Multisegmented Nanowires: a Step towards the Control of the Domain Wall Configuration

Abstract

Cylindrical nanowires synthesized by controlled electrodeposition constitute excellent strategic candidates to engineer magnetic domain configurations. In this work, multisegmented CoNi/Ni nanowires are synthesized for tailoring a periodic magnetic structure determined by the balance between magnetocrystalline and magnetostatic energies. High-resolution Transmission Electron Microscopy confirms the segmented growth and the sharp transition between layers. Although both CoNi and Ni segments have similar fcc cubic crystal symmetry, their magnetic configuration is quite different as experimentally revealed by Magnetic Force Microscopy (MFM) imaging. While the Ni segments are single domain with axial magnetization direction, the CoNi segments present two main configurations: a single vortex state or a complex multivortex magnetic configuration, which is further interpreted with the help of micromagnetic simulations. This original outcome is ascribed to the tight competition between anisotropies. The almost monocrystalline fcc structure of the CoNi segments, as revealed by the electron diffraction patterns, which is atypical for its composition, contributes to balance the magnetocrystalline and shape anisotropies. The results of MFM measurements performed under in-plane magnetic field demonstrate that it is possible to switch from the multivortex configuration to a single vortex configuration with low magnetic fields.

Figure 1

(a) Chemical mapping though XEDS allows identification of Co and Ni regions. STEM data superimposed to XEDS mapping as well as the separately XEDS maps corresponding to Co and Ni are shown. In (b) and (c) electron diffraction patterns of Co and Ni are presented, respectively. Both Co and Ni grow in the direction as shown.

Figure 2

(a) Topography, (b) geometry sketch and (c) and (d) MFM images of different NWs in as prepared magnetic state. Two different behaviours are shown by different CoNi segments. In (c) the segment in the red square shows a uniform magnetization, while in (d) a multidomain structure is displayed. (e) and (f) schematically show the expected magnetization configurations corresponding to (c) and (d) MFM images, respectively.

Figure 3

(a) Series of MFM images in a remnant state corresponding to the same piece of nanowire with different magnetic history after (a) applying a magnetic field of 1.8 T in the axial direction, (b) demagnetizing in the axial direction, (c) applying a perpendicular magnetic field of 1.8 T, and (d) demagnetizing in perpendicular direction.

Figure 4

Micromagnetic simulations show (a) a vortex along the two CoNi segments and a Ni segment with axial magnetization and (b) multivortex configurations in CoNi segments and a Ni segment with axial magnetization. In both Figures, positive and negative Mx components of the magnetization are depicted in red and blue colours and the grey colour is used for almost zero value. (c) and (d) display the corresponding simulated MFM images of the configurations shown in (a) and (b). Experimental MFM images displaying: (e) the same configuration as in (c) and (f) configuration similar to the one shown in (d).

Figure 5

(a) The sketch shows the layout of the system. (b) Explains images obtained by the non- standard advanced MFM operation mode where the magnetic field parallel to the NW axis is swept between two values while the magnetic signal is recorded along the central line over the NW. CoNi-Ni-CoNi segments are imaged by non-standard advanced MFM showing different behaviour under applied field. (c) and (d) represent two branches of the evolution of the magnetization of the Ni and CoNi segments. The profiles marked in (c) correspond to the configuration measured in the MFM images obtained at that magnetic fields presented in (e). (f) Vortex domain size decreases with the positive or negative increasing field magnitude. Data extracted from images (c) and (d).