Imaging of domain walls in small shape anisotropy dominated magnetic structures Internal Traineeship - Michael Beljaars
Jul 21, 2015
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