Review of muSR studies for SRF applications Tobias Junginger - PowerPoint PPT Presentation

Review of muSR studies for SRF applications Tobias Junginger

Acknowledgement  Experimentalists: D. Bazyl, R. Dastley, M. Dehn, D. Azzoni Gravel, S. Gehdi, Z. He, R. Kiefl, P. Kolb, R. Laxdal, Y. Ma, D. Storey, E. Thoeng, W. Wasserman, L. Yang, Z. Yao, H. Zhang (TRIUMF)  Support from Triumf Centre for Molecular & Materials Science : D. Arseneau, B. Hitti, G. Morris, D. Vyas (TRIUMF)  Support at PSI: A. Suter (PSI)  Sample Providers: D. Hall, M. Liepe, S . Posen (Cornell), A. Valente-Felenciano (JLAB), T. Tan, W. Withanage , M. Wolak, X. Xi (Temple University), G. Terenziani, S. Calatroni (CERN) Affiliations as of time of collaboration 2 T. Junginger - Review of muSR studies for SRF

μSR Facilities Around the World 3 T. Junginger - Review of muSR studies for SRF

μSR Facilities Around the World Summary: muSR is a technique that allows to measure localized magnetic fields. Using this technique we show: 1. A layer of higher T c material on niobium can push the field of first flux entry from a field consistent with H c1 to a field consistent with H sh . 2. For multilayer systems without insulator there is a wide range proximity effect to be considered 3. There is strong evidence for magnetic impurities on the surface of Nb/Cu samples 4 T. Junginger - Review of muSR studies for SRF

Outline Introduction to muSR 1. Using muSR as a local magnetometer (TRIUMF) 2. Inducing superheating in niobium by thin film coating 1. Low Energy muSR (PSI) 3. Proximity effects in NbTiN/Nb and NbTiN/AlN/Nb samples 1. Magnetic Impurities in Nb/Cu films 2. Summary 4. Outlook 5. BetaNMR 1. 5 T. Junginger - Review of muSR studies for SRF

Muon production and decay Muons are deposited ~100micron deep in a sample (bulk probe) – spin precesses with Positive muons are frequency dependent on local magnetic field produced with 100% spin polarization ~500 MeV Muon decays in  1/2 =2.2µsec - emits a positron        preferentially along the µ + spin direction u Muons are 100% spin polarized with kinetic energy of 4.1MeV 6 T. Junginger - Review of muSR studies for SRF

Muon Spin Rotation – muSR Muons are deposited one at a time in a sample • Muon decays emitting a positron preferentially • aligned with the muon spin Right and left detectors record positron • correlated with time of arrival • The time evolution of the asymmetry in the two signals gives a measure of the local field in the sample Left detector Right detector 7 T. Junginger - Review of muSR studies for SRF

Magnetic Volume Fraction Static distribution of Uniformly weakly Non-magnetic with random fields magnetic magnetic impurities 8 T. Junginger - Review of muSR studies for SRF

Using muSR as local magnetometer Meissner state • A sample is cooled in zero field - asymmetry measurements are taken as a function of applied magnetic field • The relative asymmetry at t=0 gives a measure of the volume fraction sampled by the muons that does not Intermediate state contain magnetic field • A variety of samples 1.2 Relative asymmetry and sample 1 geometries have 0.8 been characterized in 0.6 this way 0.4 Vortex state 0.2 0 Normal state 0 50 100 150 200 B (mT) 9 T. Junginger - Review of muSR studies for SRF

The field of first entry and the role of pinning in different geometries a) Transverse coin samples are sensitive to pinning - delays flux break in to the centre b) Parallel coin geometry is insensitive to pinning c) Ellipsoid samples are less sensitive • All three geometries are useful to characterize the material 1.2 Nb 800C at 2K 1 Normalized Asymmetry 0.8 0.6 0.4 Transverse Coin 0.2 Parallel Coin* Ellipsoid 0 0 0.5 1 1.5 2 2.5 H/Ho 10 T. Junginger - Review of muSR studies for SRF

The field of first entry and the role of pinning in different geometries 1.2 1.2 Nb 800C at 2K Nb 1400C at 2K 1 1 Normalized asymmetry Normalized Asymmetry 0.8 0.8 Weak Stronger 0.6 pinning 0.6 pinning Effect of pinning 0.4 0.4 Transverse Coin Transverse Coin 0.2 Parallel Coin* 0.2 Parallel Coin Ellipsoid Ellipsoid 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 H/Ho H/Ho 1400C heat treatment for three geometries 800C baked samples – pinning is clearly seen • virtually eliminates pinning from the Nb in different H entry between transverse, parallel H entry is equal for all geometries • coin and ellipsoid geometry • Our baseline substrate for thin film tests is 1400°C annealed niobium • The parallel field configuration is used to determine the field of first entry • Measurements in transverse geometry measure the pinning strength 11

Testing coated samples with muSR as a local magnetometer Muons are implanted 100 μ m deep in the bulk Coatings are between 50nm and 3.5µm Muons B Nb • Parallel field configuration. Field will first break in at the corners at 0.82 H entry and move to the center at 0.91 H entry . Only the field in the center is probed • Above Tc of niobium we measure the field of first entry of the coating only, below Tc of niobium we measure the higher H c1 or H sh 12 T. Junginger - Review of muSR studies for SRF

