instituto de cerámica y vidrio csicbenasque.org/2014magnetism/talks_contr/122_magarcia.pdf ·...

Novel Frontiers of Magnetism

Benasque 10-14 February , 2014 M. A. Garcia

Magnetism of nanoparticles: non magnetic metals, core shell, exchange bias

M. A. Garcia

Instituto de Cerámica y Vidrio

CSIC



many thanks to….. Antonio Hernando Jesus Gonzalez Puer to Morales Sabino Veintemillas Mar Garcia-Hernández Axel Hoffman Jesus Chaboy Jose Luis Vicent Lucas Perez Julio Camarero Ivan Schuller Davide Cozzoli Teresa Pellegrino Cesar de Julian Hari Srikanth German Castro Jose Costa Pietro Gambardella

Adrian Quesada Jose de la Venta Virginia Bouzas Sandra Martinez Bustos Aida Serrano Esther Enriquez Alvaro Muñoz Fernando Galvez Ana Espinosa Enrique Fernandez Pinel



Outline I. Nanoscale effects in magnetic materials -Monodomain regime -Superparamagnetism Proximity effects :Exchange bias

II: Nanoscale induced magnetism -Size effects -Proximity polarization -Surface induced magnetism

III. Measurement of nanoscale magnetism -Traditional magnetometry -XMCD & Neutrons (brief)



I. Nanoscale effects in magnetic materials

…size matters……



Size effects

NPs has similar size to relavant parameters of physical processes

Macroscopic systems (size>>> typical) periodic conditions

NP: Boundary Conditions(size typical)

HY=E Y

…… ……



Surface effects

Surface atoms are different to volume ones

0 100 200 300 400 5001E-3

0.01

0.1

1

Ato

mos

. sup/A

tom

os

volu

men

R (nm)



FM NM

M

FM FM

Interaction is of the order of few nanometers

For nanoparticles this region be a significant part of the

whole material

Proximity effects



Monodomain regime

Domains decrease the magnetostatic energy

Domains are separated by domain walls

Domain wall width (exchange correlation

length) is few nanometers



Monodomain regime

Domains walls have a certain energy

- Eexchange

- Eanisotropy

For small nanoparticles is more favourable to have no domains walls



Monodomain regime

Two different mechanisms for

magnetization rotation

Coherent spin rotation

H

Domain wall motion H



Moodomine regime

In monodomain nanoparticles there is no domain walls

Increase of the Coercive force

Hc

Particle size



Superparamagnetism

Energy Barrier = KV

Thermal Energy= KBT

Relaxation Tk

VK

Be·

·

0· =



Superparamagnetism

Time

M

Time

M

H=0 H0



Superparamagnetismo

Hc

Particle size

If relaxation time is measuring time

Coercitivity drops (Big problem for data storage)



Fingerprints of superparamagnetism

KV≈25 KBT

L(x)=coth(x)-1/x



Exchange bias

FM AFM

Junction of FM & AFM

TC>TN



H TC

TN

Cooling with field



M

H

Magnetization curve

FM FM+AFM

Increase of coercivity



Exchange bias was discovered by W. H. Meiklejohn in 1957

EB is not fully understood

It has been observed in FM/AFM FM/FiM FiM/AFM FiM/FiM FM/disordered layer



Exchange bias in core-shell nanoparticles



EB in Co/CoO nanoparticles

Co

EB in Fe3O4/Fe2O3

Co

Fe3O4

Naturally oxidized EB

EB depends critically on the shell thickness



…some reviews……..



II. Nanoscale induced magnetism



Proximity polarization

FM NM

Interface region of the NM will exhibit some magnetic polarization



Materials “close to be ferromagnetic”?

