Near-Field Nonlinear Microwave Microscopy of Bulk Nb Surfaces Tamin Tai, Behnood G. Ghamsari, Steven M. Anlage Department of Physics Center for Nanophysics and Advanced Materials University of Maryland 17 July, 2012 dBm Funding from Department of Energy / High Energy Physics Office of Naval Research Maryland Center for Nanophysics and Advanced Materials

Objective: Identify microscopic defects that cause breakdown of SRF cavities Method: 1) Examine coupons with intense, localized B RF in the superconducting state 2) Measure locally-produced harmonic generation from defects 3) Scan the probe and image the response 1  m V f + V 3f V 3f - Microwave Source Low Pass Filter High Pass Filter v 3f Spectrum Analyzer Thin Film Bulk Nb Nonlinear Superfluid Response: Superconductor P 3f resolution limit

J NL (A/m 2 ) Intrinsic Nb Dislocation tangle Hydrogen-poisoned Nb NbO x corrosion Josephson weak link Ta inclusion Quantifying the Nonlinear Response of Superconductors Schematically, we expect: P 3f resolution limit Defects with the smallest J NL will dominate the local P 3f generation

Harmonic Generation in Superconducting Thin Films Most of our work so far is on thin films… Films are flat, smooth, reproducible, produce strong signals Nb thin film (50 nm) Candidate Nonlinear Sources:  Superfluid nonlinearity as T -> T c  Vortex / Anti-vortex pair creation and motion

Bulk Nb sample From Tom Bieler, Mich. State Univ. Weld Probe

Local Harmonic Response from Bulk Nb Spectrum Analyzer Noise Floor Write-head probe nonlinearity Stronger Induced Currents Stronger Nonlinear Response

Local Harmonic Response from Bulk Nb Spectrum Analyzer Noise Floor Stronger Nonlinear Response Increasing induced current What is the role of local heating in these measurements?

Tamin Tai, Behnood Ghamsari talks in Thin Films and New Ideas Workshop on Thursday Work in Progress Fiducial Samples: Ta inclusions in Nb Film Nb Substrate

Conclusions Achieved local harmonic generation from bulk Nb surfaces under SRF operating conditions Mechanisms of harmonic generation at low temperatures not yet quantitatively understood Work in progress to: Measure magnitude of B RF vs. distance from probe Measure P 3f (x,y) in artificial Ta-in-Nb-matrix samples Image local RF critical field around defects Suggestions are welcome: What is the most critical defect to examine? What mechanisms are responsible for the observed harmonic generation?

Motivation For Our Experiment Position x Defect 1 Defect 2 J NL (x) P 3f (x) Scanning at T < T c Magnetic probe Nb film or bulk Nb J crystal grain boundary scan 500 x 200  m pit Grain Boundaries T. Bieler Mich. State Univ. welds, oxidation, hydrogen poisoning

Phase-Sensitive Harmonic Measurements: Vector Network Analyzer in Frequency Offset Mode (VNA-FOM)Measures 11

Inside Chamber Probe Assembly Experimental Setup 12/31 Permalloy shields ~2  m Cu coils Read Sensor Write Pole RF Magnetic Fields Air bearing surface 2  m Read/Write Head Thickness (~1.2 mm) Suspension Au bond pads to reader and writer Air bearing surface Cross-section (right figure)

13 Fabrication for Standard Samples Bulk Nb sample Recent Work-I

Power Dependence (P3f –Pf)

Power Dependence (P3f-Pf)

P3f (T) Excitation Freq: 5.44 GHz 10 dBm

Height Profile along the Yellow Line AFM IMAGE ~50 nm height difference Ta is rougher Ta Nb

SEM IMAGES There is a PMMA/MMA on the top of the film

T=15 K Excited Power: 16 dBm Excited Frequency: 5.33 GHz Step size: 10  m. MgB 2 Film (200 nm) P 3f Imaging mm mm Temperature Mapping mm mm P 3f (dBm) Phase Mapping mm mm degree K

21 Scanning System of the Microscope Motor of X axis Motor of Y axis Capability of scanning mode: Contact mode Y-axis: 1 full step of the motor= 0.5  m X-axis: 1 full step of the motor=50 nm Z-axis: Under Construction R. B. Dinner, REVIEW OF SCIENTIFIC INSTRUMENTS 76,

Recent Work Determine how large field is applied on the surface 22 I+I+ I-I- V+V+ PfPfPfPf V-V- Voltage source model Al 2 O 3 Nb film Substrate

Nb Josephson Weak Link Determine |B RF | from Shapiro steps on Nb Josephson weak link

