Preparation of

(1)

www.fz-juelich.de/iff/staff/Schneider_C_M/Lectures/C_M_Schneider. html

Preparation of

Magnetic Thin Film Systems

(2)

Introduction

Single magnetic films:

Ultrathin mono-crystal films grown by molecular beam epitaxy

Magnetic multilayer structures:

Polycrystalline films and coupling phenomena Complex magnetic layer stacks:

Functionality and Néel-coupling

Outline

(3)

Introduction

(4)

Single films (magnetism in reduced dimensions) Bi-/Trilayers (coupling phenomena)

Multilayers (interfacial contributions)

Complex layered structures (functionality)

Thin films systems of interest

Ta

NiFe CoFeCu CoFe IrMn

Ta SiO₂ Cu

Co

Cu SiO₂

Co Cu SiO₂

Co Cu

(5)

Molecular beam epitaxy (MBE)

Metalorganic molecular beam epitaxy (MOMBE) Magnetron sputter deposition

Ion beam sputter deposition (IBD) Ion beam assisted deposition (IBAD) Pulsed laser deposition (PLD)

Electrochemical deposition

Thin film growth techniques: Overview

(6)

• layer-by-layer growth: {001}

• island growth: {111}

Single-crystalline films grown

by molecular beam epitaxy

(7)

ultrahigh vacuum chamber several effusion cells

in-situ growth analysis (RHEED, MEED)

load-lock systems

Molecular beam epitaxy – technology

RHEED electron gun RHEED

fluorescent screen

effusion cells or

electron beam evaporators growth chamber

(ultrahigh vacuum)

substrate (T variable)

‰ kinetic energy of atoms arriving at the substrate ~100 meV

(~1200 K)

‰ rates: 1-10 monolayers/min

(8)

growth in the monolayer regime often dominated by kinetic processes at the surface

thermodynamic aspects (surface free energy) affects intermixing and interfacial sharpness

Thermodynamics vs. Kinetics

‰ thermodynamic considerations

‰ surface free energies are important

‰ kinetic aspects neglected

‰ adsorption

‰ surface diffusion

‰ nucleation

‰ interdiffusion/intermixing

(9)

homoepitaxy

layer-by-layer growth

accumulation of surface roughness oscillation of in-plane lattice spacing

Growth on (001) surfaces: Cu/Cu(001)

from J. Fassbender et al., PRB 75 (1995) 4476

(10)

growth at room temperature small island size in the first layer (heteroepitaxy)

larger island size at higher coverages (homoepitaxy)

higher tendency of double- layer islands in the first monolayer

three layers exposed on average

stabilization of metastable crystalline modification (fcc-Fe)

Morphology of growing films: Fe/Cu(001)

direct observation of growth processes with STM @ 300 K

from M.-T. Lin et al., Surf. Sci. 410 (1998) 290.

(11)

Growth along {001}

fourfold symmetric surface energetically most favorable positions:4-fold hollow sites

small lattice mismatch for 3d metals

Lattice mismatch 2.5%

Ni/Cu(100)

1.7%

Co/Cu(100)

0.6%

afm. Fe/Cu(100)

-0.8%

fm. Fe/Cu(100) Fe/Cu₃Au(100) 3%

a0(Å)

3.75

3.61

3.52 3.59 3.55 3.64 Cu3Au(100)

Cu(100)

Ni b-Co

g-Fe(afm) g-Fe(fm)

(12)

growth mode differs from Fe/Cu

initial bilayer growth

smaller islands at higher coverages

islands have edges

predominantly along [110]

epitaxial strain important

Morphology development: Fe/Cu

₃

Au(001)

0 10 20 30 40 50 60 70 80 90 100

0 1 2 3 4 5 6 7

1 layer 2 layer 3 layer 4 layer 5 layer 6 layer

layer coverage (%)

total coverage (ML)

growth

@ 300 K

1 2 3 4

5

6

sequential layer filling

(13)

magnetooptical Kerr effect in-situ measurements

perpendicular magnetization

spin-reorientation transition with increasing film thickness:

E_tot=E_shape+E_surf+E_cryst+E_me+...

