Interaction betweeen Superconductivity and Ferromagnetism ...ebm/ix/arquivos/P7.pdf · Vórtices em...
Transcript of Interaction betweeen Superconductivity and Ferromagnetism ...ebm/ix/arquivos/P7.pdf · Vórtices em...
Yutao Xing
IF-UFF
Interaction betweeenSuperconductivity and Ferromagnetism
in SC/FM nanocomposites
2
Supercondutor e ferromagneto
3
Supercondutor e ferromagneto
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
• London Theory magnetic penetration depth l
• Ginzburg-Landau Theory coherence length x
l x two kinds of superconductors!
Type I and type II SC
Vórtices
12
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
16
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
18
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)
• 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
• 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)
After annealing at 100 K:Metallic behavior with2D – weak localizationno indication for superconductivity
Annealed pure Bi-film:
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
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
A nucleação de um vórtice é possível próximo a uma região magnéticacom momento magnético suficientemente forte.
Fluxo Magnético -
Simplificação da Formação de Vórtice
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
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:
76
Obrigado!