vortex matter in superconductors with ferromagnetic dot arrays margriet j. van bael
DESCRIPTION
VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS Margriet J. Van Bael Martin Lange, Victor V. Moshchalkov Laboratorium voor Vaste-Stoffysica en Magnetisme, K.U.Leuven, Belgium A.N. Grigorenko, Simon J. Bending Department of Physics, University of Bath, United Kingdom. 1. - PowerPoint PPT PresentationTRANSCRIPT
VORTEX MATTER IN SUPERCONDUCTORS WITH FERROMAGNETIC DOT ARRAYS
Margriet J. Van Bael
Martin Lange, Victor V. MoshchalkovLaboratorium voor Vaste-Stoffysica en Magnetisme, K.U.Leuven, Belgium
A.N. Grigorenko, Simon J. BendingDepartment of Physics, University of Bath, United Kingdom
1
Artificial pinning arrays: matching effects
0
12
34
5
12
34
5 µm
50 00 A
0
Pb(500Å) film with a square antidot lattice
Strong enhancement of critical current‘matching’ effects
H1
M. Baert et al. PRL 74 (1995), V.V. Moshchalkov et al. PRB 54 (1996), PRB 57 (1998)
MAGNETIC PINNING CENTRES
Influence of magnetic moment on pinning efficiency
Field-induced superconductivity
Influence of magnetic stray fieldon pinning efficiency
Co dots with in-plane magnetization
Co/Pt dots with out-of-plane magnetization
Hybrid ferromagnetic/superconducting systemArray of magnetic dots covered with superconducting film
m
Square array of Co dipoles
d
0.36 µm
0.54 µm
1.5 µm
thickness: 380 Å
SiO2
Co (polycrystalline)AuPreparation:
e-beam lithography +molecular beam deposition +Lift-off
AFM & MFM @ H=0, RT
Enhance stray field
Not magnetizedMulti domain
MagnetizedSingle domain
M.J. Van Bael et al. PRB 59, 14674 (1999)
j c (1
07 A/m
2 )
-2 -1 0 1 2
multi - domain single - domain multi - domain single - domain multi - domain single - domain
H/H1
5
10
15
dot flux line
Triangular array of Co dots Electrical transport measurements
H1 = = 10.6 Oe
3 (1.5 m)2
0 2
H/H1 = 2 honeycomb lattice only stable for strong pinning(Reichhardt et al. PRB 57, 1998)
L. Van Look et al. Physica C 332 (2000)
T/Tc = 0.985
Magnetic dots create strong pinning potential Clear matching effects close to Tc
Better pinning for single domain dots
structural + magnetic contributions
M.J. Van Bael et al. PRB 59, 14674 (1999)
Array of Co dipoles
Pb Co C o
Flux lines pinned at Co dotsSingle domain -> better pinning
‘Tunable pinning’-6 -4 -2 0 2 4 6
0
2
4
6
323/2
1T/Tc= 0.97
M (1
0-4 e
mu)
H/H1
multi domain
no dots
single domain
M.J. Van Bael et al. PRB 59 (1999)
BUT … WHAT HAPPENS LOCALLY ??
Position of vortex on dipole ?? Superconducto
r and dipole are
not independent
Fluxoid quantizatio
n
Scanning Hall probe microscopy (SHPM)@ University of Bath
AuSTM tip
10 m
• 2DEG material for better sensitivity (2 µV/G)• Active area: 2 µm × 2 µm
0.25 µm × 0.25 µm• Spatial resolution < 1 µm• Typical sensor-surface distance: ~ 200-300 nm
probe and picture in collaboration with imec
Pb-film on square array of single domain Co dots T = 6K << Tc
Subtract dipole contribution:
Visualization of vortex lattice in magnetic dot array
- =
[dipoles + flux lines] - dipoles (T > Tc) = flux lines square vortex lattice
T = 6K, H = H1 T = 7.5 K, H = H1
Ordered vortex patterns at integer and fractional matching fields: H/H1 = 1/2, 1, 3/2, 2, …
Fluxoid quantization effects: field contrast in zero field
SHPM image at H = 0
SHPM image at H = 0
5.5 6.0 6.5 7.0 7.5 8.02.4
2.6
2.8
3.0
3.2
peak
-to-p
eak
mod
ulat
ion
(G)
T(K)
Tc = 7.16 K
S Nfie
ld c
ontra
st
(G)
field profile
contrast
M.J. Van Bael et al. PRL 86, 155 (2001)
Pb
SiO 2
0
‘Vortex–antivortex’ pair induced
T > Tc vorticesT < Tc
Pb
SiO 2
Attraction and annihilation
of negative vortex and positive fluxoidPb
SiO 2
T > Tc
+ ½H1
In applied field: position of vortex on dipole ?
