Tajima - DC and RF Measurements of Thin Film MgB2

Operated by Los Alamos National Security, LLC for NNSA

U N C L A S S I F I E D

1

DC and RF Measurements of Thin

Film MgB2

Tsuyoshi Tajima

The 4th International Workshop on: Thin Films and New

Ideas for Pushing the Limits of RF Superconductivity,

Padua, Italy,

4-6 October 2010

LA-UR-10-06587



Major contributors

Sample preparations

• Brian Moeckly (STI) and Toshiya Doi (Kagoshima Univ.)

DC measurements

• Nestor Haberkorn and Leonardo Civale (LANL)

RF measurements• Jiquan Guo and Sami Tantawi (SLAC National Accelerator

Laboratory)

Other contributors are listed at the end of this

presentation

2



Outline

Introduction

DC measurements and results

RF measurements and results

Issues and concerns

Summary and future plans

3



Outline

Introduction



Issues and concerns


4



How can we increase the Eacc to >50 MV/m, a limit associated

with the critical magnetic field of ~200 mT (2000 Oe)?

Some ideas include:

• Improving the cell design to decrease Bpeak relative to Eacc.

— This can improve the Eacc by ~10 %, but the shape might not be

appropriate for surface treatment and it is mechanically weaker than

the standard shape

• Use traveling mode instead of using standing wave mode (~42%

increase possible? FNAL is working on this.)

• Coating some thin layers of another superconductor that has

higher Hc1 and Tc than Nb (suggested by Alex Gurevich in

2005). This could increase achievable Eacc significantly

5



The key idea of using a thin film superconductor is the

fact that Bc1// increases when the thickness (d) is d< l

(magnetic penetration depth)

The RF critical magnetic field HRF in

a type-II superconductor is

somewhere between Hc1 and Hc2

The higher the Hc1//, the better to

prevent vortex penetration

Use thin films with thickness d < lL

to enhance the lower critical field

Hc, RF

[Gurevich, APL 88 (2006) 012511]

Predicted Hc1 as a function of film thickness

60 80 100 120 140200

300

400

500

600

700

800

900

Film Thickness nm

Bc1

mT

Assumptions: MgB2

Coherence length 5 nm

Penetration depth 140 nm

6



An example: Coating 105 nm MgB2 layer could sustain

355 mT, corresponding to ~100 MV/m with Bpeak /Eacc ~

3.6 mT/(MV/m) if Nb layer can sustain 170 mT

Simple single-layer example

Assumptions

Bc1(Nb) = 170 mT

λL(MgB2) = 140 nm

ξ(MgB2) = 5 nm

Hc1(MgB2) = 355 mT

d = 105 nm

The film thickness needs to be determined

so that the decayed field at the Nb surface

is below the RF critical field of Nb (~200

mT).

H0 = 355mT

Hi = 170mT

d = 105 nm

NbMgB2

Eacc ~ 100 MV/m

Dielectric

material

7



Outline

Introduction



Issues and concerns


8



Samples prepared with 2 methods have been measured

so far

9

B.H. Moeckly and W.S. Ruby, Supercond. Sci.

Technol. 19 (2006) L21–L24

Reactive co-evaporation at

Superconductor Technologies, Inc.

(STI), Santa Barbara, CA, U.S.A

At ~550 C

Co-evaporation with 2 E-beam guns

at Kagoshima Univ., Japan

At ~250 C



10

A Quantum Design Magnetic Property Measurement

System (MPMS) SQUID is used to measure Tc and Hc1

D

V

dH

dm

1*4

The Meissner slopes are different for

each orientation because each one

has a different demagnetization factor

In the Meissner State

(total expulsion of magnetic flux):

m: total magnetic moment

H: magnetic field

V: volume of the sample

D: demagnetization factor

In a superconducting thin

films:

If

then 1 – D ~ 0

And if

then 1 – D ~ 1

H

H



11

Magnetic field and magnetization alignment in thin films

VH

m IIII

4;

14 D

VHm

HaplHII

Hm

mII

)sin(cos4

2

112

Dz

VHm

If the film is exactly aligned with H (Q=0) the

Meissner slope dm/dH gives the film volume.

A calculated volume larger than the real one

indicates misalignment.

Alignment 0.1 or better is needed.



An example of Hc1 measurements at different temperatures

12

Magnetization curves as a function of applied magnetic field at various

temperatures for ~360 nm thick MgB2 film (Tc ~ 31.8 K) deposited on a

Si substrate at Kagoshima Univ. , Japan.

In this presentation, Hc1 is

defined as the H where the data

starts to deviate from the

Meissner slope, indicating vortex

entrance into the film.

This may be an overestimate of

Hc1 due to surface barriers, but it

is the relevant field for cavities.

Meissner slope



It has been found that the MgB2 thin films show Hc1

significantly higher than bulk Nb (~300 Oe) even d>l

13

0 5 10 15 20 25 30 35 400

500

1000

1500

2000 MgB2 STI

MgB2 Kagoshima #1

MgB2 Kagoshima #2

Nb rod

Hc1 [

Oe

]

T [K]

~ 300 nm

~360 nm

~400 nm

At 4.5 K, ~ 300

nm MgB2

showed Hc1 >

2000 Oe



14

Vortex entrance in MgB2 films from STI

0 5 10 15 20 25 30 35 400

500

1000

1500

2000

2500

H*

Hderiv

Hc2

H [O

e]

T [K]

H//surface (misalignment < 0.05)

thickness 300nm

H*: clearly above

vortex entrance

Hderiv: kink in

derivative indicates

sudden entrance of

large # of vortices.