Challenges for Measurements on Nb bulk materials Temperature of cold plate reaches 4.2K but Nb surface remains warmer. Rough sample surface on bulk Nb make the probe far away from sample surface. Top surface of bulk Nb (thickness: 0.1 inch) Head Gimbal Assembly (HGA) Pit on Nb Copper cold plate

Measured Input Impedance of Write Head Seagate Longitudinal Head Pole gap ~ 100 nm Coil turns 6 ~ 8 Impedance Matched to 50  Can we get microwave signal to the write head? Network Analyzer Au bond pads to writer 25

Field Image from Hard-Disk Writer Poles by the technique of High-Frequency MFM wcwc wmwm Ref. w m AC-phase HF-MFM signal U.Hartmann; Journal of Magnetism and Magnetic Materials 322 (2010) 1694– MHz 500 MHz 1 GHz 2 GHz w c :

Quenching and Localized Defects on Nb SRF Cavities These defects can lead to hot spots on accelerator cavity within operating frequency region (1-2 GHz) However, many defects are benign. How to distinguish the ‘good’ ones from the ‘bad’ ones? 500 x 200  m pit Grain Boundaries T. Bieler Mich. State Univ. welds, oxidation, hydrogen poisoning

Approach to Identifying Critical Defects GOAL: To establish links between microscopic defects and the ultimate RF performance of Nb and MgB 2 at cryogenic temperatures APPROACH: Near-Field Microwave Microscopy* 1) Stimulate Nb with a concentrated-in-space and intense RF magnetic field 2)Drive the material into nonlinearity (e.g. nonlinear Meissner effect) Why the NLME? It is very sensitive to defects… 3)Measure the characteristic field scale for nonlinearity: J NL 4) Map out J NL (x,y) → relate to previously-characterized defects *S. M. Anlage, V. Talanov, A. Schwartz, "Principles of Near-Field Microwave Microscopy," in Scanning Probe Microscopy: Vol. 1, edited by S. V. Kalinin and A. Gruverman (Springer-Verlag, New York, 2007), pages

Induce high  0 K ~ 200 mT (B c of Nb) K(x,y) sharply peaked in space ► Better spatial resolution Current distribution geometry factor Nonlinear Near-Field Microscopy of Superconductors D. Mircea, S. Anlage, Phys. Rev. B 80, (2009) + references therein P input Superconductor sample surface loop coaxial probe P 3f : NLME Nonlinearities K(x,y)

J NL (x) P 3f (x) Position x Defect 1Defect 2 What do We Learn About the Superconductor? P 3f Phys. Rev. B 72, (2005)

B RF ~ 1 Tesla (in gap) Lateral size ~ 100 nm x few-100 nm How to Generate Strong RF Magnetic Fields? Magnetic Write Head Permalloy shields ~2  m Cu coils Read Sensor Write Pole RF Magnetic Fields Air bearing surface 2  m Magnetic recording heads provide strong and localized B RF SEM picture of the magnetic write head gap Side View Bottom View Permalloy Gap Reference: IEEE Trans Magn. Vol. 37, No. 2 pp

Surface Currents under Magnetic Write Head HFSS Simulation  0 K max ~ 933 mT 3  m Frequency: 4.5 GHz Probe-sample separation: 200 nm Pole gap: 100 nm Excitation: 50 mA

Experimental Setup Goals: B RF ~ 200 mT Lateral size ~ 100 nm RF Coil on slider Superconductor Head Gimbal Assembly (HGA) We need high B RF and strongly localized field distributions Probe Superconducting Film T. Tai, et al. IEEE Trans. Appl. Supercond. 21, 2615 (2011)

MgB 2 (thickness: 50 nm) Nonlinear Measurement Measured at a fixed location T c ~36 K Samples from Prof. Xiao-Xing Xi, Temple University Noise floor Peak expected: Intrinsic NLME Strong nonlinearity saturates at low-T

Intrinsic Nonlinear Meissner Effect Near T c Near T c the superfluid density is very sensitive to small perturbations  =1*10 5 A 3 /m 2 ;T c =9.5 K (  T c =0.03K)  cutoff) = 312 nm J(cutoff) = 2.1*10 11 A/m 2 MgB 2 Nb

Temperature Depend Third Harmonic Response measurements at a fixed location Nb (thickness 50 nm) Noise floor of Spectrum Analyzer P 3f (T) at low temperatures consistent with: 1)Current loops involving Josephson weak links 2)Creation and motion of Josephson vortices 3)In MgB 2, perhaps also Josephson coupling between  and  bands > LL LL L JJ

RF Vortex Modeling a ~  G. Carneiro, PRB 69, (2004) Lattice London Calculation

A. Gurevich and G. Ciovati, Phys. Rev. B, 77, (2008) RF Vortex Modeling Predict V 3f (magnitude and phase) as a function of: RF field strength, Temperature Vortex semiloop dynamics