Upload
doannhan
View
217
Download
3
Embed Size (px)
Citation preview
Individual Magnetic Vortices Investigated by nanoSQUID Magnetometry
Javier Sesé
L. A. Rodriguez, E. Snoeck
B. Mueller, R. Kleiner, D. Koelle
Pepa Martínez-Pérez
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vórtices. Applications
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vórtices. Applications
Wernsdorfer, Adv. Chem. Phys. 118, 99-190 (2001)
Magnetic nanoparticles
Magnetizationreversalmechanisms:
Single molecule magnets
• Quantum tunneling • Quantum
superpositionSkyrmions
Nanowires
Nanoparticles
• Domain wall propagation
• Non-coherent rotation
• Uniform rotation
Nanodiscs
• Vortex states
Multi-domain
• Domain growth
Wernsdorfer, Adv. Chem. Phys. 118, 99-190 (2001)
Magnetic nanoparticles
Magnetizationreversalmechanisms:
• Domain growth• Vortex states• Domain wall
propagation• Non-coherent
rotation• Uniform rotation• Quantum tunneling • Quantum
superposition
Ultra-sensitive magnetic detection
Nano-SQUIDs:
CNT NEMs torque sensor. Sensitivity < 1µB
Ganzhorn, ACS Nano 7, 6225-6236 (2013)
Urdampilleta, NatMaterials 10, 502 (2011)
Molecular spin valve
Balasubramanian et al, Nature 455, 648 (2008)
NV microscope.Sensitivity < 10µB
Nitrogen Vacancy centers:
Carbon nanotubes:
W. Wernsdorfer, et al PRL 77 (1996)
Dc SQUID – basics
ab
Maximum critical current
Magnetic field
interference of superconductor wavefunction
Dc SQUID – basics
ab
Maximum critical current
Magnetic field
interference of superconductor wavefunction
NanoSQUIDs
Thin film SQUID Quantum Design
W. Wernsdorfer, et al PRL 77 (1996)
S1/2 ~ 0.01 - 1 µ0/Hz1/2
Spin Sensitivity
rms spectral density of flux noise:
Sµ1/2 = 50n0/Hz1/2
30 n0/µB ~ 1 µB/Hz1/2
magnetic flux µ (coupled SQUID loop)magnetic moment µ
Coupling factor:
µa~ 3000/a(nm) (n0/B)
~ 10 - 50 n0/B
NanoSQUID survey
W. Wernsdorfer, et al PRL 77 (1996)
1. Field operation
2. Temperature operation
3. Particle positioning
Problems:
Dayem bridges
S.K.H. Lam, D.L. Tilbrook, Appl. Phys. Lett. 82 (2003)
A. Blois, et al J. Appl. Phys. 114 (2013) 233907.
D. Vasyukov, et al, Nature Nan0. 8 (2013)
Jenkins,Luis et al 2015.
Martinez-Perez et al APL 99 (2011)
Drung, et al IEEE Trans. Appl. Sup. 24 (2014)
D. Gella. Master’s thesis, Zaragoza, 2015.
Awschalom, et al PRL 68 (1992). Ketchen et al APL 44 (1984)
SIS
Martinez-Perez et al. ACS Nano (2016)
SNS
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vórtices. Applications
YBCO nanoSQUIDs
Properties of YBa2Cu3O7 crystals:
Anisotropy and low coherence length:
λab ≈ 150 nm, λc ≈ 800 nm
ξab ≈ 2 nm, ξc ≈ 0.4 nm
Grain boundaries:
Josephson behavior!
grain 1
grain 2
(100)
(100)
grain 1[a,b]
grain 2[a,b]
(001)
[c]
YBCO nanoSQUIDs
GB
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
YBCO nanoSQUIDs
GB
YBCO (d=120 nm) / Au (dAu=80 nm)
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
YBCO nanoSQUIDs
GB
YBCO (d=120 nm) / Au (dAu=80 nm)
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
YBCO nanoSQUIDs
GB
Ibias
Ibias
YBCO (d=120 nm) / Au (dAu=80 nm)
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
YBCO nanoSQUIDs
GB
Imodmod
Ibias
Ibias
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
YBCO nanoSQUIDs
500 nm
Ibias
Ibias
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
YBCO nanoSQUIDs
500 nm
Imod
Ibias
Ibias
mod = Mmod • Imod
mod YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
constriction width90 nm (10 n0/B)
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
Mutual iductance 0/mA
Operation at optimum WPFLL
In-plane field B GB3T !!!
