Imaging of domain walls in small shape anisotropy dominated magnetic structures

Internal Traineeship - Michael Beljaars

Outline

Introduction Magnetic Force Microscopy Magnetic Tips Domain imaging Conclusion

Introduction

Motivationmaking fast current switchable magnetic memory (MRAM)

Methodswitching magnetization through domain wall propagation

Intr

oduc

tion

MRAM

Conventional switching with applied magnetic field

Intr

oduc

tion

fixed magnetic layer

insulating barier

switching magnetic layer

applied field applied field

Resistance of the stack depends on magnetization direction of the switching layer

magnetization direction

MRAM

Switching with domain wall propagation

current pulse

domain wall

Intr

oduc

tion

Spin torque effect

Domain wall propagation

Physical Review Letters, Vo. 92, No.7 077205-1 Intr

oduc

tion

Why single domain?

Multi domain means multiple domain walls. Propagation becomes unpredictable.

Uniform magnetization necessary for MTJ: switching between two domains

multiple domain walls single domain wall

Intr

oduc

tion

Roadmap

Create a single domain structure Induce a domain wall by means of a

external field Propagate the domain wall with current

pulses Integrate the structure with a Magnetic

Tunnel Junction (MTJ)

Intr

oduc

tion

Magnetic Force Microscopy

AC MFM First pass: topography Second pass:

magnetic force gradient

Mag

netic

For

ce M

icro

scop

y

Harmonic oscillator

0

00

F k z

k

mω

= −

=

0

0

00

0

'

''

1 1' '

' '

m

m

m m

dFk k

dz

dFkk dz

m m

k dF dF

m m dz m dz

ω

ω ω

ω ω

= +

+= =

= + = +

= +

Free vibration Additional force

Mag

netic

For

ce M

icro

scop

y

0

0

2

3 3

1

2

m

m

m

dFQ

k dz

A Q dFA

dzk

dF

k dz

ϕ

ω ω

∆ ≈

∆ ≈

∆ ≈ −

Mag

netic

For

ce M

icro

scop

y

Magnetic Tips

Nanoworld tips (bought at Nanoworld.com)

Mag

netic

Tip

s –

Nan

owor

ld T

ips

Nanoworld Tips

topographic image magnetic image

Mag

netic

Tip

s –

Nan

owor

ld T

ips

Applying a magnetic field

Mag

netic

Tip

s –

Nan

owor

ld T

ips

Field dependence

5000 0

100 50

10 025 gauss

Field

Mag

netic

Tip

s –

Nan

owor

ld T

ips

One line profile

0 gauss

50 gauss

Signal weakens dramatically

4 deg

1 deg

Mag

netic

Tip

s –

Nan

owor

ld T

ips

Explanation

Damage breaks shape anisotropy

Magnetization rotates due to the applied field

Mag

netic

Tip

s –

Nan

owor

ld T

ips

Magnetic Tips

Sputtered tips (contact tips from NT-MDT)

Mag

netic

Tip

s –

Spu

ttere

d T

ips

Sputtered Tips

Mag

netic

Tip

s –

Spu

ttere

d T

ips

Advantages

Smaller magnetic moment due to partial coating of the tip, less influencing the sample

More local interaction, leading to a higher spatial resolution

Mag

netic

Tip

s –

Spu

ttere

d T

ips

Harddisk

topographic image magnetic image

Mag

netic

Tip

s –

Spu

ttere

d T

ips

Comparison

Nanoworld tips Sputtered tips

Contrast enhancement

Higher resolution

Mag

netic

Tip

s

Conclusions on sputtered tips

Better resolution due to smaller area of interaction

Opportunities for new research to understand the behaviour of these tips and optimise the characteristics (material, layer thickness)

Domain Imaging

Dom

ain

imag

ing

E-beam sample I

Dom

ain

imag

ing

First MFM scan

topographic image magnetic image

multi domain

Dom

ain

imag

ing

~1500 nm

Domain structure at different fields

150 gauss

Dom

ain

imag

ing

160 gauss 170 gauss 0 gauss

0 gauss 50 gauss 75 gauss 100 gauss

direction of the field

E-beam sample II

0,5

μm

0,1 μ

m

0,2 μ

m

1 μm

Dom

ain

imag

ing

Microscope photographs

Dom

ain

imag

ing

structure 1 structure 2 structure 3

structure 4 structure 5 structure 6

Dimension:~55 x 55 μm

Typical width:~1 μm

Microscope photographs

Dimension:23 x 23 μmTypical width:800 – 900 nm

Dimension:27 x 27 μmTypical width:900 – 1000 nm

single domainexpected

Single domain?

topographic image magnetic image

~1

100

nm

Conclusion

Great advantage Single domain for 1 μm or smaller