Magnetic nanoparticles

Close to satisfy Stoner criterion

EF

To change the spin of one electron:

Increase of orbital energy E=1/N(EF)

Energy balance: E=1/N(EF)-I<0

N(EF)·I>1 (Stoner criterion)

N(EF)

1

Decrease of Exchange energy: I

(Stoner parameter)



Pd (FCC) is a paramagnetic metal

2.4 nm Pd NP exhibit ferromagnetism

Ferromagnetism in Pd nanoparticles

-2000 -1000 0 1000 2000

-0,04

-0,02

0,00

0,02

0,04

M (

em

u/g

)

H (Oe)

M (

em

u/g

)*10

-3

H (Oe)

T = 100K

T = 200K

T = 275K

-100 -50 0 50 100

-5,0

-2,5

0,0

2,5

5,0

5K

100K

200K

275K

B. Sampedro et al, PRL 91 (2003) 237203 0 50 100 150 200

0,00

0,01

0,02

0,03

0,04

0,05 Pd NP ( 2.4 nm)

Pd bulk

T (K)

M (

em

u/g

)*10

-3

M (

em

u/g

)

T (K)

0 50 100 150 200

6,0

6,5

7,0

7,5

Blocking T is over 300 K

N(EF)·I=0.87<1 (non ferromagnetic behaviour)

N(EF)=1.23 eV-1 spin-1atom -1

I =0.71 eV



HREM studies found a large number of twin boundaries


Twin boundaries reduce the energy of the NP



FCC metals present:

-Highly anisotropic Surface Energy

- Low formation energy for twin boundaries


Es

Es

Es Es

Es

Es Es

Es

The presence of twin boundaries reduce ESurface

The increase of Evolume is low

Total energy decreases with twin boundaries

Es Es

Es Es

Es Es

Es

Es




Only atoms close to the TB contributes to ferromagnetism Low MS (10-3 B atom-1)

Cubic symmetry splits d levels

At the twin boundaries there is a lack of symmetry No splitting of the 4d level

t2g

Local enhancement of N(EF)

Stoner criterion is satisfied !!

Lack of symmetry at the TB blocks the angular momentum

L S

Pd exhibits large spin-orbit couplingLarge K TB >300 K

4d

eg



Ferromagnetism in Pt nanoparticles

Pt is similar to Pd: - FCC metal

- N (EF)=0.9 eV-1

- I=0.85 eV atom-1

Stoner criterion almost satisfied

2-5 nm Pt NP exhibit FM

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

-1,0x10-2

-5,0x10-3

0,0

5,0x10-3

1,0x10-2

5K

50 K

300K

M (

em

u/g

Pt)

H (Oe)

Blocking T is over 300 K

0 50 100 150 200 250 300

8,5x10-3

9,0x10-3

9,5x10-3

1,0x10-2

M (

em

u/g

Pt)

T (K)

M. A. Garcia et al, Nanotech. to be publish



Ferromagnetism in Pt nanoparticles

(c)

(d)

5 nm

(b)

[101]

0.23 nm

111

111

(a)

(c)

(d)

5 nm

(b) (c)(c)

(d)(d)

5 nm

(b)

5 nm5 nm

(b)

[101]

0.23 nm

[101][101][101]

0.23 nm

111

111

(a)

111

111

111

111

(a)Pt NP also exhibit:

- a large density of twin boundaries - low MS.

The same mechanism than Pd



Effects in diamagnetic materials?

What we need for ferromagnetic behaviour?



L

S

For a single atom



In a solid……..Magnetocrystalline anisotropy

L

S

E

2 0

For an atom

KV~ eV

KBT~meV

Energy barrier= KV

Thermal energy= KBT



To observe FM we need to increase energy barriers

Exchange: Interaction between magnetic moments tending to keep them aligned

E

2 0

Barrier: E

E

2 0

Barrier n·E



We should need huge anisotropy to observe FM in single atoms

Co atom

Pt (111)

Gambardella, Rusponi et al Science 300 (2003) 1130

Co-Pt bond blocks Co L

Spin-orbit copling blocks S



Anisotropy is always increased in surfaces: Symmetry broken



Magnetic moments can be induced by bonds



Gold Nanoparticles

Holes in 5d orbital via thiol bonding

Very large spin-orbit coupling



Two set of samples:

Au-SR sample: Gold NPs capped with thiol groups

(dodecane alkyl chain)

Strong interaction with Au at surface

Au-NR sample: Gold NPs stabilized by a surfactant

(tetraoctyl ammonium bromide).

