Co/Cu = 50nm/50nm

Figure 4.5.10 shows a large field of view of a typical NW for the 50nm/50nm configuration. The phase images present circular features characteristic of the vortices previously observed. There are no significant changes between the two PL and PP

164 Chapter 4: Co/Cu multilayered nanowires

configurations for all observed wires. A total of 70 NWs were selected, of which 15 NW could be observed.

Figure 4.5.10. Reconstruction of a Co/Cu (50nm/50nm): a) hologram, b) EFTEM map, c) phase shift map for PL configuration and d) phase shift map for PP configuration.

Figure 4.5.11 shows the two remnant magnetic states of a representative portion of the NW shown in Figure 4.5.10. The diameter of this part of the NW is 90 ± 2 nm.

For this thickness, the magnetic states are the same no matters the direction of the magnetic field. The magnetization can be interpreted as rotating in the XY plane (image plane). The two circular features showed in Figure 4.5.11 represent the integrated magnetization rotating with opposite directions.

Magnetic configurations in Co/Cu nanowires 165

Figure 4.5.11. Nominal configuration Co/Cu 50nm/50nm: a) TEM image of the observed wire. b) EFTEM image of the same area to distinguish the copper (blue) and cobalt (red) layers. c) Magnetic phase shift map obtained after the application of the perpendicular saturation field with respect to the wire axis. d) Magnetic

phase shift map obtained after the application of the saturation field parallel to the wire axis.

In Figure 4.5.12, we present the fine analysis of the phase image given in Figure 4.5.11d). The simulated phase shift image is shown in the Figure 4.5.12 b), corresponding to the 3D configuration result of the micromagnetic simulation in Figure 4.5.12 d). Micromagnetic calculations demonstrate that the circular features found in the Figure 4.5.12 a) correspond to vortices with the core pointing almost perpendicular to the wire axis. Thus, this circular behaviour of the magnetization is the body of the vortex. A quantitative analysis is performed by the profiles traced over one of the circular

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features in the experimental and simulated phase shift maps. In the Figure 4.5.12 c) the agreement between both profiles can be seen. It is not possible to determine the precise direction (+ or ) of the vortex cores from experimental phase images as these cores are parallel to the electron beam. If the cores are aligned, it should be noticed that the two vortices have a different chirality. Otherwise the chirality is the same but the magnetization is reversed. Micromagnetic simulations indicate that the two vortices have an up polarization leading to a change of chirality between both vortices. However we cannot discard the possibility that both can have different polarization: the analysis of other layers shows similar behaviour.

Figure 4.5.12. Configuration Co/Cu (50nm/50nm): a) Experimental magnetic phase shift map extracted from the holograms. b) Result of the simulated phase shift map. c) Comparison of the experimental and simulated profiles of the magnetic phase shift obtained along the arrows in a) and b). d) 3D magnetic configuration of the

cobalt layers corresponding the simulated phase image in b).

Magnetic configurations in Co/Cu nanowires 167

Other typical features of the Co 50nm/Cu 50 nm system has been observed.

Figure 4.5.13 presents the experimental and simulated phase shift maps for two coupled vortices with the core almost parallel to the wire axis. This NW has a lower diameter of 62nm ± 2 nm and confirms that for this diameter the coupled vortices with the core almost parallel to the wire axis are the most stable state. One of the vortices in the Figure 4.5.13 e) shows a tilt of the core respect to the wire axis: due to the direction of the anisotropy which is lying in a cone of 20° respect to the wire axis. The qualitative agreement between the profiles traced between the experimental and simulated phase maps is presented in Figure 4.5.13 d). A statistical analysis over 15 NWs (each of them composed in average by 6 parts) of the magnetic states present in this configuration shows that about 45% of the NWs present the coupled vortices with the core almost parallel to the wire axis while 55% of them have canted vortices.

Figure 4.5.13. Configuration Co/Cu (50nm/50nm): a) EFTEM map for Co and Cu. b) Experimental magnetic phase shift map extracted from the holograms. c) Result of the simulated phase shift map. d) Comparison of the experimental and simulated profiles of the magnetic phase shift obtained along the rectangles in b) and c). e)

3D magnetic configuration of the cobalt layers corresponding the simulated phase image in c).

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By calculating different micromagnetic configurations, it is possible to conclude that when a cone of an aperture larger than 45° is used for the magnetic anisotropy, the canted vortices with the core pointing perpendicular to the axis of the wires becomes more probable. The same happens with the increase of the diameter and the Co layer thickness.

The orientation of the vortex core is very sensitive to the Co thickness layer , for: 50 more canted vortex states appears while 40 50 vortices with the core aligned along the wire axis are more probable.

For vortices aligned along the wire axis, the magnetic coupling favours a parallel alignment of the cores. On the contrary, canted vortices are less sensitive to the coupling when their cores are oriented an angle >45° with respect to the wire axis. These vortex states present the core pointing in different orientations: along the wire for diameters between 50-60 nm and others lying in angles between 45° and 90° respect to the wire axis (70-90 nm). In particular, the values of magnetocrystalline anisotropy are in the range of 100 300 10 J m⁄ . With an aperture of 30° respect to the axis for the magnetocrystalline anisotropy, most of the configurations can be achieved. The exchange constant and the magnetization of saturation were kept at 22 10 J m⁄ and 1200 10 A m⁄ 1.5T respectively as the previous cases for all the simulations performed in this 50nm/50nm configuration. Table 4.5.1 summarize the relation between the diameters, magnetic states and the magnetic anisotropy for the sample of Co 50nm/ Cu 50nm.

Magnetic configurations in Co/Cu nanowires 169

Table 4.5.1. Relation of diameters, magnetic states corresponding to the different magnetocrystalline anisotropy and the cone of aperture.

50-60 Coupled vortices (vortex core almost // to the wire axis)

100-130 20-30

70-90 Canted vortices ( vortex core almost  to the wire axis)

200-300 30