Spencer Rodgers1, Nathan Satchell2, Brian Kirby1, Norman Birge2, Julie Borchers1, and Patrick 

Quarterman1

1NIST Center for Neutron Research

2Michigan State University Department of Physics

August 6th, 2019

Probing Anomalous Field-Expulsion in 

Superconductor/Ferromagnetic Thin Films

1
Outline

2

1. Background

• What is a Ferromagnet?

• What is a Superconductor?

• S/F Heterostructure 

Proximity Effects

• Introduction to 

Reflectometry

2. Experimental Methods

• Polarized Neutron Beam

• Closed Cycle Cryostat

3. Results

• Collected X-Ray/PNR Data

• Finding Structures of 

Samples 

4. Final Thoughts

• What We Learned

• If I had More Time
What is a Ferromagnet?

• Has a net dipole 

moment from 

electron spins

https://www.askiitians.com/iit-jee-magnetism/magnetic-field/

• The material is 

magnetized, 

allowing it to create 

a magnetic field

• Magnetic domains 

align in the presence 

of external magnetic 

field

Ferromagnets are subject to 

intense study once again, 

due to their uses in nonvolatile memory storage 

devices. i.e. MRAM

3

Ferromagnet

Ferromagnet

Insulator/Tunnel 

Barrier

This low 

resistance 

reads as a 0

This high 

resistance 

reads as a 1

External Field
What is a Superconductor?

• The material must be cooled 

to a critical superconducting 

temperature

• Electrons pair up into 

Cooper pairs creating the 

superconducting state

• Lets the material have 

zero electrical 

resistance and expels 

external magnetic field

https://www.globalspec.com/learnmore/materials_chemicals_adhesives/electrical_optical_specialty_materials/superconductors_superconducting_materials

https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/23%3A_The_Transition_Elements/23.9%3A_High_Temperature_Superconductors https://www.shutterstock.com/search/thermometer+cold 4
Proximity Effects in S/F Thin Films

https://www.intechopen.com/books/superconductors-new-developments/spintronics-driven-by-superconducting-proximity-effect

Superconducting Proximity Effect

• Superconductivity leaks into the 

normal metal (N) and ferromagnet (F)

• Superconducting hetero-structures 

make Josephson junctions

5

Superconductor I/N/F Superconductor
Proximity Effects in S/F Thin Films

https://phys.org/news/2018-07-underlying-mechanism-magnetic-effect-superconducting.html 6

• Started by calculating the 

superconducting proximity effect

• Previously unexplained experimental 

results occur

• Theorists predict the electromagnetic 

proximity effect 
0.00 0.01 0.02 0.03 0.04 0.05

Reflectivity (arb. units)

Q (Å-1)

Reflectometry Explained

Substrate

Thin Film

Q

θ θ

Incident Beam Reflected Beams

Material 1

Material 2

https://alunosonline.uol.com.br/fisica/spin.html

• Gives us the depth profile of the thin film

• Constructive (peaks) 2sin(θ) = nλ

• Destructive (valleys) 2sin(θ) = (n+1

2

)λ

• PNR can directly detect magnetization

7

θ

θ

Water

Oil

Light Scattered Light

X-ray/Neutron Source Detector
0.00 0.01 0.02 0.03 0.04 0.05

Reflectivity (arb. units)

Q (Å-1)

Reading Reflectivity Data

8

• Scattering Length Density 

• Critical edge length

• Height of peaks

• Often abbreviated SLD

• Roughness

• Fall off rate

• Usable data before background

• Thickness

• Space between peaks

• 2π/d

• Magnetic Component

0.00 0.01 0.02 0.03 0.04

Reflectivity (arb. units)

Q (Å-1)

Theory ++

Theory --
Nuclear/Magnetic Depth Profile Examples

• Nuclear SLD

• Magnetization

A

C

B

Magnetization Direction

none

London Penetration Equation 

D

9

SLD SLD

Z Z

BB xx = BB0cosh � xx

λλ − ddss2λλ cosh

ddss

2λλ
PBR, a Polarized Neutron Beam 

10 http://inspirehep.net/record/1113311/plots

• Neutrons

• Spin Polarized Mirror

• Spin Flippers

• Electromagnet (0.7 T)

• Guide Fields

• Detector

• Closed Cycle Cryostat
0.05 0.10 0.15 0.20

Reflectivity (arb. units)

Q (Å-1)

R

 Theory

0.05 0.10 0.15 0.20 0.25

Reflectivity (arb. units)

Q (Å-1)

R

 Theory

From Data to Knowing Structure

11

Ni(2.8)/Nb(200) Ni(2.8)/Nb(5)/AlO(2.5)/Nb(200)

• Thin layer oscillations are prevalent

• Roughness smears out Nb oscillations 

• Compact Nb oscillations 

• Thin Ni layer smeared out by roughness

0 500 1000 1500 2000

0

10

20

30

40

50

60

 Nuclear SLD

SLD (1e-6/Å2

)