Weak interaction with Au at surface of the NP



0 1 2 3 4 50

5

10

15

20

25

30

Dm=1.4 nm

% P

art

icle

s

D (nm)

0 1 2 3 4 5 6 70

10

20

30

40

% P

artic

les

D (nm)

D=1.5-5 nm

100nm0 10 20 30 40 50 60

0

0.5

1

1.5

2

2.5

3

3.5

X[nm]

Z[n

m]

Au-NR

Au-SR

AFM Dispersed NP



-10000 -5000 0 5000 10000

-1,0

-0,5

0,0

0,5

1,0

-10000 -5000 0 5000 10000

-0,006

-0,004

-0,002

0,000

0,002

0,004

0,006

T=300K

T=5K

Au-SR

M (

em

u/g

Au

)

H(Oe)

T=5K

T=300K Au-NR

M (

em

u/g

)

H(Oe)

SR samples present ferromagnetic-like behaviour up

to 300 K

-10000 -5000 0 5000 10000

-1,0

-0,5

0,0

0,5

1,0

-10000 -5000 0 5000 10000

-0,006

-0,004

-0,002

0,000

0,002

0,004

0,006

b)

T=300K

T=5K

Au-SR

M (

em

u/g

Au

)

H(Oe)

a)

T=5K

T=300K Au-NR

M (

em

u/g

)

H(Oe)

NR Samples are diamagnetic (as bulk gold)

Magnetic properties

The capping molecule determines the magnetic behaviour of the NP

P. Crespo et al, PRL 93 (2004) 087204



Au L3 XANES Spectra

There is charge transfer from Au(5d) to S(2p).

Increase of d holes density at the surface Au atoms.

S Au

-10 0 10 20 30 400,0

0,4

0,8

1,2 Au-foil

Au-surfactant

Au-thiol

Norm

alized y

ield

(a.

u.)

E-E0



ZnO Nanoparticles

TOPO

Tryoctylphosphine

THIOL

Dodecanethiol

AMINE

Tryoctylamine

NPs capped with different molecules

-O -N -S



Crystallographic structure does not depend on the molecule

ZnO wurtzite structure

10 nm

30 40 50 600

1

2

3

4

5

TOPO

THIOL

(103)

(110)

(102)

(101)

(002)

Inte

nsity (

arb

.un

.)

2

(100)

AMINE

(a)

7 nm

(b)

0.25 nm

(c)

101

(d)1017 nm

(b)

0.25 nm

(c)

101

(d)1017 nm

7 nm

(b)

0.25 nm

(c)

101

(d)101

101

(d)101101

30 40 50 600

1

2

3

4

5

TOPO

THIOL

(103)

(110)

(102)

(101)

(002)

Inte

nsity (

arb

.un

.)

2

(100)

AMINE

(a)

7 nm

(b)

0.25 nm

(c)

101

(d)1017 nm

7 nm

(b)

0.25 nm

(c)

101

(d)101

101

(d)1011017 nm

7 nm

(b)

0.25 nm

(c)

101

(d)101

101

(d)1011017 nm

7 nm

(b)

0.25 nm

(c)

101

(d)101101

101

(d)101101

XRD TEM



XANES spectra

(Zn K-edge)

-20 0 20 40 60 80 100 1200,0

0,5

1,0

Norm

alise

d Y

ield

(a.u

)

E-Eo (eV)

THIOL

AMINE

TOPO

-20 0 20 40 60 80 100 1200,0

0,5

1,0

Norm

alise

d Y

ield

(a.u

)

E-Eo (eV)

THIOL

AMINE

TOPO

Different degree of occupation of the Zn outer orbitals depending on the molecule.

Different charge transfer for each molecule !!



Photoluminescence

The energy levels are altered by the molecule

500 550 600 6500

2

4

6

8

10

12

THIOL

AMINE

PL

In

ten

stity

(a

rb.

un

.)