YBCO nanoSQUIDs
500 nm
B
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
Operation at optimum WPFLL
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
Mutual iductance 0/mA
constriction width90 nm (10 n0/B)
In-plane field B GB3T !!!
YBCO nanoSQUIDs
500 nm
B
µ
High Temperature70 K
Operation at optimum WPFLL
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
Mutual iductance 0/mA
constriction width90 nm (10 n0/B)
-0.5 0.0 0.5
0
2
4
6
8
V (m
V)
/0
T = 300 mK
T = 70 K
In-plane field B GB3T !!!
YBCO nanoSQUIDs
500 nm
B
µ
Large coupling10 n0/µB
Operation at optimum WPFLL
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
Mutual iductance 0/mA
constriction width90 nm (10 n0/B)
High Temperature70 K
h = 70 nm
10 n0/B for h = 10 nm
(n0/B) 0
6
3
6
3
In-plane field B GB3T !!!
YBCO nanoSQUIDs
500 nm
B
µ
Operation at optimum WPFLL
optimized YBCO nanoSQUID
f (Hz)
(simulated white noise: 25 n0/Hz1/2)
50 n0/Hz1/2
YBCO (d=120 nm) / Au (dAu=80 nm)
Flux bias & modulation via current Imod („coil on chip“)
Additional constriction in SQUID loop
Focused Ion Beam Milling
YBCO on SrTiO3 bicrystal (24° grain boundary (GB))
junction width wJ 200 nm loop size: J x c 300 nm x 200 nm Extremely small inductance 4 pH
Mutual iductance 0/mA
constriction width90 nm (10 n0/B)
High Temperature70 K
Spin sensitivity5 µB/Hz1/2
Large coupling10 n0/µB
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vórtices. Applications
Co nanoparticles
Polycrystalline Co nanopillars grown by Focused Electron Beam Induced Deposition (FEBID)
FEBID Co
Co2(CO)8
100 nm 100 nm 100 nm
85 × 40 115 × 6065 × 30
Paramag.
Amorphous Co
De Teresa et al J. Phys. D: Appl. Phys. 49 (2016)
Co nanoparticles
Polycrystalline Co nanopillars grown by Focused Electron Beam Induced Deposition (FEBID)
FEBID Co
0.7 Co purity
100 nm 100 nm 100 nm
200 nm
200 nm
85 × 40 115 × 6065 × 30
Co2(CO)8
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vórtices. Applications
Co nanoparticles on YBCO nanoSQUIDsSeries of five different nanoparticles
#1 #2 #5#4#3
thickness 35 nm
20035
10035
5035
90
60
d = 60 nm
t = 40 nm
Single domain – like particles. Temperature dependence
-80 0 80 -80 0 80 -80 0 80
4.2
10
20
30
40
50
60
0.1
0
10
60
50
40
30
20
10
5
0.1
0
6
10
25
30
35
40
45
0H (mT)
0 30 60 90
20
40
60
80
0Hsw
(mT)
T (K)
#1 #2 #3
#1
#2
#3
Co nanoparticles on YBCO nanoSQUIDs
3×106 B15×106 B 1×106 B
Single domain – like particles. Temperature dependence
0 30 60 90
20
40
60
80
0Hsw
(mT)
T (K)
#1 #2 #3U0 5 104 K K 2 kJ/m3
U0 5 103 K K 1 kJ/m3
#1
#2
#3
U0 104 K K 4 kJ/m3
Classical thermally activated reversal process over an energy barrier
U0 = K
VolJ. Kurkijärvi, Phys. Rev. B 832 (1971)
Kcrys (bulk Co) 260 kJ/m3
Co nanoparticles on YBCO nanoSQUIDs
3×106 B15×106 B 1×106 B
-80 0 80
-80 0 80
0H (mT)
0H (mT)
#5#4Discs having 100 nm and 200 nm in-diameter
Vortex???
Co nanodiscs on YBCO nanoSQUIDs
9×106 B 30×106 B
~19 nm
~5 nm~11 nm
~20 nm
~5 nm~10 nm
100 nm
200 nm0.
1
0
0.25
0
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vortices. Applications
The vortex state: Magnetostatic vs exchange
Competition of a number of energies:
magnetostatic (shape)
Magneto-crystalline Exchange
Domain wall size
Single domain!
dw (A/K)1/2
dw (Co) 30 nm
D < dw
The vortex state: Magnetostatic vs exchange
Competition of a number of energies:
magnetostatic (shape)
Magneto-crystalline Exchange
Exchange lengthLE (2A/0Ms2)1/2
Magneto-crystalline
The vortex state: Magnetostatic vs exchange
Competition of a number of energies:
magnetostatic (shape) Exchange
Exchange lengthLE (2A/0Ms2)1/2
Out-of-plane
In plane
Magneto-crystalline
The vortex state: Magnetostatic vs exchange
Competition of a number of energies:
magnetostatic (shape) Exchange
Exchange lengthLE (2A/0Ms2)1/2
The vortex state contributes to magnetization reversal !
