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)