Wavelength (nm)

TOPO

(3.4 eV) (2.3 eV)

(a)

(b)

500 550 600 6500

2

4

6

8

10

12

THIOL

AMINE

PL

In

ten

stity

(a

rb.

un

.)

Wavelength (nm)

TOPO

(3.4 eV) (2.3 eV)

(a)

(b)



M. A. Garcia et al, Nano Letters 7 (2007) 1489.

Magnetic Properties

-10000 -5000 0 5000 10000

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

300 K

THIOL

AMINE

TOPO

M (

em

u/g

)

H (Oe)

(a) (b)

(c) (d)

-10000 -5000 0 5000 10000

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003 THIOL

AMINE

TOPO

5 K

M (

em

u/g

)

H (Oe)

-10000 -5000 0 5000 10000-0,006

-0,003

0,000

0,003

0,006

THIOL

AMINE

TOPO

5 K

M (

em

u/g

)

H(Oe)

-10000 -5000 0 5000 10000-0,006

-0,003

0,000

0,003

0,006

THIOL

AMINE

TOPO

M (

em

u/g

)

H (Oe)

300 K

0 100 200 300

M

T (K)

-10000 -5000 0 5000 10000

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

300 K

THIOL

AMINE

TOPO

M (

em

u/g

)

H (Oe)

(a) (b)

(c) (d)

-10000 -5000 0 5000 10000

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003 THIOL

AMINE

TOPO

5 K

M (

em

u/g

)

H (Oe)

-10000 -5000 0 5000 10000-0,006

-0,003

0,000

0,003

0,006

THIOL

AMINE

TOPO

5 K

M (

em

u/g

)

H(Oe)

-10000 -5000 0 5000 10000-0,006

-0,003

0,000

0,003

0,006

THIOL

AMINE

TOPO

M (

em

u/g

)

H (Oe)

300 K

0 100 200 300

M

T (K)



Correlation between structure and magnetism

500 550 600 6500

2

4

6

8

10

12

THIOL

AMINE

PL

In

ten

stity

(a

rb.

un

.)

Wavelength (nm)

TOPO

(3.4 eV) (2.3 eV)

(a)

(b)

500 550 600 6500

2

4

6

8

10

12

THIOL

AMINE

PL

In

ten

stity

(a

rb.

un

.)

Wavelength (nm)

TOPO

(3.4 eV) (2.3 eV)

(a)

(b)

-20 0 20 40 60 80 100 1200,0

0,5

1,0

Nor

mal

ised

Yie

ld (a

.u)

E-Eo (eV)

THIOL

AMINE

TOPO

-20 0 20 40 60 80 100 1200,0

0,5

1,0

Nor

mal

ised

Yie

ld (a

.u)

E-Eo (eV)

THIOL

AMINE

TOPO



Reproducibility?

25 nm ZnO NPs



Reproducibility

-60 -40 -20 0 20 40 60-30

-20

-10

0

10

20

30

M (

mem

u/g

)

H (KOe)

THIOL

AMINE

TOPO

5K

a

-60 -40 -20 0 20 40 60-4

-3

-2

-1

0

1

2

3

M (

mem

u/g

)

H (KOe)

THIOL

AMINE

TOPO

5 K

b

0 50 100 150 200 250 300

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

M (

mem

u/g

)

T (K)

THIOL

AMINE

TOPO

H=1000 Oe

c

-60 -40 -20 0 20 40 60-15

-10

-5

0

5

10

15

M (

me

mu

/g)

H (KOe)

THIOL

AMINE

TOPO

5K

d

-60 -40 -20 0 20 40 60-30

-20

-10

0

10

20

30

M (

mem

u/g

)

H (KOe)

THIOL

AMINE

TOPO

5K

a

-60 -40 -20 0 20 40 60-4

-3

-2

-1

0

1

2

3

M (

mem

u/g

)

H (KOe)

THIOL

AMINE

TOPO

5 K

b

0 50 100 150 200 250 300

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

M (

mem

u/g

)

