individual magnetic vortices investigated by...

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

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

NanoSQUIDs

Thin film SQUID Quantum Design


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


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






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 nanoSQUIDs

GB


Focused Ion Beam Milling


YBCO nanoSQUIDs

GB

Ibias

Ibias




YBCO nanoSQUIDs

GB

Imodmod

Ibias

Ibias


Flux bias & modulation via current Imod („coil on chip“)

Additional constriction in SQUID loop



YBCO nanoSQUIDs

500 nm

Ibias

Ibias






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)





constriction width90 nm (10 n0/B)


Mutual iductance 0/mA

Operation at optimum WPFLL

In-plane field B GB3T !!!

YBCO nanoSQUIDs

500 nm

B











YBCO nanoSQUIDs

500 nm

B

µ

High Temperature70 K










-0.5 0.0 0.5

0

2

4

6

8

V (m

V)

/0

T = 300 mK

T = 70 K


YBCO nanoSQUIDs

500 nm

B

µ

Large coupling10 n0/µB











h = 70 nm

10 n0/B for h = 10 nm

(n0/B) 0

6

3

6

3


YBCO nanoSQUIDs

500 nm

B

µ


optimized YBCO nanoSQUID

f (Hz)

(simulated white noise: 25 n0/Hz1/2)

50 n0/Hz1/2










Spin sensitivity5 µB/Hz1/2

Large coupling10 n0/µB

Outlook

1. Motivation






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






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





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



magnetostatic (shape)

Magneto-crystalline Exchange

Exchange lengthLE (2A/0Ms2)1/2

Magneto-crystalline



magnetostatic (shape) Exchange


Out-of-plane

In plane

Magneto-crystalline



magnetostatic (shape) Exchange


The vortex state contributes to magnetization reversal !

Out-of-plane

In planeVortex

Four state logic unit

LE 5 - 10 nm

The vortex state: Magnetization reversal

Vortex-state model

Mx


Mx

-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


L (nm)

-0.2 0.0 0.2

Mx (

a.u.

)

0H (T)

2R = 20 nm

Stoner Wohlfarth model: out ofplane anisotropy


LE 5 - 10 nm

16

6

4


L (nm)

-0.2 0.0 0.2

Mx (

a.u.

)

0H (T)

2R = 20 nm

Stoner Wohlfarth model: out ofplane anisotropy


LE 5 - 10 nm14

12

10

8

6

4


16

L (nm)

Vortex-state model

#4


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



-80 0 80 0H (mT)

0.2

0

5

20

10

30

40

50

60



U0

-80 0 80 0H (mT)

0.2

0

5

20

10

30

40

50

60



-80 0 80 0H (mT)

0.2

0

5

20

10

30

40

50

60



U0

Hoff

-80 0 80 0H (mT)

0.2

0

5

20

10

30

40

50

60



-80 0 80 0H (mT)

0.2

0

5

20

10

30

40

50

60



Definitions:

U0

For H = Hoff :


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 :



-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


200 nm 200 nm

200 nm 200 nm

200 nm

200 nm



-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





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)

Thank you

individual magnetic vortices investigated by...

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