Tunneling density of states as a function of thickness in superconductor/strong ferromagnet bilayers

Reymond, S.; SanGiorgio, Paul; Beasley, M. R.; Kim, J.; Kim, T.; Char, K.

doi:10.1103/physrevb.73.054505

Cited by 17 publications

(28 citation statements)

References 32 publications

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“…Equally important, as noted before, we find that once the Usadel equation in the F-layer is essentially linear (d F ≈ ξ F ) the only effect of increasing d F is to scale the DOS at the interface [10,22]. This means that none of these parameters will produce any sub-gap structure -nor, for that matter, will any other parameter in Eq.…”

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confidence: 71%

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

et al. 2008

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We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, dF . Our thinnest samples show the usual DOS peak at ±∆0, whereas intermediate-thickness samples have an anomalous "double-peak" structure. For thicker samples (dF ≥ 3.5 nm), we see an "inverted" DOS which has previously only been reported in superconductor/weak-ferromagnet structures. We analyze the data using the self-consistent nonlinear Usadel equation and find that we are able to quantitatively fit the features at ±∆0 if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the sub-gap structure through the addition of any parameter(s). Therefore, the observed anomalous sub-gap structure represents new physics beyond that contained in the present Usadel theory.The co-existence of superconductivity and ferromagnetism was first proposed by Fulde and Ferrell [1] and Larkin and Ovchinnikov [2] more than forty years ago. While some unusual materials have since been found with both superconducting and ferromagnetic transitions (e.g. Qualitative evidence for the first two of these effects is convincing, but definitive quantitative agreement with theory has been problematic. The evidence for triplet superconductivity is less certain, although a recent report by Keizer et al [7] provides tantalizing evidence for such an effect. One reason for the difficulty in achieving quantitative agreement with theory is the proliferation of physical effects that now have been incorporated into the theory, leading to a concomitant proliferation of fitting parameters, which makes discriminating fits to limited data sets nearly impossible.In order to obtain more discriminating data sets and to further explore the SF proximity effect in the case of strong ferromagnets, we have undertaken superconducting tunneling densities of state (DOS) measurements on Nb/Ni thin-film bilayers. By varying the Ni thickness, d F , we can track the spatial evolution of the behavior of the Cooper pairs diffusing into the ferromagnet. This approach gives us much more information per sample (the entire DOS spectrum) than T c or J c measurements, and is less sensitive to variations in boundary parameters. Analyzing these results with the most complete forms of the Usadel theory available has allowed us to discriminate critically for the first time the relative importance of the various physical effects now incorporated into the theory. We find that by far the most important parameter beyond the exchange field, E ex , is the degree of spin-orbit scattering (first suggested by Demler et al [8]). In addition, we find an anomalous double-peak structure in the DOS that has not been reported previously and that we have been unable to account for theoretically.We use planar tunnel junctions of the form normalinsulator-ferromagnet-superconductor. A schematic of our sample geometry is shown in the inset of Fig. 2. The deposition of our samples and characterization of the tunnel junctions has be...

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confidence: 71%

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

et al. 2008

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“…7, where substantial scatter on a plot of E ex vs bulk T ⌰ for various ferromagnetic metals in S/FM and S/FM/S systems is shown. [40][41][42][43][44] To compare the relation of E ex to bulk T ⌰ , we have expressed both quantities in kelvin units where 1 meVϵ 11.645 K. Strikingly, for some S/FM bilayers, there is a good linear relationship between E ex and T ⌰ and the constant of proportionality is close to 1 so that E ex = k B T ⌰ ; in contrast, all the junction data ͑S/FM/S͒ lie well above this line and there is a great deal of scatter. For the junctions, the constant of proportionality is as low as 1.2 for Fe and as high as 3.0 for CoFe.…”

Section: Ferromagnetic Exchange Energymentioning

confidence: 99%

Zero toπtransition in superconductor-ferromagnet-superconductor junctions

et al. 2007

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We report a systematic study of Nb/ferromagnet ͑FM͒/Nb trilayer structures in which the FM layer is one of the strong ferromagnets Co, Fe, Ni, and Ni 80 Fe 20 ͑Py͒. Accurate control of the FM layer thickness has enabled detailed studies of the magnetic and transport properties in the superconducting state. In all cases, we estimate the thickness of the magnetic dead layer and the exchange energies of the ferromagnetic layers; in doing so, we demonstrate inconsistencies between the exchange energies derived elsewhere from superconductor ͑S͒/FM bilayer experiments and from S/FM/S junction measurements compared to their bulk Curie temperatures, which may hint at further complexity in the underlying physics. We show results in support of a recent publication ͓J. W. A Robinson et al., Phys. Rev. Lett. 97, 177003 ͑2006͔͒, focus in detail on a single 0-phase transition, and show evidence for the appearance of a second harmonic in the current-phase relation at the minimum of the critical current.Thin films of Nb/ Co/ Nb, Nb/Py/Nb, Nb/ Ni/ Nb, and Nb/ Fe/ Nb were prepared on thermally oxidized ͑100͒ Si substrates with a 250 nm oxide layer on the surface in an PHYSICAL REVIEW B 76, 094522 ͑2007͒

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“…Also, it is not clear that there was no diffusion of magnetic atoms at the interface. More recent 15,16 work has focused on bilayer conductance in planar junctions with sharp interfaces and clean materials. Characteristic peaks in the conductance were observed at bias voltages corresponding to ⌬ 0 , the bulk superconducting gap energy, and an enhanced conductance at zero bias.…”

Section: Introductionmentioning

confidence: 99%

Tunneling conductance in superconductor/ferromagnet junctions: Self-consistent approach

Barsic

Valls

2009

Phys. Rev. B

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We evaluate the tunneling conductance of clean ferromagnet/superconductor junctions via a fully selfconsistent numerical solution of the microscopic Bogoliubov-de Gennes equations. We present results for a relevant range of values of the Fermi wave-vector mismatch ͑FWM͒, the spin polarization, and the interfacial scattering strength. For nonzero spin polarization, the conductance curves vary nonmonotonically with FWM. The FWM dependence of our results is stronger than that previously found in non-self-consistent calculations since, in the self-consistent case, the effective scattering potential near the interface depends on the FWM. The dependence on interfacial scattering is monotonic. These results confirm that it is impossible to characterize both the FWM and the interfacial scattering by a single effective parameter and that analysis of experimental data via the use of such one-parameter models is unreliable.

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Tunneling density of states as a function of thickness in superconductor/strong ferromagnet bilayers

Cited by 17 publications

References 32 publications

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

Zero toπtransition in superconductor-ferromagnet-superconductor junctions

Tunneling conductance in superconductor/ferromagnet junctions: Self-consistent approach

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