Direct study of domain and domain wall structure in magnetic films and nanostructures

Transcription

1 Direct study of domain and domain wall structure in magnetic films and nanostructures John Chapman, University of Glasgow Synopsis Why use Lorentz microscopy? Magnetisation reversal in soft magnetic films Single magnetic layers Exchange-biased bilayers (RO1d) Domain wall structure (ROIIIb) Domain structures in small elements Shape effects The importance of vortices (ROIIIc) Hybrid magnetic systems

2 Why Lorentz microscopy? Advantages of Lorentz microscopy: high spatial resolution (<5 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

3 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 advantages of Lorentz microscopy

4 Imaging magnetic structures by TEM electron gun electron beam horizontal field B x specimen plane Lorentz lenses rotatable specimen rod 100µm objective lens field gold wire horizontal field specimen CCD camera 2mm

5 Fresnel imaging mode 0 Oe specimen B O Z β L object plane (Fresnel) diffraction plane 3.8 Oe Fresnel intensity divergent convergent image plane x 15µm 20 nm permalloy film; H parallel to hard axis

6 Easy axis reversal of permalloy films deposited under different applied fields H a 1 µm 4 µm 1 µm 0.02 T 0.26 T 0.5 T Coercivity (Oe) Deposition field (T)

7 Variation of magnetisation dispersion during easy axis reversal NiFe, thickness 35 nm; Deposition field 200 Oe Magnetisation ripple is a consequence of the anisotropy in the grains, themselves randomly oriented in plane. Magnetisation reversal is accomplished by a single domain wall sweeping across. The wall moves sufficiently fast that it is not captured in the animated sequence. Ripple magnetisation (arbitrary units) H applied (Oe) H a 1 µm

8 Summary of reversal mechanisms in thin permalloy films Easy axis Hard axis easy axis easy axis Domain wall H a H a Small number of mobile domain walls Larger number of less mobile domain walls

9 The phenomenon of exchange biasing FM FM AFM M M 0 H 0 H H C H C H EB

10 Domains in exchange-biased bilayers 3.5nm Ta cap 10nm Ir 20 Mn 80 4nm Co 90 Fe nm Ta seed substrate H EB = 325 Oe H c = 175 Oe Domain structure midway through an easy axis reversal. Note the small average size of the domains. 2 µm

11 Magnetisation reversal in exchange-biased bilayers CoFe/IrMn bilayer Reversal in outward direction takes place over field range Oe Reversal characterised by: Low mobility walls Small scale, irregular domain structures Residual 360 walls Results suggest a high local variability of exchange bias (magnitude and/or orientation) H EB H a 2 µm

12 Dose test pattern scheme & TRIM calculations 1x x x µm 1x x10 14 TRIM Simulation 30 kev Ga + ions 6x x Ta cap Direction of incidence AFM FM Ta seed 7µm Single Dose Area 3x10 15 Zero Percent Ga + ions stopping µm Depth /nm

13 Variation of magnetisation reversal as a function of ion dose (I) CoFe/IrMn bilayer 3x10 14 Dose test pattern with 30 kev Ga + ions Reversal in outward direction takes place over field range Oe 1x10 14 H EB H a 6x µm

14 Variation of H EB & H c with dose (30 kev ions) 1.0 H EB / H 0 EB Normalised Field H C / H 0 C H EB & H C show monotonic decay with ion dose H EB & H C decay at similar rates initially Ga + dose/ x10 14 ions cm -2

15 Fine scale patterning of Ta/CoFe/Ta film by FIB (attempt 1) H a 0 Oe + 94 Oe µm 2.0 µm 1.0 µm 0.5 µm Oe Oe denotes irradiated at 1x10 15 ions cm -2 Domain walls form at the expected locations but there is effectively no control over the fields at which they form

16 Fine scale patterning of Ta/CoFe/Ta film by FIB (attempt 2) A A A A A A A B B B B B B B B 100 nm 100 nm 35 nm 70 nm 1 µm 1 µm 1 µm denotes irradiated at 5x10 14 ions cm -2 denotes irradiated at 5x10 15 ions cm -2

17 Controlled arrays of domain walls with 100 nm pitch H a H a -148 Oe 0 Oe +19 Oe + 28 Oe +38 Oe + 47 Oe + 56 Oe + 75 Oe + 84 Oe + 93 Oe Oe Oe Domain walls form at the expected locations and the structure remains stable over a field range > 25 Oe Oe Oe denotes irradiated at 5x10 14 ions cm -2

18 Why Lorentz microscopy? Advantages of Fresnel microscopy: high spatial resolution (<5 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

19 Variation of magnetisation dispersion during easy axis reversal a different imaging technique Note that intensity variations in the vicinity of the domain wall suggest it has a complex 2D structure

20 The cross-tie domain wall The cross-tie domain wall has alternating Bloch and Neel segments. The density of cross-ties depends on: film thickness, induced anisotropy, spatial constraints on the film. Note that vortices are an intrinsic part of the cross-tie domain wall.

