Localized magnetism exists on the surface of niobium, as evidenced by experiments and supported by theory. Point contact spectroscopy, SQUID, and EPR measurements show the presence of paramagnetic moments in niobium oxides that increase with mild baking and decrease with high temperature baking. Theoretical modeling of the residual resistance matches experimental data and attributes it to magnetic impurities in the niobium oxide layer, with a concentration of around 250 ppm or 6×1012/cm2. Surface treatments like baking modify the concentration of magnetic moments, providing an explanation for changes in cavity performance.

1. Localized magnetism on the Surface of Niobium: experiments and theory Th. Proslier, M. Kharitonov, M.Pellin (ANL) J.F. Zasadzinski (IIT) G. Ciovati (JLAB) N. Groll, I. Chiorescu, A. Gurevich (NHMFL) C. Antoine (CEA) A. Romanenko, L.Cooley (FNAL) Funded by ARRA-DOE, Office of science, High Energy Physics. SRF film workshop-Oct 2010

3. Theory: residual resistanceSRF film workshop-Oct 2010

4. 3 Niobium surfaces are complex, important, and currently poorly controlled at the nm level Residue from chemical processing Inclusions, Hydride precipitates Surface oxide Nb2O55-10 nm 45 nm RF depth Interface: sub oxides NbO, NbO2 often not crystalline (niobium-oxygen “slush”) Interstitials dissolved in niobium (mainly O, some C, N, H) e- flow only in the top 45 nm Clean niobium Grain boundaries Probe the surface superconductivity SRF film workshop-Oct 2010

6. The ZBC value -> Number of normal electronNormal electrons in gap => dissipation and lower Q Ideal BCS superconductor SRF film workshop-Oct 2010

7. Experimental evidences of Magnetism: Point Contact 5 Conc= 0.1-0.3% Th. Proslier, j. zasadzinski et al. APL 92, 212505 (2008) SRF film workshop-Oct 2010

8. 6 Hot and cold spots in SRF cavity (from J-lab) “Normal” spectrum Anomalous spectrum Only on hot spots Hot spots: show dissipative behavior Higher ZBC and anomalous spec. lower gap values (1.3<∆<1.55) Cold spots: “normal” dissipation Low ZBC values Normal gap values (1.5<∆<1.55) Correlates with cavities results! (once again) Origin of peculiar spectrum and dissipation? Th. Proslier, G. Ciovati to be submitted to PRST-AB (2010) SRF film workshop-Oct 2010

9. 7 Hot and cold spots in SRF cavity, Origin Temp. dep: peak at 0 mV bias increases Killing superconductivity by applying a mag. Field Nb/NbOx/Au Ta/Ta2O5/Al fits with Appelbaum theory -> Magnetic impurities in the oxides J>0 -> antiferromagnetic coupling -First time measured on Nb oxides -Same behavior observed on unbaked Nb coupons Th. Proslier, J. zasadzinski et al. accepted to PRB (2010) SRF film workshop-Oct 2010

10. 8 Zero Bias Conductance (ZBC) peak: Spin Flip Tunneling Nb-Nb2O5-Au (hot spots) Ta-Ta2O5-Ta 2 K g=2 Kondo effect Δ=g.µ B .H -> g = 3.5 Definite proof for localized paramagnetic moments in the Niobium oxide What is the origin ? SRF film workshop-Oct 2010

11. 9 Experimental evidences of Magnetism: Electron Paramagnetic resonance (EPR) On niobiumpowders (high surface/volume ratio) Nb12O29 Localized paramagnetic moments due to Oxygen vacancies. 2 values of g: g=1.2 and g=3.3 -> insolated spins -> arrangement in 1D spin chains A.Lappas, PRB 65, 134405 (2002) SRF film workshop-Oct 2010

16. Hot spot show higher concentration of Mag. Moments (PCT)

17. Correlates with Cavity results

18. Theory: surface impedance [T] 1: Cava et al. Nature 350, 598 (1991) & PRB 41, 13 (1991) & PRB 72, 033413 (2005) SRF film workshop-Oct 2010

19. Surface magnetism: Theory Model assumes: Homogeneous moment density on λ + London limit + dirty limit 3 parameters: η, α: describe effect of magnetic impurities on the superconductor -> concentration normal conductivity σ0-> mean free path, ℓ. London & dirty limit: In the limit: Density of Normal electrons: Density of superconducting electrons: SRF film workshop-Oct 2010 Solve self consistently g[T, e], f[T, e] and Δ[T] 14

20. Theory: The residual resistance 15 α=0.02 meV, η =0.5 SRF film workshop-Oct 2010

21. Theory: The residual resistance 16 α=0.033 meV, η =0.2 SRF film workshop-Oct 2010

22. Surface magnetism: Theory 17 η= 0.2 -> strong coupling

23. Theory: The residual resistance 18 η =0.2 Bake 180C Bake 120C Bake 160C Bake 90C Th. Proslier, M. Kharitonov submitted to PRL (2010) SRF film workshop-Oct 2010

24. 19 Theory: The residual resistance Δ and Tc Mean free path and λ Mean Free path, ℓ, decreases after baking consistent with BCS surface impedance fits α~2.10-2meV & η=0.2 -> 250 ppm in Nb -> 6.1012 /cm2 in Nb oxides SRF film workshop-Oct 2010

25. 20 Theory: The residual resistance conclusion London penetration depth, λ Concentration of Mag. Impurities Concentration of Magnetic impurities increase after baking consistent with SQUID data Cornell 2010 SRF film workshop-Oct 2010

26. Non-linear Meissner effect: On-chip cavity 21 60 nm Nb+5nm W 30 nm Nb+5nm W 60 nm Nb+ water 60 nm Nb+air 30 nm Nb+water 30 nm Nb+air