Z (Å)

0.10 0.12 0.14 0.16 0.18 0.20 0.22
0.1 0.2 0.3

Reflectivity (arb. units)

Q (Å-1)

R

 Theory

From Data to Knowing Structure

12

PdFe(7)/Nb(200) PdFe(7)/Nb(5)/AlO(2.5)/Nb(200)

• Clearly see compact Nb oscillations on 

the long PdFe oscillation

• Can really see all the different lengths 

of oscillations from each layer

0.08 0.09 0.10 0.11 0.12 0.13 0.14

0.05 0.10 0.15 0.20

Reflectivity (arb. units)

Q (Å-1)

 R

 Theory

0.08 0.10 0.12 0.14
200 nm Nb at 3 K, PNR Data

13

Depth Profile

• Chi2 = 1.44

• Exp. λ = 1009 ±50 Å

0 500 1000 1500 2000 • Theor. λ = 820 Å

0

2

4

 Nuclear SLD

Magnetic SLD

Z (Å)

Nuclear SLD (1e-6/Å

2

)

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

Magnetic SLD (1e-6/Å

2

)

0.01 0.02 0.03 0.04

Reflectivity (arb. units)

Q (Å-1)

R++

R--

Theory ++

Theory --

0.020 0.022 0.024 0.026 0.028

BB xx = BB0cosh � xx

λλ − ddss

2λλ cosh

ddss

2λλ
200 nm Nb at 3 K, PNR Data

14

Spin-Asymmetry

• What is Spin Asymmetry?

• Notice the oscillations of 

superconducting Nb’s SA

• Another way PNR allows 

us to view magnetic 

components of materials 

0.01 0.02 0.03 0.04

-0.10

-0.05

0.00

0.05

Spin Asymmetry (arb. units)

Q (Å-1)

SA

Theory SA

SSSS =

(↑↑−↓↓)

(↑↑+↓↓)
0.00 0.01 0.02 0.03 0.04 0.05

-0.1

0.0

0.1

0.2

0.3

0.4

Spin Asymmetry (arb. units)

Q (Å-1)

Ni/AlOx/Nb 3 K

Ni/Nb 3 K

Ni/AlOx/Nb 12 K

On-Going PNR Data Collected

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Ni/Nb/AlO/Nb at 3 K and 1.5 kOe

0.012 0.014 0.016 0.018

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

• We see a distinct and separate feature 

from just the Meissner effect

• The ferromagnetic Ni causes the 

two oscillations to split

0.01 0.02 0.03 0.04

Reflectivity (arb. units)

Q (Å-1)

R++

R--

0.028 0.030 0.032 0.034 0.036 0.038
What We Learned From Our Experiments

• Details of samples

• X-ray data showed Si-wafers are extremely rough

• Determined the structures of S/F samples

• Understanding important data

• Found Nb’s London penetration depth

• Captured abnormal feature in SA plot

• Continued analysis required

• Attempt to describe abnormal feature 

• Analysis of other sample structures required 

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If I was a SURF student for...

• Another month

• Finish collecting x-ray data

• Fit structures of each sample

• Another year

• Fit structures of PNR data with the help of x-ray fits

• Continue analysis of PNR fits to investigate electromagnetic proximity effect

• Life

• Consider other field geometries to study

• Test non-colinear magnetization in adjacent materials

• Publish paper on findings

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Thank You!

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• Acknowledgments: Patrick Quarterman

Alex Grutter

Julie Borchers

Joe Dura

NCNR Softball Team 
Hysteresis Curves M

H

1

9

• Easy Axis

• Hard Axis

• Magnetization Saturation

• Coercivity

• Remnant Field

M = Magnetization

H = Applied External 

Magnetic Field

Ex.

In plane 

magnetization

Out of plane 

magnetization

Ms

Ms

Mr

Mr
How PNR Directly Detects Magnitization

https://alunosonline.uol.com.br/fisica/spin.html

H

M

Q

• In plane 

magnetization is 

parallel to the spins 

of the neutrons 

causes them to flip

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Why We Do PNR and XRR

Neutrons X-Rays

• X-Ray SLD https://ncnr.nist.gov/instruments/magik/Periodic.html

• Neutron SLD

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Neutrons Advantage

• X-Ray https://ncnr.nist.gov/instruments/magik/Periodic.html

• Neutron SLD • Example: a layer of Niobium on Nickel

22
X-Ray Advantage

https://ncnr.nist.gov/instruments/magik/Periodic.html

• Neutron SLD • Example: a layer of Niobium on Palladium 

• X-Ray SLD 23
Bruker XRR

24

X-Ray Source

Detector 

Adjustable Stage
Closed Cycle Cryostat

https://www.arscryo.com/cryocooler-principles-of-operation 25
(Back-up Fits for Possible Questions)

26

0.01 0.02 0.03 0.04

Reflectivity (arb. units)

Q (Å-1)

R++

R--

Theory ++

Theory --