Yutao Xing

IF-UFF

Interaction betweeenSuperconductivity and Ferromagnetism

in SC/FM nanocomposites

2

Supercondutor e ferromagneto

3

Supercondutor e ferromagneto

http://www.google.com.br/url?sa=i&source=images&cd=&cad=rja&docid=GuuZMzIfMGKApM&tbnid=1yT1qADFMmkRIM:&ved=&url=http://www.9191pk.com/index.php?r=site/view&id=50d25b0cb9c66&ei=N1GlUYGHK4HM0wGFzIHACg&psig=AFQjCNHDM0L4rhI8JhKuImEQAooIc9DGzQ&ust=1369875127740895

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Kammerling Onnes (em 1911)‏

Res

istê

nci

a (Ω

)Temperatura (K)

Supercondutividade

Superconductor

CoolApplyfield

• B = 0 perfect diamagnetism: cM = -1

• Field expulsion unexpected; not

discovered for 20 years.

HHM

MHB

c

0)(0

B/0

H

-M

HHc Hc

Meissner Effect

Tc

Bc

Ic

Critical Parameters for SC

• London Theory magnetic penetration depth l

• Ginzburg-Landau Theory coherence length x

l x two kinds of superconductors!

Type I and type II SC

t

EJB

t

BE

B

E

0

0

Conductors in a Magnetic Field

Comportamento diferentes

Supercondutores dos tipo I e tipo II:

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B = 0

Estado misto:B 0

Vórtices

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Estado misto de um SC do tipo II, quando (H) é Hc1 < H < Hc2, formam-se tubos de vórtices, a variação da densidade dos pares de Cooper (ns) e a variação da densidade de fluxo.

S SN

Metallic contact between a normal metal and a superconductor

S SN

Electron-hole correlations: proximity effect

Supercurrent Andreev bound states (ABS)

Reflected hole

Incident electron

SuperconductorNormal metal (Semiconductor)

Cooper pair

Andreev reflection

Efeito de Proximidade

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Domain-wall supercondutividade

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Domain-wall supercondutividade

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Campo sempre matar SC ?

The phase boundaries Tc(Hext) for an S/F hybrid,consisting of an Al film and an array of magnetic dots, in thedemagnetized state, in the completely magnetized state in positivedirection as well as in several intermediate magnetic states

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Campo aumenta Tc

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Campo aumenta Jc

Hysteresis curves of Jc (a) at T = 6K for H k I (CoNb1500) and(b) at T = 5:5K for (CoNb1000).

H

Normal state cores Superconducting region

http://www.nd.edu/~vortex/research.html

Type II Superconductors (x < l)

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Campo dos dots

21

Campo sempre matar SC ?

• 100 nm Lead (Pb) film with varying Co particles fraction

• Here we will show results for 4% volume fraction of Co

• Co nanoparticles: ~4.5 nm diameter

Nossos resultados

Bi/Co: Sample preparation

• Narrow size distribution ~ 4.5 nm diameter

• Coercive field = 0.025 T

Co nanoparticles

-0.4 -0.2 0.0 0.2 0.4

-0.6

-0.3

0.0

0.3

0.6

Mom

ent (x

10

-4em

u)

B (T)

8K

Resistivity measurements (Bext=0)

2.5 3.0 3.5 4.0 4.5

0.00

0.05

0.10

0.15

5.5 6.0 6.5 7.0 7.50.0

0.5

1.0

1.5

2.0

2.5

T*

(

m

)

T (K)

TG B=0

(

m

)

T(K)

Pure Pb

l~ 3.1 nm ξ: ~10 nm at 3K

Spontaneous vortices

1 101E-3

0.01

0.1

1

10

Voltage (

mV

)

Current (mA)

Temperature from right to left(K):

5.5, 6.0, 6.5, 6.6, 6.7, 6.8,

6.9, 7.0, 7.05, 7.1, 7.2

Pure Pb

No vortex phase Vortex phase transition

a

Scaling analysis

t = (T - TG)/TG,TG = 3.15 K,

Critical exponents:ν = 0.25 z = 4

1 10 100

0,01

0,1

1

V/(

I(T

-TG)s

)

I/(T-TG)

2

)|(||| 3)1( JtFtE z

Not in the same universality class of the VG transition in high Tc SCs.

