Electron [and ion] beam studies of
magnetic nanostructures
John Chapman, Department of Physics & Astronomy, University of Glasgow
Synopsis
Domain wall structures
Investigation by Lorentz microscopy
Domain wall widths in soft films
Magnetisation reversal processes in soft high-moment films
Single layer films
The effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elements
Vortices
The role and elimination of metastable states
Domain wall traps
Notches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
Schematics of 180 domain wall structures
+
-
+
-
+
M
M
M
M
M
Schematic of Bloch wall
M
+
+
+
-
M
M
-
Schematic of Neel wall
+
+
+
M
M
Schematic of cross-tie wall
Fresnel imaging mode
specimen
BO
Z
object
plane
(Fresnel)
L
diffraction plane
3.8 Oe
image plane
Fresnel
intensity
convergent
divergent
x
15mm
20 nm permalloy film; H parallel
to hard axis
Differential phase contrast (DPC) imaging of a
180 domain wall in a soft magnetic film
probe-forming
aperture
scan coils
a
Bo
Bo
specimen
Direction
of
induction
mapped
L
de-scan
coils
postspecimen
lenses
quadrant
detector
5 mm
Experimental and theoretical domain wall widths
in a 8 nm thick permalloy film
a
A
B
300
300
250
250
200
200
150
150
100
100
50
50
0
0
0
b
200
400
600
800
1000
1200
1400
1600
0
200
400
600
800
1000
1200
1400
180° wall profiles in 8 nm thick permalloy: (a) as the free
layer of a spin-valve and (b) as an isolated layer
Fitting function: tanh(2x/w)
SV
w (nm)
150 + 4
300nm
isolated layer
154 + 4
From Hubert (Phys. Stat. Sol. 38, 699, 1969)
w (nm)
157
1600
Synopsis
Domain wall structures
Investigation by Lorentz microscopy
Domain wall widths in soft films
Magnetisation reversal processes in soft high-moment films
Single layer films
The effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elements
Vortices
The role and elimination of metastable states
Domain wall traps
Notches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
High-moment CoFe multilayer films
ML1
CoFe 50 nm
NiFe 1 nm
ML2
CoFe 22.5 nm
NiFe 1 nm
Al2O3 1.5 nm
ML3
CoFe 10 nm
NiFe 1 nm
Al2O3 1.5 nm
ML4
CoFe 10 nm
NiFe 1 nm
Al2O3 1.25 nm
Laminating does not significantly change the total moment of the film but it changes
the magnetisation curve and can lead to lower noise in devices.
Easy and hard axis magnetisation reversals – ML1
20
15
10
5
nWb
CoFe 50 nm
NiFe
Easy
0
Hard
-5
Hc  15 Oe
-10
-15
-20
-100 -80
-60 -40 -20
0
20
40
60
80
100
Oe
3mm
easy
axis
Ha
+25Oe
-13Oe
-14Oe
-15Oe
-20Oe
+10Oe
+3Oe
-8Oe
-10Oe
3mm
easy
axis
Ha
+41Oe
Easy and hard axis magnetisation reversals – ML2
20
15
10
CoFe 22.5 nm
NiFe
Al2O3
nWb
5
Easy
0
Hard
-5
-10
Hc  5.3 Oe
-15
-20
-60
-40
-20
0
20
40
60
Oe
2 mm
easy
axis
Ha
+45 Oe
+8 Oe
-2 Oe
-4 Oe
-41 Oe
+9 Oe
-1 Oe
-5 Oe
-21 Oe
2 mm
easy
axis
Ha
+32 Oe
Easy and hard axis reversal behaviour
Easy axis
easy
axis
Hard axis
Domain wall
easy
axis
Ha
Ha
Small number
of mobile
Larger number
of less mobile
domain walls
domain walls
Néel walls in bilayer films
1 mm
-
+
+
Schematic representation of
twin Néel walls
+1 Oe
-
+
+
-
Schematic representation of
superimposed Néel walls
Easy and hard axis magnetisation reversals – ML3
20
15
10
5
nWb
CoFe 10 nm
NiFe
Al2O3
Easy
0
Hard
-5
-10
Hc  2.8 Oe
-15
-20
-100 -80 -60 -40 -20
0
20
40
60
80
100
Oe
1mm
easy
axis
Ha
+31 Oe
+7 Oe
0 Oe
-2 Oe
-31 Oe
+15 Oe
0 Oe
-13 Oe
-32 Oe
1mm
easy
axis
Ha
+32 Oe
Easy and hard axis magnetisation reversals – ML3
20
15
10
5
nWb
CoFe 10 nm
NiFe
Al2O3
Easy
0
Hard
-5
-10
Hc  2.8 Oe
-15
-20
-100 -80 -60 -40 -20
0
20
40
60
80
100
Oe
2mm
easy
axis
Ha
+33 Oe
+14 Oe
+1 Oe
-5 Oe
-30 Oe
+25 Oe
+13 Oe
0 Oe
-33 Oe
2mm
easy
axis
Ha
+33 Oe
Easy and hard axis reversals of soft magnetic films
Easy axis – NiFeCuMo layer
Easy axis – ML4
Field range for NiFeCuMo film: ±10 Oe
Hard axis – NiFeCuMo layer
Field range for ML4: ±60 Oe
Easy and hard axis magnetisation reversals – ML4
20
15
10
5
nWb
CoFe 10 nm
NiFe 1 nm
Al2O3 1.25 nm
Easy
0
Hard
-5
-10
Hc  3.4 Oe
-15
-20
-100 -80
-60 -40
-20
0
20
40
60
80
100
Oe
3mm
easy
axis
Ha
-30 Oe
-11 Oe
0 Oe
+2 Oe
+28 Oe
-11 Oe
0 Oe
+14 Oe
+28 Oe
3mm
easy
axis
Ha
-30 Oe
Hard axis magnetisation process preserving 360°
domain walls
easy axis
H
H=0
Reduce H
unstable
so corrugates
New 3600 wall forms
High H
Small
reverse H
Corrugations
collapse
Wall disappears
Easy axis magnetisation process preserving 360°
domain walls
easy axis
H
New 3600 wall
forms here
M
small
reversed
after
switch
H
H=0
Wall
disappears
Provided there is something to pin the ends of the 360 walls, their behaviour under an
applied field and high degree of stability is readily comprehensible.