Nb3Sn on Nb Ellipsoid results 1.2 1.2 2K Nb3Sn on Nb Ellipsoid Fitting Flux Penetration Nb Normalized Asymmetry 5K 1 1 Nb3Sn 7K Linear (Nb) H nuc (T)/H nuc (0) 11K 0.8 0.8 Linear (Nb3Sn) 14K 17K 0.6 0.6 y = -0.9987x + 0.9988 R² = 0.9663 0.4 0.4   y = -0.9986x + 0.9974 2   H T T R² = 0.9813     0.2 nuc 0.2 1       0 H T nuc c 0 0 0 50 100 150 200 250 0 0.2 0.4 0.6 0.8 1 1.2 B applied (mT) (T/Tc) 2 Material H nucleate (0) T c [K] Below 9.25K we seem to measure [mT] Hsh of niobium, above 9.25K Hc1 of Niobium 227 9.36 Nb3Sn.  If the film induces superheating in Nb3Sn 37.1 17.3 niobium this should be independent on thickness 13 T. Junginger - Review of muSR studies for SRF

Testing coated samples (MgB2) Volume fraction in Meissner state at 0K 1 0,8 Niobium 1400°C annealed 0,6 MgB2 (150 nm) on Nb MgB2 (50 nm) on Nb 0,4 MgB2 (300nm) on Nb 0,2 0 0 50 100 150 200 250 Magnetic Field [mT] 14 T. Junginger - Review of muSR studies for SRF

Testing coated samples (MgB2) Volume fraction in Meissner state at 0K 1 MgB2 (300nm) on Nb 0,8 250 Again temperature Field of first entry [mT] 200 dependence as expected 0,6 150 for niobium 100 0,4 50 0 0,2 0 0,2 0,4 0,6 0,8 1 (T/9.25K)^2 0 0 50 100 150 200 250 Magnetic Field [mT] 15 T. Junginger - Review of muSR studies for SRF

Testing coated samples (Nb3Sn and MgB2) 280 Nb3Sn 270 Field of first entry [mT] MgB2 Transverse 260 Bullets 250 Theoretical H sh of Nb Disk 240 Parallel Bullet 230 Disk Disk Disk 220 210 Factor of 70 200 10 100 1000 10000 Film Thickness [nm] 16 T. Junginger - Review of muSR studies for SRF

Testing coated samples (Nb3Sn and MgB2) 280 Nb3Sn H sh =237(11) 270 Field of first entry [mT] MgB2 Transverse 260 Bullets 250 Theoretical H sh of Nb Disk 240 Parallel Bullet 230 Disk Disk Disk 220 210 Factor of 70 200 10 100 1000 10000 Film Thickness [nm] 17 T. Junginger - Review of muSR studies for SRF

Testing coated samples (Nb3Sn and MgB2) 280 Nb3Sn H sh =237(11) 270 Field of first entry [mT] MgB2 Transverse 260 Bullets 250 Theoretical H sh of Nb Disk 240 Parallel Bullet 230 Disk Disk Disk 220 No clear trend for field of first entry on material or thickness. 210 Factor of 70 Conclusion: Superheating is induced in niobium 200 10 100 1000 10000 Film Thickness [nm] 18 T. Junginger - Review of muSR studies for SRF

Outline Introduction to muSR 1. Using muSR as a local magnetometer (TRIUMF) 2. Inducing superheating in niobium by thin film coating 1. Low Energy muSR (PSI) 3. Proximity effects in NbTiN/Nb and NbTiN/AlN/Nb samples 1. Magnetic Impurities in Nb/Cu films 2. Summary 4. Outlook 5. BetaNMR 1. 19 T. Junginger - Review of muSR studies for SRF

Low energy muons • Low energy muons can be stopped in a variable depth between 0 and ~100nm • Ideal for testing layered structures • Parallel fields limited to 25mT • Has been applied to test two samples • NbTiN(80nm) on Nb • NbTiN(80nm)/AlN(20nm) on Nb 20 T. Junginger - Review of muSR studies for SRF

Field parallel to sample surface – Meissner Screening NbTiN (80nm) on Nb • Magnetic field decays with a single exponential λ =223(7) nm • 223(7) nm is short for NbTiN • Proximity effect? 2.2K 𝜇 𝑀 ∝ 1/ 𝑜 𝑇 V. Cherkez et al.- PHYSICAL REVIEW X 4, 011033 (2014) 21 T. Junginger - Review of muSR studies for SRF

Field parallel to sample surface – Meissner Screening NbTiN (80nm) on Nb • Magnetic field decays with a single exponential λ =223(7) nm • 223(7) nm is short for NbTiN • Proximity effect? London theory, no 2.2K proximity effect Naive treatment Experiment T. Kubo arXiv:1410.1248 22 T. Junginger - Review of muSR studies for SRF

Field parallel to sample surface – Meissner Screening NbTiN (80nm) on Nb Either the NbTiN layer is significantly thicker than 80 nm or long range proximity effect Fit cosh(x/ λ 0 )/cosh(d/(2 λ 0 )) λ 0 =204(18); d=135(11) 23

Field parallel to sample surface – Meissner Screening NbTiN (80nm) on Nb At 8 K a vortex must have entered the niobium 11 K λ 0 =204(18); d=135(11) 8 K λ 0 =190(15); d=157(13) 24