Out-of-plane
In planeVortex
Four state logic unit
LE 5 - 10 nm
-0.2 0.0 0.2
Mx (
a.u.
)
0H (T)
2R = 20 nm
Stoner Wohlfarth model: in plane anisotropy
LE 5 - 10 nm
6
4
The vortex state: Magnetization reversal
L (nm)
-0.2 0.0 0.2
Mx (
a.u.
)
0H (T)
2R = 20 nm
Stoner Wohlfarth model: out ofplane anisotropy
Stoner Wohlfarth model: in plane anisotropy
LE 5 - 10 nm
16
6
4
The vortex state: Magnetization reversal
L (nm)
-0.2 0.0 0.2
Mx (
a.u.
)
0H (T)
2R = 20 nm
Stoner Wohlfarth model: out ofplane anisotropy
Stoner Wohlfarth model: in plane anisotropy
LE 5 - 10 nm14
12
10
8
6
4
The vortex state: Magnetization reversal
16
L (nm)
Vortex-state model
#4
Co nanodiscs on YBCO nanoSQUIDs
9×106 B
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
100 nm in-diameter co disc: temperature dependence & energy barriers
U0
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
U0
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
U0
Hoff
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence & energy barriers
Definitions:
U0
For H = Hoff :
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60 Definitions:
0 60 120 180 240 300-90
-60
-30
0
30
60
90
0H (m
T)T (K)
U0
For H = Hoff :
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence
-80 0 80 0H (mT)
0.2
0
5
20
10
30
40
50
60 Definitions:
0 60 120 180 240 300-90
-60
-30
0
30
60
90
0H (m
T)T (K)
U0
For H = Hoff :
U0/kB = 2 × 104 Ka = 5 n = 2
Discs with different diameters and different nominal thicknesses: Electron Holography
500 nm 100 nm 20 nm
200 nm 50 nm 10 nm
1000 nm 1000 nm
500 nm 200 nm
Co nanodiscs on YBCO nanoSQUIDs
200 nm 200 nm
200 nm 200 nm
200 nm
200 nm
Co nanodiscs on YBCO nanoSQUIDs
100 nm in-diameter co disc: temperature dependence
-40 -20 0 20 40 60 80
0
20
40From numerical simulations
U/k
B (1
04 K)
0H (mT)
From fitting parameters
U0
0 60 120 180 240 300-90
-60
-30
0
30
60
90
0H (m
T)T (K)
Ua
Un
U0/kB = 2 × 104 Ka = 5 n = 2
Outlook
1. Motivation
2. NanoSQUID: fabrication and properties
3. FEBID Co nanoparticles:
4. Measurements: quasi single-domain particles
5. Measurements: Flux closure states
6. Magnetic vortices. Applications
Experimental observation of the vortex circulation and core
The vortex state: Observation
J. Appl. Phys 92, 1466 (2002)
Lorentz M
Science 298, 577 (2002)
STM
Science 289, 930 (2000)
MFM
200 nm
E Holography
PRB 83, 212402 (2011)
TXM
500 nm
The vortex state: Observation
2002 J. Phys.: Condens. Matter 14 R1175
Lorentz m MFM
APL 91, 202501 2007
SQUID MOKE
APL 86, 072501 2005
Experimental observation of the vortex-assisted magnetization reversal
Micro-Hall
APL 96, 112501 201030
0 nm
Nano-SQUID
PRB 53 3341 1996
The vortex state: Four-state logic unit
Polarity, circulation and handedness
P = +1; C = -1
P = -1; C = -1 P = -1; C = +1
P = +1; C = +1
0 3 6 9 12 15
FTT
f (GHz)
The vortex state: Spin-Wave Modes
Radial modes:
15 GHz
Gyrotropic mode:
1.5 GHz
H disc
11.5 GHz
Azimuthal modes:
14 GHz
H// disc
The vortex state: Gyrotropic mode
Gyrotropic mode:
1.5 GHz
Waeyenberge Nature 44 (2006)
POLARITY REVERSAL!!
The vortex state: Gyrotropic mode
Locatelli et al Scientific Reports 5 (2015)
SPIN-TRANSFER OSCILLATORS
CANCER CELL DESTRUCTION
Kim et al NATURE MAT 9 (2010)