Mechanism of Localization of the Magnetization Reversal in 3 nm Wide Co Nanowires

The mechanism of magnetization reversal has been studied in a model system of self-assembled cobalt nanowires with a 3 nm diameter. The structure, orientation and size of grains within the nanowires could be determined by high resolution transmission electron microscopy. The magnetic properties were probed using static and dynamic magnetization measurements. Micromagnetic modeling based on the structural analysis allows us to correlate the structure and the magnetic behavior of the wires, revealing competition between shape anisotropy, magnetocrystalline anisotropy and exchange in the localized reversal within Co hcp oriented grains. These results provide direct experimental evidence of the link between anisotropy fluctuations and reversal localization in nanowires.

Figure 1

(a) Localization of the magnetization reversal due to anisotropy ($K$) fluctuations along the wire axis, $\varphi$ is the angle of the local magnetization with respect to the FNW axis during reversal. (b) Schematic of the FNW assembly studied in this Letter. (c) X-ray absorption spectra at the Co $K$ edge of the FNW assembly and of a reference Co hcp foil. (d) Magnetic hysteresis cycles at 20 K with field parallel (circles) and perpendicular (squares) to the wires.

Figure 2

(a) HRTEM cross-sectional image of an embedded FNW taken along the ceria [$1\overline{1}0$] zone axis. (b)–(d) Local diffraction patterns obtained by FFT in zones labeled 1–3 in (a), with the indexation of these patterns considering multiple diffraction in ceria and hcp Co with $c$ axis $\parallel$ [111] (b), fcc Co aligned with ceria (c), hcp Co with $c$ axis $\parallel$ [$\overline{1}\overline{1}1$] (d). (e),(f) Filtered images (on 0002 Co hcp spots) revealing the distribution of tilted hcp grains inside the FNW.

Figure 3

(a) Variation of the coercive field with the temperature (disks) and fits assuming a temperature independent energy barrier for reversal (black line $m=3/2$ and dashed line $m=2$). (b) Width of the switching field distribution ${\mu }_0\Delta H_{\mathrm{sw}}$ (squares) and magnetocrystalline anisotropy ($K_{1u}$) of hcp Co (thick gray line) as a function of the temperature. Inset: switching field distribution at $T=20\mathrm{K}$. (c) Magnetic viscosity as a function of the temperature. Inset: magnetization decay obtained at 300 K and 0.0625 T. (d) Variation of the activation volume for reversal as a function of the temperature. (e) Schematic illustrating the directions of the $c$ axis in two hcp grains within FNWs and their projection in the (001) plane. (f) Angular dependence of the resonance field recorded by FMR in the (001) plane.

Figure 4

(a) Model anisotropy distribution used to compute the nucleation field. (b) Typical energy filtered plane view TEM image used to measure the mean distance between wires. (c) Size distribution of the hcp Co grains.