Magnetism: Fe/Cu

₃

Au(001)

-100 -50 0 50

longitudinal Kerr intensity (arb. units)

H (Oe)

Fe/Cu₃Au(100) RT-growth t (ML):

7.3 4.9 3.9 3.8 3.5 2.5

(b) Longitudinal

-200 -100 0 100

polar Kerr intensity (arb. units)

H (Oe)

Fe/Cu₃Au(100) RT-growth t (ML):

1.7

4.9 3.9 3.8 3.6 3.5 3.4 3.0 2.9 2.7 2.5 2.2 2.1

(a) Polar

perpendicular magnetization

in-plane

magnetization

(14)

growth mode determined by film strain

initial bilayer growth mode (missing intensity maximum)

The role of epitaxial strain: NiPd films

4

3

2

1

0

MEED-intensity[arb.units)

750 500

250 0

Ni_xPd_1-x/Cu₃Au(100) growth at T = 300 K

Ni₁₀₀Pd₀ Ni₈₀Pd₂₀ Ni₆₀Pd₄₀ Ni₄₈Pd₅₂ Ni₃₀Pd₇₀ Ni₂₀Pd₈₀

time [sec]

3.5 3.7 3.9

3.6 3.8

0 20 40 60 80 100

u

Pd concentration [%]

Pd_xNi_1-x

Cu₃Au

Cu

compressive strain

tensile strain

latticeparameter[Å]

Medium Energy Electron Diffraction on Ni_xPd_1-x films with varying Pd concentration

(15)

Initial bilayer growth

transition layer-by-layer to island growth

layer-by-layer growth in the “strain-free” regime

The role of epitaxial strain: NiPd films

6 ML Ni

9 ML Ni₈₄Pd₁₆ 9 ML Ni₂₇Pd₇₃

9 ML Pd/Cu₃Au(100)

50 nm x 50 nm

tensile

compressive

(16)

Low coverage: double-layer triangular island, etch pits High coverage: large roughness, 12 layers exposed

Island (Vollmer-Weber) growth mode

Co/Cu(111): Epitaxial growth

50 x 50 nm

0.3 ML 5 ML

from J. Camarero et al., Phys. Rev. Lett., 76 (1996) 4428.

(17)

fcc stacking fault of adjacent islands (ABA vs ACA)

rough films

nucleation on islands

large fraction of hcp stacking percolation only at higher

coverages

Growth on (111) surfaces: Twinning

[111]

[111]–

––

–

twin formation island formation

(18)

surfactant diffuses to surface

low surface free energy (Pb, Bi, Sb, ...) low chemical reactivity

structural regulation magnetically neutral

Surfactant behavior during growth

surfactant

Surfactant properties

(19)

Pb floats on both the Co and Cu film surface 1.5 ML of Pb is optimum coverage

Surfactant action of Pb: Cu(111)

(20)

Medium energy

electron diffraction no intensity

oscillations without surfactant

intensity oscillations with Pb surfactant

initial bilayer growth suppressing first

monolayer intensity maximum

Diffraction results during growth along {111}

(21)

Low coverage: smaller islands (~25Å) of single layer height, higher density (3x10¹²cm^-2), no etch pits

High coverage: only three layers exposed, layer growth

Morphology of surfactant-grown films

50 x 50 nm

0.3 ML 4 ML

(22)

Microscopic mechanism

from J. Camarero et al., PRL 81 (1998) 850

adsorption

exchange into terrace

diffusion by exchange

push-out at step edge

(23)

Antimony has similar effect as Pb, but possibly other microscopic mechanism

Ni/[Sb]Cu(111)

(24)

• Co/Cu multilayers

• interlayer coupling

• giant magnetoresistance

Polycrystalline multilayers grown by magnetron

sputtering

(25)

Rich variety of magnetic coupling phenomena due to electronic and magnetostatic (dipolar) mechanisms

Bi- and trilayer systems: magnetic coupling

parallel alignment

antiparallel alignment

?

(26)

Flat interfaces promote interlayer coupling in {111} oriented trilayer systems

Magnetic properties of (111) oriented films

longitudinal Kerr effect

antiparallel coupling!