Negative field:

Positive field:

Coext BBB

|||||| Coext BBB

BB

10 20 301E-4

1E-3

0.01

0.1

1

10

0 H

+ H

- H

Vo

lta

ge (

mV

)

Current (mA)

(b)

Magnetic field polarity

• Particles aligned at T>TB

• Field (<0.01 T) turned on for T=2.9K (<TG)

Magnetic field polarity

1 101E-3

0.01

0.1

1

10

Sample 3 at 3.1 K

+ B

0 B

- B

(a) V

(m

V)

I (mA)

• Field (<0.01 T) turned on for T=3.1K (~TG)

Magnetic field polarity

1 101E-3

0.01

0.1

1

10 V

(m

V)

Sample 3 at 3.2 K

+ B

0 B

- B

(a)

I (mA)

Field (<0.01 T) turned on for T=3.2 K (>TG)

Phase Diagram for Pb/Co 4 vol%

2,5 3,0 3,5 4,00,0

0,1

0,2

0,3

PVL

Tc

T*

VL

H

(T

)

T (K)

SVS

N

TG

2,5 3,0 3,5 4,0

0,00

0,05

0,10

0,15

(

m

)

T(K)

Y.T. Xing et al, PRB 78 224524 (2008)

1 2 3 4 5 6 7 8-16

-14

-12

-10

-8

-6

-4

-2

0

2

M

om

ent

(x 1

0-5 e

mu)

Temperature (K)

FC 10 Oe

FC 20 Oe

ZFC 10 Oe

ZFC 20 Oe

Magnetic properties

Bext MCo (FC)1 2 3 4 5 6 7 8 9 10 11

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Mom

ent

(x 1

0-5 e

mu)

Temperature (K)

FC 10 Oe

FC 20 Oe

Magnetic properties

4 5 6 7 8 9 10-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

Mo

me

nt (x

10

-5 e

mu

)

Temperature (K)

10 Oe

20 Oe

FC

Magnetic properties

MCo

Origin of PME: Interaction between Co and Pb

Bext

Y.T. Xing et al, PRB 80, 224505 (2009)

Tc as function of Co-volume fraction

1. Low critical Co-volume fraction due to the formation of spontaneous vortices.

2. Annealing leads to a decrease of critical Co-volume fraction

Explanation:

Increase of mean free path , gives increase in :

ξ(as prepared) = 10 nm

ξ(annealed) = 25 nm

Y.T. Xing et al, Eur. Phys. J. B 76, 353 (2010).

0 1 2 3 4 5 60

1

2

3

4

5

6

7

Tc (

K)

v (vol% Co)

as-prepared

after annealing

from magnetic

measurements after annealing

Hext

= 0

Co atoms

Cu clusters

Shorter coherence length? -Bi film

Bi: ξ ≈ 7 nm Much shorter than ξPb

Bulk Bi:Rhomboedral structure, semimetal with very small EF ~ 27.2 meV and Fermi surface (10-5

times Brilloin zone)

Problem:

Bulk Bi: not superconducting!

High pressure phases of Bi: superconducting

Amorphous Bi (a-Bi): superconducting Tc = 6 K

Bi-clusters: superconducting Superconductivity in granular systems built from well-defined

rhombohedral Bi-clusters: Evidence for Bi-surface superconductivity.