T (K)

THIOL

AMINE

TOPO

H=1000 Oe

c

-60 -40 -20 0 20 40 60-15

-10

-5

0

5

10

15

M (

me

mu

/g)

H (KOe)

THIOL

AMINE

TOPO

5K

d

Different dependence on the molecule

Same correlation electronic structure -magnetism



Origin of the magnetism

Magnetism in ZnO (and other 3d oxides) is explained as due to unbalanced 3d holes with

some exchange interactions

1s

2s

2p

3s

3p

4s

3d

Zn Zn O



XMCD

XMCD ZnO L3 edge (2p→3d)

APS

XMCD ZnO K edge (1s→4p)

http://www.anl.gov/



XMCD

Magnetism is at the 4p -> CONDUCTION BAND



III. Measurement of nanoscale magnetism



Magnetic measurements with lab magnetometers



Los sistemas de magnetometría SQUID, VSM miden por VSM& SQUID detect ALL the magnetic flux across the coil



Valores de Imanación: Fe: 220 emu/g

Co: 160 emu/g

Fe3O4: 83 emu/g

10-5 emu=5·10-8 g de Fe !!!

A 15 μm particle could account for this signal



Kapton tape

-10 -5 0 5 10-6

-4

-2

0

2

4

6

M (

e

mu

/cm

)

H (kOe)-10 -5 0 5 10

-40

-20

0

20

40

M (

e

mu

/cm

)

H (kOe)

(a) (b)

(d)(c)

-10 -5 0 5 10

-15

-10

-5

0

5

10

15

m (

e

mu

)

H (kOe)

-20 -10 0 10 20

-10

0

10

5K

300 K

m (

e

mu

)

H (kOe)

-10 -5 0 5 10-6

-4

-2

0

2

4

6

M (

e

mu

/cm

)

H (kOe)-10 -5 0 5 10

-40

-20

0

20

40

M (

e

mu

/cm

)

H (kOe)

(a) (b)

(d)(c)

-10 -5 0 5 10

-15

-10

-5

0

5

10

15

m (

e

mu

)

H (kOe)

-20 -10 0 10 20

-10

0

10

5K

300 K

m (

e

mu

)

H (kOe)



Kapton tape

-10 -5 0 5 10-6

-4

-2

0

2

4

6

M (

e

mu

/cm

)

H (kOe)-10 -5 0 5 10

-40

-20

0

20

40

M (

e

mu

/cm

)

H (kOe)

(a) (b)

(d)(c)

-10 -5 0 5 10

-15

-10

-5

0

5

10

15

m (

e

mu

)

H (kOe)

-20 -10 0 10 20

-10

0

10

5K

300 K

m (

e

mu

)

H (kOe)

-10 -5 0 5 10-6

-4

-2

0

2

4

6

M (

e

mu

/cm

)

H (kOe)-10 -5 0 5 10

-40

-20

0

20

40

M (

e

mu

/cm

)

H (kOe)

(a) (b)

(d)(c)

-10 -5 0 5 10

-15

-10

-5

0

5

10

15

m (

e

mu

)

H (kOe)

-20 -10 0 10 20

-10

0

10

5K

300 K

m (

e

mu

)

H (kOe)

Cinta Kapton sin bolsa: m>2μemu ~60-70%

Cinta Kapton en bolsa hermética: m>2μemu ~10-20%



Kapton tape

-10 -5 0 5 10-6

-4

-2

0

2

4

6 As received

30 min exposed to air

M

(e

mu

/cm

)

H (kOe)



Kapton tape

-10 -5 0 5 10

-15

-10

-5

0

5

10

15 Stored in air

Removing edges

m (

e

mu

)

H (kOe)



Metallic twizzers

(a)(b)

-10 -5 0 5 10-200

-100

0

100

200

Clean

Iron tweezers pressed

m (

e

mu

)

H (kOe)

-10 -5 0 5 10

-100

-50

0

50

100

m (

e

mu

)

H(kOe)

(a)(b)

