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VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS Margriet J. Van Bael

VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS Margriet J. Van Bael Martin Lange, Victor V. Moshchalkov Laboratorium voor Vaste-Stoffysica en Magnetisme, K.U.Leuven, Belgium A.N. Grigorenko, Simon J. Bending Department of Physics, University of Bath, United Kingdom. 1.

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VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS Margriet J. Van Bael

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  1. VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS Margriet J. Van Bael Martin Lange, Victor V. Moshchalkov Laboratorium voor Vaste-Stoffysica en Magnetisme, K.U.Leuven, Belgium A.N. Grigorenko, Simon J. Bending Department of Physics, University of Bath, United Kingdom 1

  2. Pb(500Å) film with a square antidot lattice f0 Strong enhancement of critical current ‘matching’ effects H1 Artificial pinning arrays: matching effects M. Baert et al. PRL 74 (1995), V.V. Moshchalkov et al. PRB 54 (1996), PRB 57 (1998)

  3. Influence of magnetic stray field on pinning efficiency ? Influence of magnetic moment on pinning efficiency Field-induced superconductivity ? m MAGNETIC PINNING CENTRES Hybrid ferromagnetic/superconducting system Array of magnetic dots covered with superconducting film • Co dots with in-plane magnetization • Co/Pt dots with out-of-plane magnetization

  4. thickness: 380 Å Au Co (polycrystalline) SiO2 d Not magnetized Multi domain Enhance stray field Magnetized Single domain Square array of Co dipoles 0.36µm 0.54µm Preparation: e-beam lithography + molecular beam deposition + Lift-off 1.5µm AFM & MFM @ H=0, RT M.J. Van Bael et al. PRB 59, 14674 (1999)

  5. M.J. Van Bael et al. PRB 59, 14674 (1999) f0 2 H1 = = 10.6 Oe 15 3 (1.5 mm)2 ) 2 A/m 10 7 (10 c j 5 single single single - - - domain domain domain dot  multi multi multi - - - domain domain domain flux line ¡ - 2 - 1 0 1 2 H/H1 structural + magnetic contributions • Magnetic dots create strong pinning potential • Clear matching effects close to Tc • Better pinning for single domain dots L. Van Look et al. Physica C 332 (2000) Triangular array of Co dots • Electrical transport measurements T/Tc = 0.985 H/H1 = 2 honeycomb latticeonly stable for strong pinning (Reichhardt et al.PRB 57, 1998)

  6. M.J. Van Bael et al. PRB 59 (1999) single domain multi domain no dots Flux lines pinned at Co dots Single domain -> better pinning ‘Tunable pinning’ Array of Co dipoles BUT … WHAT HAPPENS LOCALLY ?? Position of vortex on dipole ?? Fluxoid quantization Superconductoranddipolearenot independent

  7. 10 mm Au STM tip Scanning Hall probe microscopy (SHPM) @ University of Bath • 2DEG material for better sensitivity (2 µV/G) • Active area: 2µm × 2µm 0.25 µm × 0.25 µm • Spatial resolution < 1 µm • Typical sensor-surface distance: ~ 200-300 nm probe and picture in collaboration with imec

  8. Visualization of vortex lattice in magnetic dot array Pb-film on square array of single domain Codots T = 6K << Tc • Subtract dipole contribution: T = 6K, H = H1 T = 7.5 K, H = H1 - = [dipoles + flux lines] - dipoles (T > Tc) = flux lines square vortex lattice • Ordered vortex patterns at integer and fractional matching fields: H/H1 = 1/2, 1, 3/2, 2, …

  9. field profile contrast Tc = 7.16 K SHPM image at H = 0 S N field contrast (G) f0 Fluxoid quantization effects: field contrast in zero field SHPM image at H = 0 ‘Vortex–antivortex’ pair induced M.J. Van Bael et al. PRL 86, 155 (2001)

  10. vortices T > Tc T < Tc vortices T > Tc T < Tc f0 f0 - f0 + ½H1 + ½H1 Attraction and annihilation of negative vortex and positive fluxoid In applied field: position of vortex on dipole ? - ½H1 Field polarity dependent pinning Confirmed by theoretical model (Milosevic et al. PRB 69 (2004)) M.J. Van Bael et al. PRL 86, 155 (2001)

  11. Co/Pt (111) SiO2 MFM magnetized H> 0 MFM magnetized H< 0 MFM demagnetized 0.4m 1m single-domain all down single-domain random up - down single-domain all up Array of Co/Pt dots with out-of-plane magnetization Preparation 270 Å e-beam lithography + molecular beam deposition + lift-off AFM

  12. strong pinning weak pinning weak pinning strong pinning parallel parallel antiparallel antiparallel Co/Pt dots as artificial pinning centers m < 0 m > 0 M.J. Van Bael et al. PRB 68, 014509 (2003)

  13. total current: screening current js vortex current jv Line energy vortex (~f2) stray field outside SC (dot + vortex) magnetic moment in vortex field -m.bz Stray field of dot is screened below Tc  js Attractive interaction when field and moment are parallel Strong on-site pinning Attractive interaction when field and moment are parallel Strong on-site pinning bz js jv bz Repulsive interaction when field and moment are antiparallel Weak interstitial pinning jv Interaction between vortex and magnetic dot Einteraction = Ekinetic + Efield + Emoment m dot vortex M.J. Van Bael et al. PRB 68, 014509 (2003)

  14. Vortices between dots Vortices pinned by dots Asymmetric pinning in magnetized Co/Pt dot array Dots magnetized in negative direction T = 6.8 K H = -1.6 Oe <0 T = 6.8 K H = 1.6 Oe >0 Vortex-dot interaction: attractive for parallel alignment repulsive for anti-parallel alignment M.J. Van Bael et al. PRB 68, 014509 (2003)

  15. Schematic sample cross-section Case of larger dots What if the dots induce flux quanta ? larger dots Co/Pd Diameter 0.8 µm Period 1.5 µm

  16. Dots magnetized down Dots magnetized up Pb Pb T = 7.10K T = 7.15K T = 7.18K T = 7.10K T = 7.15K T = 7.18K m < 0 m > 0 • Pinning is strongly field-polarity dependent • Maximum critical current shifted to non-zero field cfr. M.V. Milosevic and F.M. Peeters, PRL 93, 267006 (2004) Magnetized state: Critical current

  17. m = 0 N mz < 0 S m = 0 m = 0 m = 0 N N mz > 0 S mz > 0 mz < 0 S mz < 0 N H-T phase diagram • For magnetized dots • Phase diagram asymmetric • Shift of maximum Tc • Superconductivity induced by magnetic field (~ 2 mT) Magnetoresistivity M. Lange et al. PRL 90, 197006 (2003)

  18. Applied field H = 0 Applied field H = 2H1 Stray field of dots destroys superconductivity between and below dots ~2f0 per unit cell Between the dots, the stray field compensates the applied field (2H1= 1.84 mT) and superconductivity emerges Cond-mat/0209101 Field compensation effects M. Lange et al. PRL 90, 197006 (2003)

  19. CONCLUSION • Artificial pinning arrays Very efficient pinning Induce particular geometry of vortex lattice • Magnetic pinning centers • Magnetism provides extra parameter • Fundamental interaction between pinning center and flux line ? • Domain state and stray field important • Field polarity dependent pinning • Magnetic dots can create vortex-antivortex pairs • Field compensation effects and field-induced superconductivity

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