21 Some magnetic imaging modes in the (S)TEM Fresnel image Foucault images DPC images Coherent Foucault image

22 Foucault imaging mode specimen B O Z β L object plane (Fresnel) diffraction plane Foucault intensity image plane x

23 Rectangular elements - the importance of end domains Permalloy elements, 2000 nm long vortex-type end domains In high aspect ratio (acicular) elements: the magnetisation aligns (predominantly) along the axis a single domain occupies most of the element the switching field is >100x the coercivity of the unpatterned film the switching field depends strongly on element width

24 Differential phase contrast (DPC) imaging probe-forming aperture scan coils B o α B o specimen β L postspecimen lenses de-scan coils 200nm quadrant detector 1000 x 200 nm 2 permalloy element, 40 nm thick

25 Reversal of rectangular permalloy element 4000 x 750 nm 2 permalloy element, 30 nm thick remanent states

26 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 250 nm wide permalloy element, 25 nm thick edge structure encourages further domain formation 0 Oe 320 Oe 0 Oe 160 Oe 250nm Elimination of domain walls is difficult; wall structures can and do modify

27 Reversal of 1000 x 250 x 15 nm 3 permalloy element: rectangular shape 250 nm 295 Oe 0 Oe -101 Oe -249 Oe -295 Oe 300 Oe 0 Oe -100 Oe -200 Oe -300 Oe H

28 Field parallel to the short axis: response of 1000 x 250 x 15 nm 3 permalloy element with rectangular shape 500 nm 590 Oe 204 Oe 0 Oe -100 Oe -305 Oe -590 Oe 500 Oe 200 Oe 0 Oe -420 Oe -440 Oe -500 Oe

29 Field parallel to the short axis: response of 1000 x 250 x 15 nm 3 permalloy element with elliptical shape 500 nm 590 Oe 404 Oe 100 Oe 0 Oe -100 Oe -404 Oe -590 Oe 500 Oe 400 Oe 100 Oe 0 Oe -100 Oe -400 Oe -500 Oe

30 Remanent state: vortex or uniformly magnetised? S C 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 nm 2, energies of S and C states are very similar.

31 Field parallel to the short axis: response of 1000 x 250 x 15 nm 3 permalloy element with rectangular shape (revisited) 500 Oe 200 Oe 0 Oe -100 Oe -300 Oe -500 Oe 500 nm 590 Oe 204 Oe 0 Oe -100 Oe -305 Oe -590 Oe

32 Favouring S state formation by field misalignment 500 nm 590 Oe 204 Oe 0 Oe -240 Oe -590 Oe 10 o H

33 DPC images of the remanent states in the elliptical element 250 nm 50 nm

34 Comparison of vortex structures: experiment and simulation nm Distance nm nm Integrated induction (Tnm) Distance (nm) Line width (nm) Distance nm

35 Ultra-small elements - imaging and simulation nickel elements 40 nm wide, 12 nm thick Simulation courtesy of Thomas Schrefl 200nm out of plane (up) out of plane (down) in-plane 50nm

36 Property modification of Co/Pt using Ga ions θ = 0 Untilted: θ = -30 in-plane magnetic lines revealed; no contrast variation within triangles 1 µm Tilted: aligned in-plane magnetic lines, random perpendicular magnetic triangles

37 Co/Pt multilayer, tilted through 30 with lines written at ions cm -2 Perpendicular magnetisation random, in-plane magnetisation random 1 µm Perpendicular magnetisation random, in-plane magnetisation (horizontal) aligned

38 Co/Pt multilayer with aligned perpendicular magnetisation θ = +30 θ = 0 θ = ions/cm 2 φ θ = 30, φ = -30 θ = 0, φ = ions/cm 2 θ = 30, φ = 0 θ = 0, φ = ions/cm 2 1 µm

39 Why Lorentz microscopy? Advantages of Fresnel and DPC microscopy combined: high spatial resolution (<5 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

40 Acknowledgements Stephen McVitie, Chris Wilkinson, Pat Nicholson, Katherine Kirk, Aurelie Gentils, Maureen MacKenzie, Damien McGrouther, Xiaoxi Liu, Yingang Wang Russell Cowburn (Imperial College, London) Frederik Vanhelmont (Philips Research) Thomas Schrefl, Josef Fidler (TU Wien) Jacques Ferre (Universite Paris-sud) Engineering & Physical Sciences Research Council EU