Quantitative magneto-optical imaging of S/F hybrid structures

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Quantitative magneto-optical

investigation of S/F hybrid structures

Jérémy Brisbois

Experimental Physics of Nanostructured Materials University of Liège, Belgium

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Collaborators

Prof. Alejandro Silhanek Dr. Gorky Shaw

Sylvain Blanco Alvarez Jonas Müller

Prof. Benoît Vanderheyden Prof. Philippe Vanderbemden

Dr. Roman Kramer Dr. Nora Dempsey Dr. Thibaut Devillers Prof. Klaus Hasselbach Jeroen Scheerder

Prof. Joris Van de Vondel

Lincoln Pinheiro Prof. Maycon Motta Prof. Wilson Ortiz

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Introduction

Why study S/F hybrids? Technique

Quantitative magneto-optical imaging Results

Superconducting film + magnetic material

Outline

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Perfect conductivity disappears as vortices move under

the action of a current.

F

= J×B

B J

F

When a current is applied, a force pushes the vortex:

E

The variation of B induces an electric field in the vortex core.

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K. Matsumoto, SUST 23, 14001 (2009)

• Random pinning: dislocations, grain boundaries, impurities…

In all cases, once the pinning potential is defined, it cannot

be changed.

Vortices can be anchored by defects inside the superconductor.

• Artificial pinning:

irradiation, implantation, nanofabrication…

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6 J eddy currents B V super-conductor conductor J B V magnetic material magnetization hysteresis

Vortex motion can be influenced through the stray field outside the superconductor.

 Unaltered superconductor

 Flexible damping

Affecting vortex motion by external structuring

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Why study S/F hybrid structures?

superconductor + diluted magnetic semiconductor

M. Berciu, Nature 435, 71 (2005)

1. Superconducting devices

Vortices can either control or be controlled through another

material in the vicinity of the superconductor.

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Why study S/F structures?

8 Smooth flux penetration Flux avalanches vortex motion

H increased dissipation in normal core

efficient heat removal T raises locally

2. Vortex damping - avoid flux avalanches

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Avalanches are harmful to superconductivity and practical

applications.

→ observe, control and avoid flux avalanches.

The system undergoes a dramatic transition to a state of lower energy.

T might largely rise over T_c, thus threatening superconductivity. v_avalanches ~ 100 km/s

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Introduction

Why study S/F hybrids?

Technique

Quantitative magneto-optical imaging

Results

Outline

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GGG transparent substrate (500 µm) Bi:YIG Faraday active layer (3 µm)

Al mirror (100 nm)

sample

sample holder

Faraday effect

Rotation of the linear polarization of light:

𝛼 = 𝑉𝑑𝐻

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Magneto-optical imaging (MOI) setup

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Examples of magneto-optical images

Magnetic Co bars Pb bulk FeSe

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Conversion from I to B

Conversion from I to B: compare the intensity with the

calibration curve at every pixel.

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Conversion from I to B

I (a.u.) 100 µm 1471 578 B (mT) 0 8.7 20 µm I B

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Introduction

Why study S/F hybrids?

Technique

Quantitative magneto-optical imaging

Results

Outline

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Nb film + Co magnetic disk

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Nb film + NdFeB ferromagnetic layer

G. Shaw et al., RSI 89, 23705 (2018).

B_z (mT)

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The direction of magnetization is easily controlled with in-plane fields ~ 1 mT.

J. Brisbois et al., Sci. Rep. 6, 27159 (2016)

Nb film + permalloy (Py) layer

B_z (mT) 0.4 -0.4 0 permalloy (NiFe) 500 µm Py (50 nm) M

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Idea: use a magnetic layer to record the vortex trajectories.

Imprinting magnetic fields

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Printings are stable, even up to room temperature!

Imprinting flux avalanches

J. Brisbois et al., Sci. Rep. 6, 27159 (2016)

Before, T = 10 K µ₀H = 0 mT T = 4 K µ₀H = 4.8 mT After, T = 10 K µ₀H = 0 mT M 500 µm

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Imprinting works also at room temperature → tune the magnetic landscape at will

Thermomagnetic pattern

Imprinting

Room temperature imprinting

500 µm

Py (460 nm)

T = 6 K, µ₀H = 4.8 mT

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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

Conclusion and perspectives

 Tune the magnetic landscape at will to guide flux/avalanches  Contributions to magnetic damping: hysteresis, magnons…

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𝐄

𝐁

𝐯

𝐅

_L r 𝜌

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0

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