Colossal proximity effect in a superconducting triplet spin valve based on halfmetallic ferromagnetic CrO2

Ferromagnets can sustain supercurrents through the formation of equal spin triplet Cooper pairs and the mechanism of odd-frequency pairing. Since such pairs are not broken by the exchange energy of the ferromagnet, superconducting triplet correlations are long-ranged and spin-polarized, with promises for superconducting spintronics devices. The main challenge is to understand how triplets are generated at the superconductor (S)/ ferromagnet (F) interface. Here we use the concept of a so-called triplet spin valve (TSV) to investigate the conversion of singlets in a conventional superconductor to triplets in the halfmetallic ferromagnet CrO_2. TSV's are composed of two ferromagnetic layers (separated by a thin normal metal (N) layer) and a superconductor (F_1/N/F_2/S). The package F_1/N/F_2 generates triplets in F_1 when the magnetization directions of the F_{1,2}-layers are not collinear. This drains singlet pairs from the S-layer, and triplet generation is therefore signalled by a decrease of the critical temperature $T_c$. Recently, experiments with TSV's were reported with Co draining layers, using in-plane fields, and finding T_c-shifts up to 100~mK. Using CrO_2 instead of Co and rotating a magnetic field from in-plane to out-of-plane, we find strong T_c variations of almost a Kelvin up to fields of the order of a Tesla. Such strong drainage is consistent with the large lengths over which supercurrents can flow in CrO_2, which are significantly larger than in conventional ferromagnets. Our results point to the special interest of halfmetals for superconducting spintronics.Comment: 6 pages, 5 figures; supplementary information separat

High-Quality CrO2 Nanowires for Dissipation-less Spintronics

Quantum Matter and Optic

Emergence of the stripe-domain phase in patterned permalloy films

The occurrence of stripe domains in ferromagnetic permalloy (Py = Fe20Ni80) is a well-known phenomenon which has been extensively observed and characterized. This peculiar magnetic configuration appears only in films with a thickness above a critical value (dcr), which is strongly determined by the sputtering conditions (i.e., deposition rate, temperature, magnetic field). So far, dcr has usually been presented as the boundary between the homogeneous (H) and stripe-domain (SD) regime, respectively, below and above dcr. In this work we study the transition from the H to the SD regime in thin films and microstructured bridges of Py with different thicknesses. We find there is an intermediate regime, over a quite significant thickness range below dcr, which is signaled in confined structures by a quickly changing domain-wall configuration and by a broadening of the magnetoresistance dip at the coercive field. We call this the emerging stripe-domain (ESD) regime. The transition from the ESD to the SD regime is accompanied by a sharp increase of the magnetoresistance ratio at the thickness where stripes appear in MFM

Direct-write printing of Josephson junctions in a scanning electron microscope

Quantum Matter and Optic

Controlling supercurrents and their spatial distribution in ferromagnets

Quantum Matter and Optic

Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr<inf>2</inf>RuO<inf>4</inf>

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 JP17H04848AbstractThe 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.</jats:p

Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr<inf>2</inf>RuO<inf>4</inf>

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 JP17H04848AbstractThe 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.</jats:p

Spin-triplet supercurrents of odd and even parity in nanostructured devices

Triplet superconductivity refers to a condensate of equal-spin Cooper pairs (pairs of electrons with equal spin). While exceptionally rare in nature, triplet pairing of electrons can occur if either the temporal or spatial component of the superconducting wavefunction can be represented by an odd function. These are often referred to as odd-frequency and odd-parity triplets, respectively. We use hybrid magnetic devices to study the former, while the latter is investigated in mesoscopic structures of strontium ruthenate (Sr\tss{2}RuO\tss{4}).Quantum Matter and Optic