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]

M M M M M M M M M M Schematic of Bloch wall Schematic of Neel wall Schematic of cross-tie wall Schematics of 180 domain wall structures

Fresnel imaging mode 20 nm permalloy film; H parallel to hard axis 15 m 3.8 Oe

B o B o L probe-forming aperture scan coils specimen de-scan coils post- specimen lenses quadrant detector Direction of induction mapped 5 m Differential phase contrast (DPC) imaging of a 180 domain wall in a soft magnetic film

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) SVisolated layer w (nm) 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 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 CoFe 50 nm NiFe 1 nm ML1 CoFe 22.5 nm NiFe 1 nm Al 2 O nm ML2 CoFe 10 nm NiFe 1 nm Al 2 O 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. ML4 CoFe 10 nm NiFe 1 nm Al 2 O nm

CoFe 50 nm NiFe Easy and hard axis magnetisation reversals – ML1 easy axis HaHa -20Oe +25Oe -13Oe -14Oe 3 m -15Oe easy axis HaHa -10Oe -8Oe +41Oe +10Oe +3Oe 3 m H c 15 Oe

Easy and hard axis magnetisation reversals – ML2 +32 Oe+9 Oe-1 Oe-5 Oe-21 Oe 2 m easy axis HaHa CoFe 22.5 nm NiFe Al 2 O Oe -41 Oe -4 Oe -2 Oe +8 Oe easy axis HaHa 2 m H c 5.3 Oe

Easy axis Hard axis easy axis HaHa HaHa 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

Easy and hard axis magnetisation reversals – ML3 CoFe 10 nm NiFe Al 2 O 3 H c 2.8 Oe easy axis HaHa +31 Oe -31 Oe -2 Oe 0 Oe+7 Oe 1 m easy axis HaHa +32 Oe+15 Oe0 Oe-13 Oe-32 Oe 1 m

Easy and hard axis magnetisation reversals – ML3 +33 Oe+25 Oe+13 Oe0 Oe-33 Oe 2 m easy axis HaHa CoFe 10 nm NiFe Al 2 O Oe -30 Oe -5 Oe +1 Oe+14 Oe easy axis HaHa 2 m H c 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 nm NiFe 1 nm Al 2 O nm easy axis HaHa -30 Oe-11 Oe0 Oe+14 Oe+28 Oe 3 m easy axis HaHa -30 Oe-11 Oe0 Oe+2 Oe+28 Oe 3 m H c 3.4 Oe

New wall forms Wall disappears Small reverse H Reduce H H = 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 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 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 250 nm 50 nm 12 Distance nm 9 nm

Metastable states in rectangular elements On application of field H C-state S-state C C S S 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 M 0 Oe +25 Oe -15 Oe-40 Oe +H 250nm Dimensions of central section: 1000 x 200 nm 2

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 Thickness 20 nm Thickness 30 nm

Wires with constrictions – the reversal process Field range: -250 Oe to +250 Oe then back to –250 Oe

H ext Oe Oe Oe Oe- 230 Oe Wires with constrictions – the reversal process w l w = 500 nm = 100, 150 nm l = 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 ODonnell (Seagate Technology) Bob McMichael, Bill Egelhoff (NIST)