Wall watching: the progress of domains

1
Wall watching the progress of domains in small
elements
John Chapman, University of Glasgow, Glasgow, UK
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
2
Imaging magnetic structures by TEM
3
Why Lorentz microscopy?

• Advantages of Lorentz microscopy
• high spatial resolution (lt10 nm has been
  demonstrated)
• information on domain and domain wall
  structures
• straightforward image interpretation (usually)
• sensitive to induction (so contrast arises from
  specimen magnetisation and stray fields)
• quantitative information on spatial
  distribution of integrated induction components
• suitable for real time studies involving field
  and temperature variation
• availability of complementary (perfectly
  registered) nanostructural information

4
But..

• Limitations of Lorentz microscopy
• the best that can be achieved from a single
  image is recovery of the phase of the electron
• wave
• phase shifts can be of magnetic or
  electrostatic origin, leading to severe problems
  when
• the latter contribution dominates
• there is no information about components of
  induction parallel to the direction of electron
• travel
• for multilayers, there is no way of separating
  contributions to the images arising from
• individual layers
• there is no contrast from antiferromagnets
• time resolution is poor (typically 1s, best
  20ms)
• There is no single imaging technique that
  overcomes these limitations whilst offering the

5
Fresnel imaging mode
3.8 Oe
15mm
20 nm permalloy film H parallel to hard axis
6
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
7
Domain structures in permalloy films subjected
to different fields
8
Domain structures in permalloy based multilayers
9
Reversal of the free layer of a spin-valve with
crossed anisotropy - variation with field
orientation
10
Asymmetric reversal on the outward and return
paths of the magnetisation cycle
11
Summary of observations on continuous films

• Domain walls form very easily in
  nanopolycrystalline magnetic thin films
• In a single layer film
• reversal frequently involves domain walls
• the behaviour of the walls depends on the
  applied field orientation
• more complex domain structures develop if
  pinning sites area present
• extremely complex magnetisation distributions
  can form in buckled films
• In a multilayer film
• interactions between different magnetic layers
  leads to further complexity
• much smaller scale domain structures may be
  supported
• the reversal mechanism can change significantly
  with applied field orientation
• the reversal mechanism on the outward and
  return parts of a cycle can differ

12
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
13
Modified Stoner Wohlfarth model for insight into
the free layer reversal mechanism

• Modified Stoner-Wohlfarth theory (following
  Labrune and Kools)
• no domains
• coupling between free and pinned layers
  accounted for in
• terms of an offset field Hi
• key parameters are (i) coupling strength hj
  Hi/HK and (ii)
• orientation of applied field Ha with respect
  to the bias
• direction
• Simplified energy equation (assuming a fixed
  pinned layer)
• E K sin2(? - ?) - ?oM.(Ha Hi)
• anisotropy Zeeman

M
H
a
q
j
e
H
i
easy axis
14
Reversal of spin-valve free layer - ? 179

• seed/PtMn/CoFe/Ru/CoFe/Cu/CoFe/NiFe/cap
• bottom spin-valve
• crossed anisotropy, with ? 87
• HK 8 Oe, Hi 12 Oe

Ha
15
Reversal of spin-valve free layer - ? 175

• seed/PtMn/CoFe/Ru/CoFe/Cu/CoFe/NiFe/cap
• bottom spin-valve
• crossed anisotropy, with ? 87
• HK 8 Oe, Hi 12 Oe

Ha
16
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
17
Reversal of exchange-biased layers -
variation with AF thickness
18
Reversal of exchange-biased layers -
variation with AF thickness
19
Summary of domain and domain wall behaviour in
multilayer films
The magnetisation reversal of the free layer of a
SV (or STJ) can proceed by magnetisation
rotation, domain processes or a combination of
both. Domains and domain walls in multilayers
need not behave as do their counterparts in
single layer films. Despite having only one
variable parameter, a modified Stoner-Wohlfarth
model provides insight into the way the
magnetisation reversal proceeds. In some cases,
usually when the coupling between the free and
the other magnetic layers is appreciable, domain
walls are observed at unexpected
orientations. The magnetisation reversal of a
layer exchange-biased to an antiferromagnet
depends critically on the thickness of the
antiferromagnet (and the temperature).
20
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
21
Effect on magnetic properties of patterning
ferromagnetic
antiferromagnetic
non-magnetic

• Contributions to the behaviour of continuous
  films
• Easy axes in individual layers
• Interlayer coupling (exchange or magnetostatic)

• Additional effects from patterning
• Shape anisotropy
• Magnetostatic coupling at edges of layers
• Possible change of materials properties,
  especially near edges

22
Patterning techniques used
23
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
24
Differential phase contrast (DPC) imaging
25
Reversal of rectangular spin-valve element
26
Rectangular elements - the importance of end
domains
Permalloy elements, 2000 nm long

