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Why are flux avalanches deflected by a metallic layer?

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Why-are-flux-avalanches-deflected-by-a-metallic-layer7

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Sudden- avalanches- of- magnetic- flux- bursting- into- a- superconducting- sample- undergo- deflections- of- their-

trajectories-when- encountering- a- conductive- layer- deposited- on- top- of- the- superconductor:- RemarkablyH- in- some- cases- the- flux-

is-

totally-excluded-from-the-area-covered-by-the-conductive-layer:-We-present-a-simple-classical-model-that-accounts-for-this-

behaviour-and-considers-a-magnetic-monopole-approaching-a-semiCinfinite-conductive-plane:-This-model-suggests-that-electromagnetic-braking-is-an-important-mechanism-responsible-for-avalanche-deflection:

(xperimental4Physics4of4

Nanostructured4=aterials

)cknowledgments

Electromagnetic-braking

What-are-flux-avalanches7-Why-to-avoid-them7

èv4The-polarization-of-light-is-

----rotated-proportionally-to-the---local-magnetic-field4in4the4xi9Yij

4444mFaraday-effectSv4

0v47ight4is4reflected4by4the48l4mirror

4444and4send4back4to4the4objectivev

Tv4The4beam4crosses4an4analyzer

4444oriented4at4–L°4with4respect4to4the

4444polarizerv4

MOI-gives-real-time-images-of-the-magnetic-

field-at-the-surface-of-the-sample:-The4penetration4of4magnetic4flux4in4a4superconducting4film4depends4on4the4thermal4

and4magnetic4diffusivity4Y

T

4and4Y

=4

94

Smooth flux

penetration

DM-00-DT DT-00-DM

Flux avalanches

Flux-motion Local-T-rises Heat-dissipation 'ritical-current- and-pinning-decrease

Observe-avalanches2-MagnetoCoptical-imaging

[n4 avalanchesP4 flux4 can4 move4 at4 velocities4as4high4as4èLL4kmés4and4 the4 temperature4 can4 go4 well4 over4 the4critical4temperature4Tc4[è]v44

Flux-avalanches-destroy-superconductivity-and-are-therefore-

harmful-to-applications:-FortunatelyH-flux-avalanches-can-be-prevented-by-coating-the-superconductor-with-a-metallic-layer:

This4has4been4widely4exploited4in4

devicesP4with4the4following4benefits4[0]9

U4reduce4the4size4and4speed4of4flux

44jumps4melectromagnetic-brakingS

U4increase4the4thermal4conductivity

44mheat-removalS4

U4provide4protection4against4quenching

jbrisbois(ulg:ac:be

www.mate.ulg.ac.be

Jérémy-qrisbois-University-of-Liège

'ontact

)lux4avalanches4in4a40LL4nmUthick4Nb4film4 m04mm4x404mmS4at40vE4K4in4a4èL4Oe4 perpendicular4field4[ã]v4The4sample4was4 cooled4before4the4field4was4appliedv

[é]-Rv4jv4=ints4and48v47v4RakhmanovP4Rev. Mod. Phys.-[UP4EEè4mè–6èS

[B]4Rv4xv4°arrisonP4]v4Pv4Pendrys4and47v4Sv4WrightP4J. Low Temp. Phys.-é5P4èèT4mè–ãRS

[U]4=v4Rv4Koblischka4and4Rv4]v4WijngaardenP4Supercond. Sci. Technol.-5P4è––4mè––ES4

[è]4]v4Rv4ReitzP4J. Appl. Phys.-èéP40Líã4mè–ãLS

[[]-Wv4SaslowP4Am. J. Phys.-1JP4í–T4mè––èS

[1]47v4:v4Yavis4and4]v4Rv4ReitzP4J. Appl. Phys.-èBP4Rèè–4mè–ãèS

[]]4]v4xrisbois4et al.P4New J. Phys.-é1P4èLTLLT4m0LèRS

References

MagnetoCoptical-imaging-IMOID-is-a-technique-that-allows-in

situ-visualisation-of-the-magnetic-field-distribution-[U]:4[t4works4as4follows9

microscope objective linearly

In-first-approximationH-each-pixel-intensity-I

is-thus-proportional-to-the-local-magnetic-field-H2

V4is4the4Verdet4constantP4depending4on4the4

indicatorP4and4d4is4the4thickness4of4the4xi9Y[j4

layerv

(xample4of4a4setup4using4a4°e4closedUcycle4cryostat4 and4a4homeUmade4coil4to4apply4a4magnetic4fieldv Same4sample4as4above4under4the4same4 conditionsP4but4with4an4additional4ELL4nmU thick4:u4layer4[ã]v4The4avalanches4are4 completely4excluded4from4the4capped4areav

)valanches-deflection

Yeflection4of4avalanches4in4the4same4Nb4sampleP4where4the4 :u4triangle4has4been4replaced4by4a4ringv484èL4Oe4field4was4 applied4before4cooling4the4sampleP4and4was4then4set4to4zerov When4the4field4is4increased4to40L4OeP4 avalanches4eventually4start4to4penetrate4 the4covered4region4[ã]v4Some4avalanches4 change4their4trajectories4at4the4inferfacev4 The4studied4sample4has4a4critical4 temperature4Tc4of46vT4K4[ã]v4[t4is4patterned4 with4a4periodic4square4array4of4antidots4to4 ease4the4triggering4of4avalanchesv

(lectromagnetic4braking4comes4from4

eddy-currents4induced4in4the4metalv4

[t4can4be4explained4in4terms4of4MaxwellOs-images-method94currents4are4described4

by4receding4image4magnets4[RPE]v

8ssumptions9 U48valanche4front4w4single4vortex4mmagnetic-monopoleS4 U4:onstant4distance4zL4from4an4infinite4conducting4plate U4Overdamped4medium4mconstant-driving-forceS

)-lift-force-F

L

-and-a-drag-force-F

D

-

appear-from-the-interaction-between-the-magnet-and-the-eddy-currents2

The-drag-force-is-non-linear-and-non-monotonous-at-high-velocities:

The-avalanche-deflection-observed-in-

the-experiments-arises-from-the-

repulsive-force-appearing-at-the-metal-border-[1]:4This4force4

comes4from4the4

assymetry4in4the4eddy4

current4distributionv

The-trajectories-for-different-incident-

angles-θ-reproduce-the-two-regimes-

we-observed-in-the-experiments2-exclusion-and-deflection-[]]:

84magnet4appears4and4 induces4a4receding4image4 magnet4meddy4currents4decayS 84moving4magnet4leaves4a4 trail4of4receding4images4and4 interacts4with4themv

These4forces4are4also4valid4for4a4

magnetic4monopole4mC4is4differentSv

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