0 500 1000 1500 2000 2500-200

-150

-100

-50

0

m

[e

mu

]

H [Oe]

H*

T=30K

0 500 1000 1500 2000 2500-0.2

-0.1

0.0

0.1 T=30K

dm

/dH

[e

mu

/Oe

]

H [Oe]

Hderiv



15

0 5 10 15 20 25 30 35 400

500

1000

1500

2000

H

[Oe

]

T [K]

Thickness ~ 300nm

Thickness ~ 500nm

Comparison of Hder with two MgB2 films of different thickness:

Vortex entrance in the thinner film occurs at a higher H

We are writing a paper

on these results with

more detailed analysis.

These results are

significantly different from

the curve obtained from

two-fluid model. It seems

that the thickness effect

starts even d>l.

STI films coated on sapphire



Outline

Introduction



Issues and concerns


16



RF measurements of 2-inch (~5 cm) diameter wafers (~1

mm thick) have been carried out at SLAC National

Accelerator Laboratory using 11.4 GHz system

Hemi-spherical TE013–

mode cavity with magnetic

fields in parallel with the

sample surface

Sample: <1.5 mm thick

Cold headTemperature sensor

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Typical distribution of

superconducting and normal-

conducting regions after quench

Radial H profile



0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

0 20 40 60 80 100 120

Q0

Sample temperature (K)

#1: MgB2(100nm)/B(10nm)/Nb#2: MgB2(1000nm)/Nb#3: MgB2(200nm)/B(200nm)/Nb#4: MgB2(300nm)/SapphireNb (single crystal RRR~300)Copper

#4: MgB2(300nm)/Sapphire limited by the Q0 of copper dome

18

1

/Rs, L

ow

er

su

rfa

ce

re

sis

tan

ce

A summary of low-power tests of various samples:

So far, only STI has been able to produce 2-inch

samples, thus all the data herein are on STI samples.

Warning: some data are misleading since the effects of ambient magnetic field and cracks are also included.



MgB2(300nm) showed a clear quench at 25 mT (250 Oe)

at 3 K, which is significantly lower than the Hc1

measured by DC measurements (> 200 mT)

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

0 5 10 15 20 25 30

Q0

Bpeak (mT)

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Outline

Introduction



Issues and concerns


20



Slide 21

Planned

Actual

Bulk Nb

B

MgB2 200 nm

200 nm

B

MgO

Nb

Mg

Note the increase of O and Mg

Problem: B\MgB2 or Al2O3\MgB2 layers coated on bulk Nb at STI

at 550 C caused inter-diffusion of Mg and O such as shown

here. See more details at Roland Schulze’s talk.

Auger depth profile

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Clues to solve the inter-diffusion problem have been

obtained

Recently, MgB2(200nm)/Alumina(300nm)/Nb showed a

result comparable with MgB2(300nm)/Sapphire, i.e.,

Alumina coating can prevent the inter-diffusion with

low RF losses. An optimization of necessary Alumina

layer thickness needs to be done.

UHV baking of Nb substrates at 800 C for 4 hours

have been tried and it seems to help reduce the

oxygen diffusing from the bulk Nb and clean the

surface according to the XPS surface analysis. See

the detail at Roland Schulze’s talk.

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Outline

Introduction



Issues and concerns


23




DC measurements have shown that Hc1 of MgB2 thin films is

higher than bulk Nb even d>l. (>200 mT at 4.5 K)

RF measurements at 11.4 GHz, however, have shown that the

Hc, RF is low (~25 mT at 3 K)

We need to identify the source of this discrepancy

We plan to do DC and RF measurements of Hc1 // with thinner and

multi-layer films. Recently, three 2-inch samples of 4 layers of MgB2

(50nm)/Al2O3(10nm) have been prepared with the help from ANL for

ALD Al2O3 (Thomas Proslier, Mike Pellin)

We will need to develop a technique to coat cavity inner surfaces. The

good news is, now that we know Hc1 of even ~300 nm thick film could

exceed bulk Nb performance, it might not be as difficult as we thought

to realize the concept of increasing the sustainable magnetic field on

the cavity surface.

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Thanks for your attention!

Many thanks to the Defense Threat

Reduction Agency (DTRA) for the funding

and a number of people listed in the

following slides.

25



Acknowledgments

LANL

• AOT-MDE: A. Canabal (now at U. Maine), G. Eremeev (now at

JLab), et al.

• MPA-STC: R. DePaula, A. Apodaca

• MST-6: R. Schulze, A. Zocco, R. Edwards

• MST-7: B. Day

• MST-8: M. Hawley, P. Hosemann, V. Livescu

• P-DO: P. Turchi

• P-25: S. Greene, C. Morris

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Acknowledgments (cont.)

Other National Labs

• SLAC

— V. Dolgashev, D. Martin, C. Yoneda

• JLab

— G. Eremeev

• ORNL

— I. Campisi

• ANL

— T. Proslier, M. Pellin

Industry

• Cabot Microelectronics Polishing Company (CBPC)

— S. Lesiak

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Acknowledgments (cont.)

Collaborators in Japan

• National Institute for Materials Science (NIMS)

— Superconducting Materials Center: A. Matsumoto, H. Kumakura, M.

Tachiki,

— Advanced Electronic Materials Center: H. Abe

— Nanotechnology Innovation Center: E. Watanabe

• Kagoshima Univ.

— T. Nishikawa, T. Nagamine, K. Yoshihara

• KEK

— H. Inoue

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Technology

Tajima - DC and RF Measurements of Thin Film MgB2