• In high aspect ratio (acicular) elements
• the magnetisation aligns (predominantly) along
  the axis
• a single domain occupies most of the element
• the switching field can be gt100x the coercivity
  of the unpatterned film
• the switching field depends strongly on element
  width

the coercivity is gt100x the
coercivity of an unpatterned film
vortex-type

end domains

the switching field depends
strongly on element width
27
Reversal of rectangular permalloy element
4000 x 750 nm2 permalloy element, 30 nm thick
remanent states
28
Persistence of domains into elements with
sub-250 nm widths
200 nm wide permalloy element, 25 nm thick end
domains are clearly visible - and can still be
seen in elements 100 nm wide
Elimination of domain walls is difficult wall
structures can and do modify
29
Reversal of 1000 x 250 x 15 nm3 permalloy
element rectangular shape
H
30
Reversal of 1000 x 250 x 15 nm3 permalloy
element gently curved ends
31
Reversal of permalloy elements - rectangular
and with gently curved ends
2500 x 500 nm2 permalloy element, 10 nm thick
temperature 150 C
s-shaped domains
vortices
32
Reversal of 1000 x 250 x 15 nm3 permalloy
elements elliptical ends
33
Hysteresis loop of permalloy elements 1000 nm
long - field parallel to long axis
34
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
35
Field parallel to the short axis response of
1000 x 250 x 15 nm3 permalloy element with
rectangular shape
590 Oe
204 Oe
0 Oe
-100 Oe
-305 Oe
-590 Oe
500 Oe
200 Oe
0 Oe
-420 Oe
-440 Oe
-500 Oe
36
Field parallel to the short axis response of
1000 x 250 x 15 nm3 permalloy element with
elliptical shape
37
Remanent state vortex or uniformly magnetised?

• If an S state forms initially, the remanent
  state is uniformly magnetised.
• If a C state forms initially, the remanent
  state contains a vortex.
• For 15 nm thick permalloy elements with
  dimensions 1000 x 250 nm2, energies of S and
• C states are very similar.

38
Field parallel to the short axis response of
1000 x 250 x 15 nm3 permalloy element with
rectangular shape (revisited)
39
Favouring S state formation by field
misalignment
10o
H
40
Hysteresis loops of permalloy elements 1000 nm
long - field parallel to short axis
41
DPC images of the remanent states in the
elliptical element
42
Comparison of vortex structures experiment and
simulation
43
Synopsis Imaging magnetic structures by
TEM Domains in continuous small grain
polycrystalline films single layer, spin-valve
and spin-tunnel junction films the modified
Stoner-Wohlfarth model exchange-biased
bilayers Domains in patterned small grain
polycrystalline films rectangular elements the
importance of end domains switching behaviour as
a function of element shape the effect of fields
parallel to the short axes of elements vortices,
S states and C states vortices in
elliptically shaped elements ultra-small
elements Summary and acknowledgements
44
Ultra-small elements - imaging and simulation
nickel elements 40 nm wide, 12 nm thick
45
Ultra-fine patterning by FIB
Co/Pt multilayer patterned into 1000 nm squares
by FIB - separation of squares 60 nm lines
support in-plane magnetisation, squares support
perpendicular magnetisation
46
Summary
Domains still exist in elements whose widths are
less than the domain wall width in the equivalent
continuous films in general elimination of
domains is difficult! The properties of elements
with sub-micron widths depend strongly on element
geometry. For an element of fixed size, the
switching field can be controlled by selection of
element thickness and end shape. When fields are
applied parallel to the short axis of the
element, different magnetic statesresult
according to the precise field orientation.
Whether an S state or a C state
forms initially determines the remanent state of
the element. Metastable states play a major role
in the behaviour of elements with sub-micron
dimensions. In ultra-small elements the effects
of grain size become increasingly apparent. Ion
beam irradiation offers an exciting way to tailor
the properties of magnetic films.
47
Summary and acknowledgements
Lorentz electron microscopy provides a wealth of
experimental data on magnetisation configurations
in films and elements. Studies of how the
configurations evolve under an applied magnetic
field can be carried out with comparative
ease. The behaviour of domain walls is rarely
predictable although, once observed, an
explanation is frequently found. Wall watching
plays - and will continue to play - a key role in
the advancement of our understanding and
effective use of magnetic films and elements.
Acknowledgements Stephen McVitie, Chris
Wilkinson, Xiaoxi Liu , Katherine Kirk, Peter
Aitchison, Chee Lim, Philippe Gogol, Patrick
Warin, Helene Ardhuin, Gary Yi (University of
Glasgow) Alan Johnston, Denis ODonnell (Seagate
Technology) Frederik Vanhelmont, Murray Gillies,
Jos van Haaren (Philips Research
Laboratories) Jurgen Fassbender, Burkard
Hillebrands (University of Kaiserslautern) Jacques
Ferre, Claude Chappert, Rhonda Hyndman
(Universite Paris-sud) Bill Doyle (University of
Alabama) Thomas Schrefl, Joseph Fidler (Technical
University of Vienna)