IINTERLAYERNTERLAYER E EXCHANGEXCHANGE C COUPLINGOUPLING, P, PAIRAIR BBREAKINGREAKING & 2D V & 2D VORTEX ORTEX DDYNAMICSYNAMICS ININ

FFERROMAGNETERROMAGNET-S-SUPERCONDUCTORUPERCONDUCTOR HHETEROSTRUCTURESETEROSTRUCTURES

R. C. BUDHANI

Indian Institute of Technology, Kanpur

Collaborator :- K. Senapati

Funding :- D.R.D.O. Govt. of India

Pair-breaking effects of ferromagnetic boundaries in Pair-breaking effects of ferromagnetic boundaries in cuprates.cuprates.

Influence of ferromagnetic boundaries on the vortex Influence of ferromagnetic boundaries on the vortex state of ultra-thin YBCO films.state of ultra-thin YBCO films.

Exchange coupling between two ferromagnets Exchange coupling between two ferromagnets separated by an exotic superconductor.separated by an exotic superconductor.

ObjectivesObjectives :- :-

dFM --- 300 ÅdY --- 50 - 500 Å

dNM --- 100 Å

[100] STO

LSMO

LSMO

YBCO (c-axis)

[100] STO

PBCO

PBCO

YBCO (c-axis)

dNM

dY

dFM

dY

dFM dNM

THIN FILM STRUCTURES

-200 0 200

-2

0

2

(b)

Field (Oe)

H || [110]

M (

10-

4 em

u)

-2

0

2

-200 0 200

(a)H || [100]

50 100 150 200 250 300 3500

1x10-4

2x10-4

M (

emu)

Temperature (K)

with 500 Oe

ZFC FC

LSMO film 600 Å

-: STATE OF MAGNETIZATION IN LSMO:-

Soft Magnetic Films with in-plane Magnetization

MS(T)/MS(0) = 1 - AT3/2

J ~ 2 meV

75 150 225 300 3750

400

800

1100 Å

550Å

450 Å

X= 350Å

LSMO (X) / YBCO (100 Å) / LSMO (X)

Res

ista

nce

(O

hm

s)

Temperature (K)

20 40 60 80 1000

25

50

Res

ista

nce

(Ohm

s)

Temp (K)

50 100 150 200 250 300 350

-8.0

-4.0

0.0

LSMO / YBCO ( X ) / LSMO

ZFC500 Oe

x = 100 Å x = 200 Å x = 300 Å x = 500 Å

M ( 1

0 -4

emu)

Temperature (K)

100

40

60

80

40

60

80

Senapati et al., Phys. Rev. B 70, 174506 (2004)

dY (Å )

Tc (

K)

NM Boundary FM Boundary

0 interfaceC Y C C Y C YtotalT d T T d T d

ln

( )

2 20

4

1 12 2 2

20

2

C C

C Y

C

eff Y C

TT d

zz

d d

0

A

MOST OF THE DYNAMICS SCALES WITH THE EFFECTIVE THICKNESS

interface pair breakingC Y C Y C Ytotal

T d T d T d

Effective Tc due to pair breaking (PB)

Inhomogeneous stress field Hole concentration Reduced Tc

Exchange field Spin injection

Interface transparencyEnhanced PB in a reduced

carrier system

SuperconductingmagnetVacuum canDrive coil

SampleHall sensor

G-10 base

Brass rod

Heating element

-: VORTEX DYNAMICS :-

0.00

0.08

20 40 60 80

4T

|TH

3|

dY = 100 Å / FM boundary[a]

0

121 Hz / 1 Oe

1E-3

10

R (

)

20 40 60 800.0

0.5

1.0 [b]

T (K)

TH'

Irreversibility temperature and field TH3 Inverse sheet inductance (1/L) TH’

TH’ = 1- (4a2/c2d) (1/L) [d = thickness , a = effective radius]

1/L = ln(1/Z)THERMALLY ACTIVETED INVERSE SHEET INDUCTANCE

Senapati et al. Rev. Sci. Instrum. 75, 141 (2004)

0.00 0.02 0.04

-0.4

-0.2

0.0

0.8 1.00

2

4

ln (

T C

(H) /

TC

(0) )

H1/2 / T

C(H) ( T1/2K-1 )

deff

(Å)

H (

T)

T

C(H) / T

C(0)

150 300

30

60

90

Hp

b (

0) (

T)

Tirr Tc(H) Critical temperature at which ac Loss of pinning Loss of phase stiffness

c

c B c

πT HLn =

T 0 4k T (H)

Clean limit pair breaking :

( )0

1/2

0Fk H

k H

1/2c(H)

c(0) c(H)

PB

vh2 = ; = , where l = l 2πH

T Hln T T

slope Pair breaking field H

HPB(0) scales with deff

Two regimes of thermal activation separated by a characteristic temperature T*

0.04 0.08

1

0.1

10.05 0.10 0.15

dY= 75 Å

2 T

0

log

( T

H' )

1/T (K -1)

2 T

10 kHz

21 Hz

log

(T

H')

1 / T ( K -1 )

0.02 0.04

0.1

1

1/T (K -1)

dY= 75 Å

0

4 T

log

( T

H' )

LSMO-YBCO-LSMO

Frequency : 121 Hz

PBCO-YBCO-PBCO

Frequency : 121 Hz

d2P

d0

1/22 2

0

0d

0

d

exp - . nT

, .

n ( ) exp 2

ln

<<

Id f

I C

d

pd

v R

R R R Characteristic length for two dislocation

RT aT

103 104 105

102

104

106

0 250 5000

10

300 / FM 75 / FM 75 / FM 35 / FM

U (

0, H

) (

K )

Field (Oe)

dY(Å)

U0( 1

04 K )

deff

( Å )

High T (> T*) --------- U0(T) Energy of creation of dislocationsLow T (< T*) --------- U0(T) Energy required to move preexisting dislocations

50 100 150

0.05

0.10

0.15

100 200 3000.05

0.10

0.15

J 1 (er

g/cm

2 )

Temperature (K)

75100300

dY(Å)

100 K

20 K

J 1 (er

g/cm

2 )

dY(Å)

Summary

• The dynamics of the vortex state in 2D films of YBCO sandwiched between FM-boundaries has been studied over a wide range of temperature, frequency and magnetic field strength.

• Flux motion is controlled by creation of dislocations near Tm, and by the motion of preexisting dislocations at T<Tm.

•The irreversibility temperature in the vortex state maps onto a clean-limit pair-breaking temperature Tc(H), and the pair-breaking field scales with the thickness of the films.

-1.5

0.0

1.5

-250 0 250

HS

120 K

-250 0 250

-1.5

0.0

1.5

HS

Field (Oe)

100 K

-1.5

0.0

1.5

HS

80 K

-1.5

0.0

1.5

HS

M (

10-4 e

mu)

60 K

-250 0 250-2.5

0.0

2.5

HS

40 K

-250 0 250-4

0

4

HS

20 K

LSMO – YBCO – LSMO(300 Å) – (100 Å) – (300 Å)

M-H curves ; Tc = 62 K

K. Senapati and R. C. Budhani, Submitted to Phys. Rev. B.