Quantitative magneto-optical investigation of S/F hybrid structures

Transcription

1 Quantitative magneto-optical investigation of S/F hybrid structures Jérémy Brisbois Experimental Physics of Nanostructured Materials University of Liège, Belgium

2 Collaborators Prof. Alejandro Silhanek Dr. Gorky Shaw Sylvain Blanco Alvarez Jonas Müller Prof. Benoît Vanderheyden Prof. Philippe Vanderbemden Jeroen Scheerder Prof. Joris Van de Vondel Lincoln Pinheiro Prof. Maycon Motta Prof. Wilson Ortiz Dr. Roman Kramer Dr. Nora Dempsey Dr. Thibaut Devillers Prof. Klaus Hasselbach 2

3 Outline Quantitative magneto-optical investigation of S/F hybrid structures Introduction Why study S/F hybrids? Technique Quantitative magneto-optical imaging Results Superconducting film + magnetic material 3

4 Superconducting flux vortices J E B F When a current is applied, a force pushes the vortex: F = J B The variation of B induces an electric field in the vortex core. Perfect conductivity disappears as vortices move under the action of a current. 4

5 Pinning vortices by defects Vortices can be anchored by defects inside the superconductor. Random pinning: dislocations, grain boundaries, impurities Artificial pinning: irradiation, implantation, nanofabrication In all cases, once the pinning potential is defined, it cannot be changed. K. Matsumoto, SUST 23, (2009) 5

6 Affecting vortex motion by external structuring Vortex motion can be influenced through the stray field outside the superconductor. B superconductor B V J V J eddy currents conductor magnetization hysteresis magnetic material Unaltered superconductor Flexible damping 6 Baker and Rojo, Phys. Rev. B 64, (2001) A. Palau et al., Phys. Rev. Lett. 98, (2007).

7 Why study S/F hybrid structures? 1. Superconducting devices superconductor + diluted magnetic semiconductor Vortices can either control or be controlled through another material in the vicinity of the superconductor. 7 M. Berciu, Nature 435, 71 (2005)

8 Why study S/F structures? 2. Vortex damping - avoid flux avalanches H increased vortex motion dissipation in normal core T raises locally Flux avalanches efficient heat removal Smooth flux penetration 500 µm 500 µm 8

9 The system undergoes a dramatic transition to a state of lower energy. v avalanches ~ 100 km/s T might largely rise over T c, thus threatening superconductivity. Avalanches are harmful to superconductivity and practical applications. observe, control and avoid flux avalanches. 9

10 Outline Quantitative magneto-optical investigation of S/F hybrid structures Introduction Why study S/F hybrids? Technique Quantitative magneto-optical imaging Results Superconducting film + magnetic material 10

11 Faraday effect Rotation of the linear polarization of light: α = VdH Typical values: α ~ 1 for B ~ 3 mt GGG transparent substrate (500 µm) Bi:YIG Faraday active layer (3 µm) Al mirror (100 nm) sample sample holder 11

12 Magneto-optical imaging (MOI) setup 12

13 Examples of magneto-optical images Magnetic Co bars Pb bulk FeSe LCCO Nb + Cu Nb + Py 13

14 Conversion from I to B Conversion from I to B: compare the intensity with the calibration curve at every pixel. M. Roussel, Ph.D. thesis, Univ. of Wollongong (2007). G. Shaw et al., RSI 89, (2018). 14

15 Conversion from I to B I (a.u.) 100 µm B (mt) I B 20 µm 15 G. Shaw et al., RSI 89, (2018).

16 Outline Quantitative magneto-optical investigation of S/F hybrid structures Introduction Why study S/F hybrids? Technique Quantitative magneto-optical imaging Results Superconducting film + magnetic material 16

17 Nb film + Co magnetic disk 17 G. Shaw et al., RSI 89, (2018).

18 Nb film + NdFeB ferromagnetic layer 200 µm B z (mt) 500 µm 18 G. Shaw et al., RSI 89, (2018).

19 J. Brisbois et al., Sci. Rep. 6, (2016) Nb film + permalloy (Py) layer permalloy (NiFe) Py (50 nm) B z (mt) 0.4 M µm -0.4 The direction of magnetization is easily controlled with in-plane fields ~ 1 mt. 19

20 Guiding vortex motion 20

21 Imprinting magnetic fields Idea: use a magnetic layer to record the vortex trajectories. magnet + iron fillings vortex + magnetic layer 21

22 J. Brisbois et al., Sci. Rep. 6, (2016) Imprinting flux avalanches Before, T = 10 K µ 0 H = 0 mt 500 µm T = 4 K µ 0 H = 4.8 mt After, T = 10 K µ 0 H = 0 mt M Printings are stable, even up to room temperature! 22

23 Room temperature imprinting Thermomagnetic pattern Py (460 nm) 500 µm T = 6 K, µ 0 H = 4.8 mt Imprinting Imprinting works also at room temperature tune the magnetic landscape at will 23 G. Shaw et al., RSI 89, (2018).

24 Conclusion and perspectives Quantitative MOI allows to isolate the magnetic field of a superconductor, even when it is buried in a stronger magnet field. Magnetic flux can be guided and imprinted in a magnetic layer. Tunable pinning landscapes can be obtained by imprinting a magnetic template in a Py layer. Perspectives: Improve the magnetic recording Tune the magnetic landscape at will to guide flux/avalanches Contributions to magnetic damping: hysteresis, magnons 24

25 Thank you for your attention!

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