B. Weitzel and H. Micklitz, Phys. Rev. Lett. 66, 389 (1991)

Bi films covering Co clusters

Different Co-volume (area) fractions

Pure Bi-films (as-prepared)

After annealing at 100 K:Metallic behavior with2D – weak localizationno indication for superconductivity

Annealed pure Bi-film:

Bi-film with Co clusters

RT

RJ

RTRJ

RT

RJ

Tunneling resistance with Coulomb barrier

Thermal activated tunneling between islands (or grains)

Electron tunneling (RT) and Cooper pair tunneling (RJ)

…

Explanation

1/2

0 0exp( / )R R T T

Explanation

Co(24 A%)/Bi(5nm)

Annealed Bi-Layer

Due to annealing opening of gap in DOS of 2D crystalline Bi-layer

This is caused by (?):(i) Magnetic

impurities at surface of Bi

(ii) Stress at the interface between crystalline Bi and Co-clusters.

(iii) Disorder

Interpretation of result

D. V. Haviland et. al., Phys. Rev. Lett. 62,2180 (1989)A. T. Bollinger et al., Nature, 472, 458 (2011),

SC-I transition in 2D systems

We have:

??

Near QCP

Vórtices em supercondutores

A nucleação de um vórtice é possível próximo a uma região magnéticacom momento magnético suficientemente forte.

Fluxo Magnético -

Simplificação da Formação de Vórtice

Corrente – entrando do plano

X

Dinâmica de Vórtice

Corrente – entrando do plano

Força de Pinning

X

Dinâmica de Vórtice

Corrente – entrando do plano

Corrente – entrando do plano

Corrente – entrando do plano

Sample structure

Nanodisk of NiFe

Nanodisk of NiFe

Laser ablation system

Presure: 3x10-7 torr in one hour

Laser system: 600 mJ

Ni Plasma

Sample preparation

Thin films: in vacuum

Nanoparticles: in Ar with a pressure of 1 torr

Ni nanoparticles prepared in Ar

Ni nanoparticles prepared in Ar

Ni nanoparticles prepared in Ar

Ni nanoparticles prepared in O2

Ni nanoparticles prepared in O2

XPS spectra of Ni nanoparticles

XRD of Ni nanoparticles

0 30 60 90 120 150 180 210 240 270 300

0,0

6,0x10-8

1,2x10-7

1,8x10-7

0 20 40 60 80 1000,0

6,0x10-8

1,2x10-7

1,8x10-7

x (

em

u/O

e)

Temperatura (K)

x (

em

u/O

e)

Temperatura (K)

100 Oe

1kOe

2kOe

0 50 100 150 200 250 300

0,00E+000

5,00E-008

1,00E-007

1,50E-007

2,00E-007

2,50E-007

3,00E-007

H = 100 (Oe)

H = 500 (Oe)

H = 1K (Oe)

c (

em

u/O

e)

Temperatura (K)

Magnetic susceptibility

Sample prepared

2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4 NinpAr/Nb/NinpAr

H 0T

H 0.2T

H 0.4T

H 0.6T

H 0.8T

H 1T

H 1.2T

H 1.4T

H 1.6T

H 1.8T

H 2T

H 2.5T

Re

sis

tên

cia

(O

hm

s)

Temperatura (K)2,0 2,5 3,0 3,5 4,0

0,0

0,5

1,0

1,5

2,0

2,5

NinpO2/Nb/NinpO

2

B 0T

B 0.2T

B 0.4T

B 0.6T

B 0.8T

B 1T

B 1.2T

B 1.4T

B 1.6T

B 1.8T

B 2T

Re

sis

tên

cia

(O

hm

s)

Temperatura (K)

Transporte meausrements

1 2 3 4

0,002

0,004

0,006

0,008

0,01

Vo

lta

ge

m (

V)

Corrente (mA)

NinpO2/Nb/NinpO

2

1 10

1E-4

1E-3

0,01

Vo

lta

ge

m (

V)

Corrente (mA)

NinpAr/Nb/NinpAr T 3.5 a 4.7 K

Transporte meausrements

Magnetic measurements

SQUID

From Google Image:

SQUID em Português:

LulaNa google Imagem:

Magnetic measurements

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Obrigado!

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