(27)

Magnetron sputtering: principle

‰ kinetic energy of atoms arriving at the substrate ~10 eV

‰ higher mobility

‰ rates: 1-10 nm/min

discharge @ 10^-3-10^-2 mbar Ar magnetic field enhances

ionization yield

DC (metals) or RF-stimulated discharge (insulators)

reactive sputtering with gas mixtures (Ar/O₂, Ar/N₂)

(28)

Optimized for multilayer growth

Turntable at constant speed

Deposition of each source controlled by shutter

Magnetron sputtering – methodical aspects

Cu Co

NiO

Pt

FeMn FeNi

Ru CoFe

– 8 independent sputtering sources (DC / RF) – base pressure < 10^-7 mbar

– operating pressure (Ar) ~2-5 x 10^-3 mbar

(29)

layer stacking invisible in focused mode perfect layer structure within grains

{111} texturized grains (–> x-ray diffraction) grains reach through the entire layer stack

structural quality improves with distance to substrate interface

Co/Cu multilayers: morphology

defocused focused energy filtered TEM

Co Cu Transmission electron microscopy

(30)

structural Bragg peak according to multilayer periodicity

“Kiessig” fringes due to reflection at the substrate

Low-angle X-ray diffraction

– 3 nm structural periodicity

– 6 nm magnetic periodicity (afm coupling) – 1. afm max.: [2nm Co / 1nm Cu]

– 2. afm max.: [1nm Co / 2nm Cu]

(31)

Co/Cu multilayers: soft x-ray diffraction

‰ transverse magnetic field

‰ "T-MOKE" geometry

‰ suppression of half order reflex

‰ afm ordered state is broken up in the external field

5 10 15 20 25 30 35

10^-2 10^-1 10⁰ 10¹ 10² 10³

B_x=-41mT B=0 B_x=+41m T

scattering angle q [deg]

intensity[arb.units]

1st order magn. peak

3rd order magn. peak 1st order

chem. peak

2nd order chem. peak

‰ antiparallel magnetic alignment of neighboring Co layers

‰ 1. afm coupling maximum (t_Cu~1nm)

‰ incomplete removal of afm order

at the maximum magnetic field of 41 mT

off-resonant scattering resonant scattering

(32)

Co/Cu multilayers: soft x-ray diffraction

‰ high brilliance beamline (UE-52)

‰ p-pol. light @ Co L₃ edge (776 eV)

‰ 3. and 5. order magnetic peaks indicate excellent multilayer quality

‰ combined structural and magnetic information

10^-12 10^-11 10^-10 10^-9 10^-8 10^-7

reflectedintensity[arb.units]

80 60

40 20

0

angle of incidence q [°]

1.magnetic 3.magnetic 5.magnetic

1.struct. 2.struct. 3.struct.

Co/Cu multilayer 1. afm maximum Co L₃ edge

(33)

Sputtered Co/X multilayers

Interlayer coupling can be observed

with almost every nonmagnetic

metallic spacer Oscillatory

interlayer coupling is a very general phenomenon

Particularly strong coupling occurs with Ru as a spacer

material

Interlayer coupling

from S.S.P. Parkin et al., Phys. Rev. Lett., 67 (1991) 3598.

(34)

afm coupling only for > 10 periods

Co/Cu multilayers: “dead” interfacial layers

-30 -20 -10 0 10 20 30

m0H / mT -40

-20 0 20 40

m/µemu

n = 2

-30 -20 -10 0 10 20 30

m0H / mT -50

0

50 n = 4

-30 -20 -10 0 10 20 30

m0H / mT -50

0 50

n = 5

-30 -20 -10 0 10 20 30

m0H / mT -150

-100 -50 0 50 100 150

n = 10

-30 -20 -10 0 10 20 30

m0H / mT -200

-100 0 100

200 n = 20

-30 -20 -10 0 10 20 30

m₀H / mT -400

-200 0 200 400

n = 35

m/µemu m/µemum/µemu m/µemum/µemu

(35)

coherently strained multilayer up to 600°C

as grown layer stack with {111}

orientation

change of texture {111} –> {100}

at higher sample temperature

Co/Cu multilayers: structural aspects

46 48 50 52 54 56 58 60 62 64 66 10^-2

10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸ 10⁹ 10¹⁰ 10¹¹

Cu {200}

Co {200}

Co {111}

Cu {111}

Co/Cu {200}

Co/Cu {111}

740˚C 640˚C 550˚C 430˚C 380˚C 330˚C

290˚C

Co/Cu [30] ML, in-situ meas. Co K_a

Intens.[cps]

