Upload
owen-cochran
View
214
Download
1
Tags:
Embed Size (px)
Citation preview
Electron [and ion] beam studies of magnetic nanostructuresJohn Chapman, Department of Physics & Astronomy, University of Glasgow
Synopsis
Domain wall structuresInvestigation by Lorentz microscopyDomain wall widths in soft films
Magnetisation reversal processes in soft high-moment filmsSingle layer filmsThe effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elementsVorticesThe role and elimination of metastable statesDomain wall trapsNotches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
M
M
M M
++
+--
-
M M
++
+-
--
+
++
M M
-
--
M M
Schematic of Bloch wall
Schematic of Neel wallSchematic of cross-tie wall
Schematics of 180 domain wall structures
Fresnel imaging mode
20 nm permalloy film; H parallel to hard axis
15m
3.8 Oe
BoBo
L
probe-formingaperture
scan coils
specimen
de-scancoils
post-specimen
lenses
quadrantdetector
Direction of induction mapped
5 m
Differential phase contrast (DPC) imaging of a 180 domain wall in a soft magnetic film
A B
a
b
300nm
0
50
100
150
200
250
300
0 200 400 600 800 1000 1200 1400 16000
50
100
150
200
250
300
0 200 400 600 800 1000 1200 1400 1600
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 isolated layer
w (nm) 150 + 4 154 + 4
From Hubert (Phys. Stat. Sol. 38, 699, 1969) w (nm) 157
Experimental and theoretical domain wall widths in a 8 nm thick permalloy film
Synopsis
Domain wall structuresInvestigation by Lorentz microscopyDomain wall widths in soft films
Magnetisation reversal processes in soft high-moment filmsSingle layer filmsThe effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elementsVorticesThe role and elimination of metastable statesDomain wall trapsNotches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
High-moment CoFe multilayer films
CoFe 50 nmNiFe 1 nm
ML1
CoFe 22.5 nmNiFe 1 nmAl2O3 1.5 nm
ML2
CoFe 10 nmNiFe 1 nm Al2O3 1.5 nm
ML3
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.
ML4CoFe 10 nmNiFe 1 nmAl2O3 1.25 nm
-20
-15
-10
-5
0
5
10
15
20
-100 -80 -60 -40 -20 0 20 40 60 80 100
Oe
nW
b
Easy
HardCoFe 50 nmNiFe
Easy and hard axis magnetisation reversals – ML1
easy axis
Ha
-20Oe+25Oe -13Oe -14Oe
3m
-15Oe
easy axis
Ha -10Oe-8Oe+41Oe +10Oe +3Oe
3m
Hc 15 Oe
-20
-15
-10
-5
0
5
10
15
20
-60 -40 -20 0 20 40 60
Oe
nW
b Easy
Hard
Easy and hard axis magnetisation reversals – ML2
+32 Oe +9 Oe -1 Oe -5 Oe -21 Oe
2 m
easy axis
Ha
CoFe 22.5 nmNiFe Al2O3
+45 Oe -41 Oe-4 Oe-2 Oe+8 Oe
easy axis
Ha
2 m
Hc 5.3 Oe
Easy axis Hard axis
easy axis
Ha
easy axis
Ha
Domain wall
Small number of mobile domain walls
Larger number of less mobile domain walls
Easy and hard axis reversal behaviour
Néel walls in bilayer films
+-
+-
+ -
+-
Schematic representation of superimposed Néel walls
Schematic representation of twin Néel walls
1 m
+1 Oe
-20
-15
-10
-5
0
5
10
15
20
-100 -80 -60 -40 -20 0 20 40 60 80 100
Oe
nW
b
Easy
Hard
Easy and hard axis magnetisation reversals – ML3
CoFe 10 nmNiFe Al2O3 Hc 2.8 Oe
easy axis
Ha +31 Oe -31 Oe-2 Oe0 Oe+7 Oe
1m
easy axis
Ha +32 Oe +15 Oe 0 Oe -13 Oe -32 Oe
1m
-20
-15
-10
-5
0
5
10
15
20
-100 -80 -60 -40 -20 0 20 40 60 80 100
Oe
nW
b
Easy
Hard
Easy and hard axis magnetisation reversals – ML3
+33 Oe +25 Oe +13 Oe 0 Oe -33 Oe
2m
easy axis
Ha
CoFe 10 nmNiFe Al2O3
+33 Oe -30 Oe-5 Oe+1 Oe+14 Oe
easy axis
Ha
2m
Hc 2.8 Oe
Easy and hard axis reversals of soft magnetic films
Easy axis – NiFeCuMo layer
Hard axis – NiFeCuMo layer
Easy axis – ML4
Field range for NiFeCuMo film: ±10 Oe
Field range for ML4: ±60 Oe
Easy and hard axis magnetisation reversals – ML4
CoFe 10 nmNiFe 1 nm
Al2O3 1.25 nm
easy axis
Ha-30 Oe -11 Oe 0 Oe +14 Oe +28 Oe
3m
easy axis
Ha-30 Oe -11 Oe 0 Oe +2 Oe +28 Oe
3m
Hc 3.4 Oe-20
-15
-10
-5
0
5
10
15
20
-100 -80 -60 -40 -20 0 20 40 60 80 100
Oe
nW
b Easy
Hard
New 3600 wall forms
Wall disappears
Small reverse H
Reduce HH = 0
unstable
so corrugates
High H
Corrugations collapse
easy axis
H
Hard axis magnetisation process preserving 360° domain walls
Easy axis magnetisation process preserving 360° domain walls
H=0
H
small reversed
after switch
New 3600 wall
forms here
Wall disappears
M
easy axis
H
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
Growth direction
20nm
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 structuresInvestigation by Lorentz microscopyDomain wall widths in soft films
Magnetisation reversal processes in soft high-moment filmsSingle layer filmsThe effects of lamination – desirable and otherwise!
Magnetisation reversal processes in magnetic elementsVorticesThe role and elimination of metastable statesDomain wall trapsNotches (constrictions) in magnetic wires
[Domain wall modification by ion irradiation]
Vortex structures: experiment and simulation
250 nm
50 nm
1 2
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
9 n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
9 n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
9 n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
9 n m
- 1 0 0 - 5 0 0 5 0 1 0 0
- 1 . 0
- 0 . 5
0 . 0
0 . 5
1 . 0
D i s t a n c e n m
Distance nm
Distance nm
9 nm
Metastable states in rectangular elements
SC EE On application of field
H
C-state C-stateS-state S-state
C
C
S
S Flux Closure
mμ1
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
M
0 Oe +25 Oe
-15 Oe -40 Oe
+H
250nm
Dimensions of central section:1000 x 200 nm2
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
200 nm
Wire width: 500 nm; wire thickness: 20 nm
Reversing “domain wall packets” in permalloy wires at constrictions of different geometry
200 nm
200 nm
Thickness 20 nm
Thickness 30 nm
500-
100
500-
150
500-
200
Wires with constrictions – the reversal process
Field range:
-250 Oe to +250 Oe
then back to –250 Oe
H ext
500-
100
500-
150
0 Oe - 116 Oe - 174 Oe
- 182 Oe - 230 Oe
Wires with constrictions – the reversal process
w
l
w = 500 nm= 100, 150 nml = 750 nm
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)