-10 -5 0 5 10-200

-100

0

100

200

Clean

Iron tweezers pressed

m (

e

mu

)

H (kOe)

-10 -5 0 5 10

-100

-50

0

50

100

m (

e

mu

)

H(kOe)



Metallic twizeers



-10 -5 0 5 10-3

-2

-1

0

1

2

3

Nylon sieve

Iron sieve

H (kOe)

M (

me

mu

/g)

2.5 m

KeV

Co

un

ts

KeV

Co

un

ts

(a)

(c) (d)

(b)

-10 -5 0 5 10-3

-2

-1

0

1

2

3

Nylon sieve

Iron sieve

H (kOe)

M (

me

mu

/g)

2.5 m

KeV

Co

un

ts

KeV

Co

un

ts

(a)

(c) (d)

(b)

Metallic twizzers



Cotton

-20 -10 0 10 20-8

-4

0

4

8

-0.2

-0.1

0.0

0.1

0.2

M (

em

u/g

)

m

(m

em

u)

H (KOe)



Silver paint

-8 -4 0 4 8-1.0

-0.5

0.0

0.5

1.0

M(e

mu

/g)

H (KOe)

Tintas de rotulador (roja !!!) ~4 x 10-5 emu



Plastic sampleholders

-10 -5 0 5 10

-4

-2

0

2

4

New straw

Deformed straw

m (

e

mu

)

H (KOe)

300K



Sample position

dR

m

0.0 0.2 0.4 0.6 0.8 1.0

1.0

1.5

2.0

2.5

3.0

Ma

gn

etic f

lux (

arb

. u

nits)

d/R

(b)(a)

dR

m

0.0 0.2 0.4 0.6 0.8 1.0

1.0

1.5

2.0

2.5

3.0

Ma

gn

etic f

lux (

arb

. u

nits)

d/R

(b)(a)



Sample position: Anisotropy?

-10 -5 0 5 10

-100

-50

0

50

100

H perpendicular

H parallel

M (

e

mu

)

H (kOe)-10 -5 0 5 10

-200

-100

0

100

200 H perpendicular

H parallel

M (

e

mu

)H (kOe)

(a)

(b)

-10 -5 0 5 10

-100

-50

0

50

100

H perpendicular

H parallel

M (

e

mu

)

H (kOe)-10 -5 0 5 10

-200

-100

0

100

200 H perpendicular

H parallel

M (

e

mu

)H (kOe)

(a)

(b)



Element specific techniques are required for confirmation -XMCD .Neutrons

For reliability

For a better understanding



XMCD in Au Nanoparticles



XMCD & Neutrons in Au Nanoparticles



XMCD in Au Nanoparticles



XMCD in ZnO nanoparticles

XMCD ZnO L3 edge (2p→3d)

APS

XMCD ZnO K edge (1s→4p)

http://www.anl.gov/



XMCD

Magnetism is at the 4p -> CONDUCTION BAND



….in summary……

Magnetic materials modify their properties at the nanoscale

New magnetic effects appear at the nanoscale

Experimental measurement of nanomagnetism requires

specific techniques and protocols.



APS + IPNS

http://www.bonsai-project.eu

EU 6th FP- Proyecto BONSAI …and money came from….

…..and user programs of:

Proyecto FIS-2008-469

Proyecto PIF

2007-50I015

http://www.anl.gov/

http://www.bonsai-project.eu/

http://images.google.es/imgres?imgurl=http://www.dfu.min.dk/dfu/data/filemanager/protect/$00000092_EU_Flag.jpg&imgrefurl=http://www.mpa-eu.net/&h=1067&w=1600&sz=154&hl=es&start=9&um=1&tbnid=ctjUlOD4DB5LqM:&tbnh=100&tbnw=150&prev=/images?q%3Deu%26um%3D1%26hl%3Des%26rlz%3D1T4GZHY_esFR227FR227%26sa%3DN

instituto de cerámica y vidrio csicbenasque.org/2014magnetism/talks_contr/122_magarcia.pdf ·...

Documents