} Am^{-2} over distances as long as 600 nm. These CrO_{2}-nanowire-based JJs thus provide a realistic route to creating a scalable device platform for dissipation-less spintronics
Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator CaRuO
The surprisingly low current density required for inducing the insulator to
metal transition has made CaRuO an attractive candidate material for
developing Mott-based electronics devices. The mechanism driving the resistive
switching, however, remains a controversial topic in the field of strongly
correlated electron systems. Here we probe an uncovered region of phase space
by studying high-purity CaRuO single crystals, using the sample size as
principal tuning parameter. Upon reducing the crystal size, we find a four
orders of magnitude increase in the current density required for driving
CaRuO out of the insulating state into a non-equilibrium (also called
metastable) phase which is the precursor to the fully metallic phase. By
integrating a microscopic platinum thermometer and performing thermal
simulations, we gain insight into the local temperature during simultaneous
application of current and establish that the size dependence is not a result
of Joule heating. The findings suggest an inhomogeneous current distribution in
the nominally homogeneous crystal. Our study calls for a reexamination of the
interplay between sample size, charge current, and temperature in driving
CaRuO towards the Mott insulator to metal transition
Superconducting Triplet Rim Currents in a Spin-Textured Ferromagnetic Disk
Since the discovery of the long-range superconducting proximity effect, the interaction between spin-triplet Cooper pairs and magnetic structures such as domain walls and vortices has been the subject of intense theoretical discussions, while the relevant experiments remain scarce. We have developed nanostructured Josephson junctions with highly controllable spin texture, based on a disk-shaped Nb/Co bilayer. Here, the vortex magnetization of Co and the Cooper pairs of Nb conspire to induce long-range triplet (LRT) superconductivity in the ferromagnet. Surprisingly, the LRT correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnet, despite its trivial band structure. We show that these robust rim currents arise from the magnetization texture acting as an effective spin–orbit coupling, which results in spin accumulation at the bilayer–vacuum boundary. Lastly, we demonstrate that by altering the spin texture of a single ferromagnet, both 0 and π channels can be realized in the same device.peerReviewe
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Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr 2 RuO 4
Funder: JSPS-EPSRC Core-to-Core program (A. Advanced Research Network)Funder: JSPS research fellow (KAKENHI Grant No. JP16J10404)Funder: Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Netherlands Organisation for Scientific Research); doi: https://doi.org/10.13039/501100003246Funder: Grant-in-Aid JSPS KAKENHI JP26287078 and JP17H04848Abstract: The chiral p-wave order parameter in Sr2RuO4 would make it a special case amongst the unconventional superconductors. A consequence of this symmetry is the possible existence of superconducting domains of opposite chirality. At the boundary of such domains, the locally suppressed condensate can produce an intrinsic Josephson junction. Here, we provide evidence of such junctions using mesoscopic rings, structured from Sr2RuO4 single crystals. Our order parameter simulations predict such rings to host stable domain walls across their arms. This is verified with transport experiments on loops, with a sharp transition at 1.5 K, which show distinct critical current oscillations with periodicity corresponding to the flux quantum. In contrast, loops with broadened transitions at around 3 K are void of such junctions and show standard Little–Parks oscillations. Our analysis demonstrates the junctions are of intrinsic origin and makes a compelling case for the existence of superconducting domains