A spin triplet supercurrent through the half-metallic ferromagnet CrO2

Keizer, R. S.; Goennenwein, Sebastian T. B.; Klapwijk, T. M.; Miao, Guo‐Xing; Xiao, Gang; Gupta, Arunava

doi:10.1038/nature04499

Cited by 752 publications

(726 citation statements)

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“…Measurements[19] on a SQUID, in which a phase change of π in the order parameter is found after inversion, indicate an odd-parity state. Very recently, a Josephson current through a strong ferromagnet was observed, indicating the existence of a spin triplet state [20] induced …”

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confidence: 99%

“…Experimental results that may argue for the existence of long range triplet pairing of superconductors through a ferromagnet have been obtained in superlattices [18] with ferromagnetic spacers, and in two superconductors coupling through a single ferromagnet [19,20]. Measurements[19] on a SQUID, in which a phase change of π in the order parameter is found after inversion, indicate an odd-parity state.…”

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confidence: 99%

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Odd Triplet Pairing in Clean Superconductor/Ferromagnet Heterostructures

2007

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We study triplet pairing correlations in clean Ferromagnet (F)/Superconductor (S) nanojunctions, via fully self consistent solution of the Bogoliubov-de Gennes equations. We consider FSF trilayers, with S being an s-wave superconductor, and an arbitrary angle α between the magnetizations of the two F layers. We find that contrary to some previous expectations, triplet correlations, odd in time, are induced in both the S and F layers in the clean limit. We investigate their behavior as a function of time, position, and α. The triplet amplitudes are largest at times on the order of the inverse "Debye" frequency, and at that time scale they are long ranged in both S and F. The zero temperature condensation energy is found to be lowest when the magnetizations are antiparallel.PACS numbers: 74.45.+c, 74.25.Bt, 74.78.Fk The proximity effects in superconductor/ferromagnet (SF) heterostructures lead to the coexistence of ferromagnetic and superconducting ordering and to novel transport phenomena [1,2]. Interesting effects that arise from the interplay between these orderings have potential technological applications in fields such as spintronics [3]. For example, the relative orientation of the magnetizations in the F layers in FSF trilayers can have a strong influence on the conductivity [4,5,6,7,8], making them good spin valve candidates. Such trilayers were first proposed[9] for insulating F layers and later for metallic [10,11] ones. This interplay also results in fundamental new physics. An outstanding example is the existence of "odd" triplet superconductivity. This is an s-wave pairing triplet state that is even in momentum, and therefore not destroyed by nonmagnetic impurities, but with the triplet correlations being odd in frequency, so that the equal time triplet amplitudes vanish as required by the Pauli principle. This exotic pairing state with total spin one was proposed long ago [12] as a possible state in superfluid 3 He. Although this type of pairing does not occur there, it is possible in certain FSF systems [1,2,13,14] with ordinary singlet pairing in S. This arrangement can induce, via proximity effects, triplet correlations with m = 0 and m = ±1 projections of the total spin. If the magnetization orientations in both F layers are unidirectional and along the quantization axis, symmetry arguments show that only the m = 0 projection along that axis can exist.Odd triplet pairing in F/S structures has been studied in the dirty limit through linearized Usadel-type quasiclassical equations [2,13,14,15]. In this case, it was found that m = 0 triplet pairs always exist. They are suppressed in F over short length scales, just as the singlet pairs. The m = ±1 components, for which the exchange field is not pair-breaking, can be long ranged, and were found to exist for nonhomogeneous magnetization. For FSF trilayers [2,16,17], the quasiclassical methods predict that the structure contains a superposition of all three spin triplet projections except when the magnetizations of the F layers are collinear, in wh...

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Odd Triplet Pairing in Clean Superconductor/Ferromagnet Heterostructures

2007

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“…Evidence for the LRSTPE has also been observed in other systems, including: Pd 0.88 Ni 0.12 and Pd 0.987 Fe 0.013 coupled to Co in S/F1/N/F2/N/F1/S-type junctions 2 ; Co nanowires with W superconducting leads 10 ; and CrO 2 , a half-metallic oxide coupled to NbTi or MoGe superconducting leads 11,12 . In these systems the origin of LRSTPE is more likely to be related to artificially created inhomogeneity, either through the difference in spin scattering generated by the choice of thin films in the multilayer stack or though the particular arrangement that a polycrystalline random alignment of grains may present to the superconducting interface.…”

Section: Introductionmentioning

confidence: 86%

Evidence for spin mixing in holmium thin film and crystal samples

et al. 2011

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In a number of recent experiments, holmium has been shown to promote spin triplet pairing when in proximity to a spin singlet superconductor. The condition for the support of spin triplet pairing is that the ferromagnet should have an inhomogeneous magnetic state at the interface with the superconductor. Here we use Andreev reflection spectroscopy to study the properties of single ferromagnet/superconductor interfaces formed of holmium and niobium, as a function of the contact resistance of the junction between them. We find that both single crystal and c-axis oriented thin film holmium show unusual behavior for low junction contact resistance, characteristic of spin mixing type properties, which are thought necessary to underpin spin triplet formation. We also explore whether this signature is observed when the junction is formed of Ni 19 Pd 81 and niobium.

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“…16 However, a number of recent experimental studies have demonstrated an unexpectedly long-ranged proximity effect in mesoscopic superconductor-ferromagnet hybrid structures. [17][18][19][20][21][22] There are a number of theoretical scenarios which can explain this phenomenon. In most of the scenarios pair correlations inside the ferromagnet are attributed to some type of equal-spin triplet pairing.…”

Section: Introductionmentioning

confidence: 99%

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

Takei

Galitski

2012

Phys. Rev. B

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Motivated by the recent experiment of Wang et al. [Nature Physics 6, 389 (2010)], who observed a highly unusual transport behavior of ferromagnetic Cobalt nanowires proximity-coupled to superconducting electrodes, we study proximity effect and temperature-dependent transport in such a mesoscopic hybrid structure. It is assumed that the asymmetry in the tunneling barrier gives rise to the Rashba spin-orbit-coupling in the barrier that enables induced p-wave superconductivity in the ferromagnet to exist. We first develop a microscopic theory of Andreev scattering at the spin-orbit-coupled interface, derive a set of self-consistent boundary conditions, and find an expression for the p-wave mini-gap in terms of the microscopic parameters of the contact. Second, we study temperature-dependence of the resistance near the superconducting transition and find that it should generally feature a fluctuation-induced peak. The upturn in resistance is related to the suppression of the singleparticle density of states due to the formation of fluctuating pairs, whose tunneling is suppressed. In conclusion, we discuss this and related setups involving ferromagnetic nanowires in the context of one-dimensional topological superconductors. It is argued that to realize unpaired end Majorana modes, one does not necessarily need a half-metallic state, but a partial spin polarization may suffice. Finally, we propose yet another related class of material systems -ferromagnetic semiconductor wires coupled to ferromagnetic superconductors -where direct realization of Kitaev-Majorana model should be especially straightforward.

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A spin triplet supercurrent through the half-metallic ferromagnet CrO2

Cited by 752 publications

References 23 publications

Odd Triplet Pairing in Clean Superconductor/Ferromagnet Heterostructures

Odd Triplet Pairing in Clean Superconductor/Ferromagnet Heterostructures

Evidence for spin mixing in holmium thin film and crystal samples

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

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