The fact that walls form in particular locations suggests that their origin is closely
related to the local microstructure of the laminated films.
Cross-sectional TEM images of ML1 and ML3
20nm
20nm
Growth direction
Growth direction
Summary of the behaviour of the high-moment CoFe
multilayer films
•
180○ domain walls with cross ties were observed in the single layer films with a
seedlayer, consistent with their 50nm thickness.
•
Much improved magnetisation curves were found for the laminated films.
However, defect areas and 360○ domain walls were also frequently present in
structures with many layers. The comparatively low contrast suggested they did not
exist in all the layers in the multilayer stack.
•
The behaviour and resilience to annihilation of the 360○ domain walls requires strong
pinning at the ends; normal TEM imaging reveals nothing unusual about the regions
where the ends were located.
•
Cross sectional TEM revealed decreasing grain size but increasing roughness with
increasing number of spacer layers. The former is the probable origin of the
decreasing coercivity and the latter of the complex local inhomogeneous
magnetisation distributions that form. Local fields >100 Oe are expected where the
roughness is greatest.
Synopsis
Domain wall structures
Investigation by Lorentz microscopy
Domain wall widths in soft films
Magnetisation reversal processes in soft high-moment films
Single layer films
The effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elements
Vortices
The role and elimination of metastable states
Domain wall traps
Notches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
Vortex structures: experiment and simulation
9 nm
1.0
50 nm
9 nm
1.0
0.5
0.5
0.0
-100
9 nm
Distance nm
-50
0
50
100
Distance
nmnm
Distance
0.0
-0.5
-100
-50
0
0.5
0.0
-100
-50
0
0.0
-0.5
-100
-50
0
50
100
-0.5
-1.0
250 nm
-1.0
1.0
1.0
0.5
1
2
50
Distancenm
nm
Distance
50
Distance nm
-0.5
-1.0
100
100
Metastable states in rectangular elements
1μm
S-state
C-state
S-state
C-state
EC  ES
On application of field
S
C
H
S
C
Flux Closure
Domain wall traps
Unlike simply shaped magnetic elements that to a zeroth order approximation are single
domain structures, domain wall traps are (to the same approximation) two domain
structures separated by a head-to-head domain wall.
Various geometries are
possible for the domain
wall packet separating the
oppositely magnetised
domains.
In the traps we have
studied, magnetic vortices
are found frequently.
Domain wall trap based on a compliant vortex
domain wall structure
+25 Oe
0 Oe
Dimensions of central section:
1000 x 200 nm2
+H
M
-15 Oe
-40 Oe
250nm
Movement of domain wall packet in a trap
Field variation from 0 Oe to
-40 Oe to +40 Oe and back
several times
Field variation from 0 Oe to
-100 Oe and back
Reversing “domain wall packet” in a permalloy
wire close to and at a constriction
Wire width: 500 nm; wire thickness: 20 nm
200 nm
Reversing “domain wall packets” in permalloy
wires at constrictions of different geometry
200 nm
Thickness 20 nm
200 nm
Thickness 30 nm
Wires with constrictions – the reversal process
Field range:
-250 Oe to +250 Oe
then back to –250 Oe
Wires with constrictions – the reversal process
d
w
l
0 Oe
w = 500 nm
d= 100, 150 nm
l = 750 nm
- 116 Oe
- 174 Oe
- 182 Oe
- 230 Oe
Summary
•
Magnetic vortices are found frequently in wall structures in small elements; their
core is typically <10 nm in extent.
•
Metastable domain configurations that occur in elements with high symmetry can be
eliminated by lowering the element symmetry and/or by the introduction of notches
leading to more reproducible switching behaviour.
•
An alternative bi-state element is the domain wall trap; reproducible behaviour and
lower switching fields can be obtained, but at the expense of a larger element area.
•
Notches (constrictions) in wires also act as local pinning sites; the structure of head-to
-head domain walls in their vicinity is rarely simple and differs from that in the
uniform parts of the wire.
Acknowledgements
Stephen McVitie, Beverley Craig, Craig Brownlie, Aurelie Gentils, Damien McGrouther, Nils
Wiese, Xiaoxi Liu, Chris Wilkinson (University of Glasgow)
Alan Johnston, Denis O’Donnell (Seagate Technology)
Bob McMichael, Bill Egelhoff (NIST)