2q [deg]

100 200 300 400 500 600 700 800

10⁰ 10¹ 10²

morph. transition multilayer -> granul.

texture transition

<111> -> <200>

{200}

{111}

integralintens.[cps]

T [˚C]

(36)

parallel alignment: low R

antiparallel alignment: high R

spin-dependent transport through nonmagnetic metallic (GMR) or insulating layer (TMR)

Giant magnetoresistance (GMR)

d << l_S parallel (p)

antiparallel (ap) M

M

Å Ç

(37)

finite information depth of MOKE (~100Å)

antiparallel coupling of Fe layers through Cr interlayer antiparallel alignment is stable ground state

Interlayer coupling: single-crystalline Fe/Cr

from G. Binasch et al., Phys. Rev. B, 39 (1989) 4828.

120 Å Fe/

10 Å Cr/

120 Å Fe

(38)

Bell-shaped resistivity characteristics

GMR in polycrystalline multilayers

10 20 30 40

50 GMR

(R-Rs)/Rs/%

-200 -100 0 100 200 -1.0

-0.5 0.0 0.5 1.0

MOKE

I Kerr/I sat

magnetic field / mT

continuous rotation of the layer magnetization in the applied magnetic field

gradual reduction of the antiparallel ground state antiparallel ground

state = high resistivity state

(39)

Oscillatory interlayer exchange coupling

GMR and interlayer coupling

0 1 2 3 4

t_Cu [nm]

0 50 100

GMR[%]

1. afm-max.

2. afm-max.

3. afm-max.

T=4.2 K o RT

•

+

+ –

interlayer thickness [arb. units]

AFMinterlayercouplingstrength[arb.units]

1. afm max.

2. afm max.

‰ s. lecture by D. Bürgler

(40)

small clusters in the interfacial region

Co/Cu multilayers: Low-temperature behavior

-5 0 5 10 15

B/T

0 5 10 15

GMR/%

T/K 300 250 200 170 140 125 80 50 30 17 9 4.2

-5 0 5 10 15

µ₀H / T -6

-4 -2 0

[r(H=0)–r(H=0)]/µWcm

(Co/Cu)¥60 t_Co=3.5 Å t_Cu=7.6 Å

T = 300 K T = 200 K T = 80 K T = 4.2 K

-1 0 1 2 3 4 5

m₀H / T 0

200 400 600

m/µemu

(Co/Cu)¥60

t_Co= 3.5 Å t_Cu= 7.6 Å T = 10 K T = 300 K

0 1 2 3 4 5

m0H / T 0.8

0.85 0.9 0.95 1

T = 10K T = 300K m(µ0H)/m(5T)

V T>100K:

‚ long “tails”, possibly

superparamagnetic behavior

V T<100K, µ₀H<5T

‚ transition to GMR

(orientation of “canted” Co clusters (< 20 atoms) at the interface ?)

V high field magnetization:

‚ Co is almost fully oriented parallel to the external field

‚ saturation increases at low temperature (Curie

temperature)

(41)

Interlayer coupling very sensitive to interfacial roughness

Role of the interfaces

-1 -0.5 0 0.5 1 1.5

m₀H / T 0

10 20 30 40 50 60 70

GMR/%

(Co/Cu)¥50 tCo/Å tCu/Å EA 11.1 11.5 FA 11.0 11.4 GA 10.8 11.2 HA 10.5 10.9 IA 10.0 10.5 JA 9.6 9.9 KA 9.0 9.3

