µsquid microscopy C. Veauvy(1)*, V. Dolocan(1), D. Mailly(2) and K.Hasselbach(1) (1)CRTBT CNRS Grenoble, (2)LPN CNRS Marcoussis.

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1 µsquid microscopy C. Veauvy(1)*, V. Dolocan(1), D. Mailly(2) and K.Hasselbach(1) (1)CRTBT CNRS Grenoble, (2)LPN CNRS Marcoussis. * now Colorado School of Mines, Golden, Co, USA, cveauvy@mines.edu C. Veauvy et al. Review of Scientific Instruments. 73, 3825, 2002

2 outline 1. The µsquid-fm microcope 2. The magnetic probe : the µsquid 3. The topographic probe: the tuning fork 4. Magnetic sensitivity and spatial resolution 5. Observation of vortices in a microstructured Al film disordered low temperature phase ordering of the interstitial vortices agregation of the interstitial vortices surface superconductivity Conclusion and Outlook

3 Vortex Transitions and the phase diagram H(T) Vortex in a Nb film with blind holes. H supraconductivité localisée H c2 H c3 normal T=4.2 K H=6.37 G vortex A. Bezryadin et al., J. Low Temp., 102, 73, 1996 H c1 supra T c T

4 Magnetic microcopy Lorentz Microscopy, MFM, Bitter decoration, magneto optics, Sensibilité magnétique (G) Hall probe microscopy, SQUID microscopy microscopie 10 2 électronique MFM Bitter SHPM Magnéto-optique 0,001 0,01 0, Résolution spatiale (µm) microscopie à µsquid microscopie à SQUID 10-8 F F F F 0 1F 0 S. Bending, Advances in Physics, 48, 449, 1999

5 High resolution magnetic probes Sonde de Hall, AsGa, Microscope STM-Hall, S. Bending µsquid/membrane SiC A. Madouri, D. Mailly

6 SQUID = Superconducting QUantum Interference Device. the msquid Flux F is quantized in the loop in units of F 0 : F 0 = h 2e = Vs Al msquid fabricated by D. Mailly at LPN-CNRS Flux transferfunction I c (F) of an Al msquid:

7 Principle and sensitivity of the msquid measurement V The V(I) characteristic of a µsquid is hysteretic, due to heating. I cmin I c I Courant 1500 I c I - e c Dt Dt count µs < T < 1.6 ms temps I c (µa) With a maximal sampling frequency of 10 khz, the flux sensitivity of the µsquid is: 10-5 F 0 / Hz

8 Magnetic coupling between µsquid and sample Mutual inductance, M, calculation between the µsquid and the vortex: F SQUID = M I vortex (1) inductance mutuelle (H) (2) (3) -1, ,5-1 -0,5 0 0,5 1 1,5 2 distance (µm) z = 1 µm z = 500 nm z = 250 nm z = 125 nm z = 16 nm micro-squid de largeur = 1 µm vortex de largeur 200 nm

9 Simple cutting by a dicing machine Precision: 5-7 µm Making of msquid tips Deep reactive Ion etching and cutting by a dicing machine Precision: 1-2 µm

10 Force sensor: Tuning Fork Diapason = piezo-electric resonator made of quartz. Spring constant: k=25500 N.m -1 Mechanical excitation with a thickness mode piezo plate. SQUID silicium µsquid diapason L ~ 5 mm Piezo-electric plate for the mechanical excitation

11 Diapason characteristics Resonance: f 0 ª 25kHz and Q = at 4.2 K under vacuum. Harmonic oscillator. Power: ~1 pw: well suited for cryogenic use. amplitude d'oscillation (nm) phase ( ) fréquence (Hz)

12 Approach and retraction curves Distance < 100 nm: interaction tip-sample. Control of tip sample distance plaque piézo-électrique (excitation mécanique) ~ 5-10 distance < 100 nm échantillon T = 4 K fréquence de résonance (Hz) distance pointe-échantillon (nm)

13 Scheme of the SQUID-FM microscope

14 Dilution refrigerator Reversed Dilution refrigerator (Sionludi) (A. Benoit and M. Caussignac CRTBT-CNRS Commercialized by Air Liquide). 35 cm µsquid microscope 800 mk stage 4 K stage 20 K stage 80 K stage Heat exchanger He 4

15 µsquid-fm Microscope

16 Resolution of the SQUID-FM microscope Nb film 200 nm thick T=0.9 K, H=1G 28 µm Spatial resolution : 1-2 µm Scanning speed 5 µm/ s. magnetic resolution : F 0 / Hz C. Veauvy et al. Review of Scientific Instruments. 73, , Technoparade Marcoussis LPN: Contribution Veauvy, Dolocan, Mailly, Hasselbach déplacement Collab. CRTBT (µm) CNRS et LPN flux magnétique (en unité de F 0 dans la boucle du SQUID) 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 0,01 0,008 0,006 0,004 0,002 0 h=1 µm l=200 nm h=3 µm l=200 nm

17 Vortices in a perforated Al film: the sample 10 µm 10 µm T c = 1.23 K thickness=170 nm x(0) = 240 nm

18 Field cooled at the matching field Image (28µm x 28µm) T=0.4K H= 0.14 G. Each hole has captured one flux quantum.

19 Hole diameter decides on the pinning probability of vortices 28 µm T = 0.4 K, H = 0.7 H M T = 1.16 K, H = 0.7 H M field lines are first captured in bigger holes

20 Interstitial vortices, low field Images (28 x 28 µm 2 ) field cooled de 0.14 G T=0.4 K T=0.4 K Surface barrier, Pinning of the interstitial vortices, T=1.07 K Thermal activation and ordering.

21 Interstitial vortices, higher field 1.14 G 0.4 K 1.18 K

22 Hole diameter is an important parameter for the appearance of surface superconducivity H H c3 depends on the size (R) of the hole: R 0: H c3 H c2 H c3 =1.69 H c2 R : H c H c2 H c3 (1.5 µm) H c3 (1 µm) H=5.7 H M H c3 (0.5 µm) H c2 T = 1.2 K, H = 5.7 H M

23 Vortex lattice evolution as function of temperature 400 mk 1.18 K = 0.95T/Tc 1.2 K = 0.97T/Tc Applied field= 5.7 H M = 1.14 G Image size: 28*28 µm 2. x increases T close T c : unpinning of the vortices. Surface superconductivity H c2 <H<H c3 (H c3 =1.69 H c2 ) température (K) champs critiques (G) H c2 H c3

24 Conclusion SQUID-FM microscope: - Association of µsquid and piezo-electric force sensor. - Magnetic sensibility of F 0 / Hz for a µsquid of 1µm diameter. - Spatial resolution of 1-2 µm. First observation vortices and their pinning in Aluminum. - ordering of the vortices close to T c due to pinning array - role of hole diameter for the pinning efficiency. First direct observation of surface superconductivity. Acknowledgements: J. R. Kirtley, B. Pannetier, A. Benoit, JL Bret, A.Madouri, T. Fournier, T. Crozes. Support by DGA, CNRS, Région Rhône Alpes.

25 ongoing work Individual and collective Pinning, unconventional superconductors, nanocircuits and frustrated lattices. Spatial resolution: use of the etched µsquid tips. magnetic resolution: shunted µsquids. 30µm 60µm Sr 2 RuO 4 T=0.4 K H+0.15G 14/03 12h58 Réseau d anneau jonction pi/2pi W. Guichard, M. Aprili, et al.