- ½H1
Field polarity dependent pinningConfirmed by theoretical model (Milosevic et al. PRB 69
(2004)) M.J. Van Bael et al. PRL 86, 155 (2001)
vorticesT < Tc
+ ½H1
0.4 m
1 m
MFM magnetized H> 0
single-domain all up
MFM magnetized H< 0
single-domain all down
MFM demagnetized
single-domainrandom up - down
Array of Co/Pt dots with out-of-plane magnetization
x [ m ]
0
0.51.
01 .5
y [m
]
00.5
1.01 .5
AFM
Preparatione-beam lithography + molecular beam deposition + lift-off
SiO2
Co/Pt (111) 270 Å
m > 0m < 0Co/Pt dots as artificial pinning centers
strong pinning
strong pinning
parallel parallel
weak pinning
weak pinning
antiparallel antiparallel-3 -2 -1 0 1 2 3
-4
-2
0
2
4
M (1
0-4 e
mu)
H/H1
T = 7.00 K T = 7.10 K
-3 -2 -1 0 1 2 3
-4
-2
0
2
4
M (1
0-4 e
mu)
H/H1
T = 7.00 K T = 7.10 K
M.J. Van Bael et al. PRB 68, 014509 (2003)
total current:screening current js
vortex current jv
Line energy vortex (~2)stray field outside SC
(dot + vortex)
magnetic moment in vortex field
-m.bz
Interaction between vortex and magnetic dot
Einteraction = Ekinetic + Efield + Emoment
Stray field of dot is screened below Tc js
js
m
jv
bz
Attractive interaction when field and moment are parallelStrong on-site pinning
vortexdot
Repulsive interaction when field and moment are antiparallelWeak interstitial pinning
jv
bz
Attractive interaction when field and moment are parallelStrong on-site pinning
M.J. Van Bael et al. PRB 68, 014509 (2003)
SC
T = 6.8 K H = 1.6 Oe >0T = 6.8 K H = -1.6 Oe <0
Asymmetric pinning in magnetized Co/Pt dot arrayDots magnetized in negative direction
Vortex-dot interaction: attractive for parallel alignment repulsive for anti-parallel alignment
S C
Vortices pinned by dots
Vortices between dots
M.J. Van Bael et al. PRB 68, 014509 (2003)
Schematic sample cross-section
Case of larger dotsWhat if the dots induce flux quanta ?
larger dots Co/PdDiameter 0.8 µmPeriod 1.5 µm
Magnetized state: Critical currentDots magnetized down
Pb
m < 0T = 7.10KT = 7.15KT = 7.18K
Dots magnetized up
Pb
m > 0T = 7.10KT = 7.15KT = 7.18K
Pinning is strongly field-polarity dependent Maximum critical current shifted to non-zero field
cfr. M.V. Milosevic and F.M. Peeters, PRL 93, 267006 (2004)
7.18 7.20 7.22 7.24
4
2
0
-2
-4
HH
/ 1
T (K)
N
S
N m = 0
7.18 7.20 7.22 7.24T (K )
4
2
0
-2
-4
HH
/ 1
mz < 0N
S
7.18 7.20 7.22 7.24T (K )
4
2
0
-2
-4H
H /
1
mz > 0
N
S
H-T phase diagramFor magnetized dots• Phase diagram asymmetric• Shift of maximum Tc
• Superconductivity induced by magnetic field (~ 2 mT)
-4 -2 0µ H0 (mT)
2 4
n
1.0
0.8
0.6
0.4
0.2
0
mz > 0-4 -2 0
µ H0 (m T)2 4
n
1.0
0.8
0.6
0.4
0.2
0
mz < 0
-4 -2 0µ H0 (mT)
2 4
n
1.0
0.8
0.6
0.4
0.2
0
m = 0Magnetoresistivity
-4 -2 0µ H0 (mT)
2 4
n
1.0
0.8
0.6
0.4
0.2
0
m = 0
M. Lange et al. PRL 90, 197006 (2003)
-4 -2 0µ H0 (mT)
2 4
n
1.0
0.8
0.6
0.4
0.2
0
mz < 0
-4 -2 0µ H0 (mT)
2 4
n
1.0
0.8
0.6
0.4
0.2
0
m = 0
Field compensation effectsApplied field H = 0
Stray field of dots destroys superconductivitybetween and below dots ~20 per unit cell
Applied field H = 2H1
Between the dots, the stray field compensates the applied field (2H1= 1.84 mT) and superconductivity emerges
Cond-mat/0209101M. Lange et al. PRL 90, 197006 (2003)
CONCLUSION Artificial pinning arrays
Very efficient pinning Induce particular geometry of vortex lattice
Magnetic pinning centersMagnetism provides extra parameterFundamental interaction between pinning center and flux line ?
Domain state and stray field important
Field polarity dependent pinning Magnetic dots can create vortex-antivortex pairs Field compensation effects and field-induced superconductivity