Distinguishing Local Demagnetization Contribution to the Magnetization Process in Multisegmented Nanowires

Abstract

Cylindrical magnetic nanowires are promising materials that have the potential to be used in a wide range of applications. The versatility of these nanostructures is based on the tunability of their magnetic properties, which is achieved by appropriately selecting their composition and morphology. In addition, stochastic behavior has attracted attention in the development of neuromorphic devices relying on probabilistic magnetization switching. Here, we present a study of the magnetization reversal process in multisegmented CoNi/Cu nanowires. Nonstandard 2D magnetic maps, recorded under an in-plane magnetic field, produce datasets that correlate with magnetoresistance measurements and micromagnetic simulations. From this process, the contribution of the individual segments to the demagnetization process can be distinguished. The results show that the magnetization reversal in these nanowires does not occur through a single Barkhausen jump, but rather by multistep switching, as individual CoNi segments in the NW undergo a magnetization reversal. The existence of vortex states is confirmed by their footprint in the magnetoresistance and 2D MFM maps. In addition, the stochasticity of the magnetization reversal is analysed. On the one hand, we observe different switching fields among the segments due to a slight variation in geometrical parameters or magnetic anisotropy. On the other hand, the stochasticity is observed in a series of repetitions of the magnetization reversal processes for the same NW under the same conditions.

Keywords: magnetic force microscopy; magnetic nanowires; magnetization reversal processes; magnetoresistance.

Figure 1

(a) AFM and (b) MFM image of an individual Co₈₅Ni₁₅/Cu nanowire. Scale bar = 2 μm. (b) Magnetoresistance curves of a nanowire that is under a current of 0.5 mA. Forward direction corresponds to the field applied from −60 to +60 mT, and backward from +60 to −60 mT). Arrows indicate positions of some jumps in magnetization reversal.

Figure 2

(a) Sketch of 2D mode acquisition process; (b) 2D MFM map corresponding to a variation of the field between −9 and +37 mT. (c) Sketch of the magnetic configuration expected for segments in the regions named as I to VI in (b) illustrated using mumax3 data.

Figure 3

(a,b) The 2D modes MFM images corresponding to NW measured by MFM in top panel of (a). Scale bar is 2 μm. The reconstructed hysteresis curves (measured along the profiles marked in (a,b)) are presented in (c,d).

Figure 4

(a) A series of 3D MFM images obtained by sweeping the in-plane magnetic field from 8 to −29 mT. (b) Profiles obtained along the dashed lines (upper panel) and solid lines (bottom panel). The average switching field is marked by a dashed pink line. (c) Successive magnetoresistance curves measured under the same conditions.

Figure 5

Micromagnetic simulations of a tri-segment nanowire following hysteresis cycle with field applied parallel to NW axis. Nanowire consisted of three CoNi segments spaced between Cu layers (a) Magnetization in the z direction of a nanowire at different magnetic field values. (b) Cross-sections of the segments are marked in blue in (a) for fields of 0 and −130 mT.