T = 300K

-1 -0.5 0 0.5 1 1.5 2

m₀H / T -300

-200 -100 0 100 200 300

magneticmomentm/µemu

-1.5 -1 -0.5 0 0.5 1 1.5

ComagnetizationMCo/T

-50 0 50

m₀H / mT

-250 0 250

m/µemu

M_sat(Co_fcc) = 1.62 T

-1 -0.5 0 0.5 1 1.5 2

-1.5 -1 -0.5 0 0.5 1 1.5

-1 -0.5 0 0.5 1 1.5 2

m₀H / T -300

-200 -100 0 100 200 300 /

M_sat(Co_fcc) = 1.62T

-500 0 500

m0H / mT -250

250

-1 -0.5 0 0.5 1 1.5 2

-1.5 -1 -0.5 0 0.5 1 1.5

-1 -0.5 0 0.5 1 1.5 2

m0H / T -200

-100 0 100

200 Msat(Cofcc) = 1.62 T

-500 0 500

m0H / mT -250

0 250

! 90% Co afm coupled

magneticmomentm/µemu ComagnetizationMCo/T

m/µemu

0

magneticmomentm/µemu ComagnetizationMCo/T

m/µemu

40..50% Co afm coupled

V large volume fraction changes from afm to fm coupled state

V nearly constant GMR

‚ interface scattering more important than bulk scattering

(42)

• Tunneling magnetoresistance contacts

Complex magnetic layer

stacks

(43)

Magnetic tunneling junction: Principle

M

H

H_C1 < H_C2

H_C1 H_C2

R

H Each magnetic electrode provides a different functionality

“Hard” layer: magnetic reference

“Soft” layer: magnetic sensor

(44)

Switching fields differ strongly with interfacial conditions

Magnetic tunneling contacts: Morphology

(45)

Interfacial roughness generates local magnetic stray fields, leading to an effective ferromagnetic coupling Coupling strength depends on geometrical and

magnetic properties of each layer

Néel coupling at rough interfaces

H_d = p²h²M_H

√2lt_S exp(−2p√

2 d

l ) × [1 − exp(−2p√

2t_S

l )] × [1 − exp(−2p√

2t_H l )]

correlated roughness

interlayer “soft” layer “hard” layer

‰ insulating barrier excludes sizable interlayer exchange coupling

‰ magnetic tunneling systems are best suited to demonstrate influence of Néel coupling

(46)

Néel-coupling in MTJ important

Independent switching of electrodes is hindered TMR signal is reduced and takes triangular shape

Roughness-induced magnetic coupling

(47)

Modern growth techniques allow a film thickness control in the submonolayer range

Magnetic and spin transport properties are extremely sensitive to interfacial structure and morphology

“Interface” engineering of magnetic systems Access to new magnetic coupling phenomena Basis for technological developments

Preparation of

Preparation of

Magnetic Thin Film Systems

Outline

Introduction

Thin films systems of interest

Thin film growth techniques: Overview

• layer-by-layer growth: {001}

• island growth: {111}

Single-crystalline films grown

by molecular beam epitaxy

Molecular beam epitaxy – technology

Thermodynamics vs. Kinetics

Growth on (001) surfaces: Cu/Cu(001)

Morphology of growing films: Fe/Cu(001)

Growth along {001}

Morphology development: Fe/Cu

Au(001)

Magnetism: Fe/Cu

Au(001)

The role of epitaxial strain: NiPd films

The role of epitaxial strain: NiPd films

Co/Cu(111): Epitaxial growth

Growth on (111) surfaces: Twinning

Surfactant behavior during growth

Surfactant action of Pb: Cu(111)

Diffraction results during growth along {111}

Morphology of surfactant-grown films

Microscopic mechanism

Ni/[Sb]Cu(111)

• Co/Cu multilayers

• interlayer coupling

• giant magnetoresistance

Polycrystalline multilayers grown by magnetron

sputtering

Bi- and trilayer systems: magnetic coupling

?

Magnetic properties of (111) oriented films

Magnetron sputtering: principle

Magnetron sputtering – methodical aspects

Co/Cu multilayers: morphology

Low-angle X-ray diffraction

Co/Cu multilayers: soft x-ray diffraction

Co/Cu multilayers: soft x-ray diffraction

Interlayer coupling

Co/Cu multilayers: “dead” interfacial layers

Co/Cu multilayers: structural aspects

Giant magnetoresistance (GMR)

Å Ç

Å Ç

Interlayer coupling: single-crystalline Fe/Cr

GMR in polycrystalline multilayers

GMR and interlayer coupling

Co/Cu multilayers: Low-temperature behavior

Role of the interfaces

• Tunneling magnetoresistance contacts

Complex magnetic layer

stacks

Magnetic tunneling junction: Principle

Magnetic tunneling contacts: Morphology

Néel coupling at rough interfaces

Roughness-induced magnetic coupling

Summary