Influence of Local Icosahedral Short-Range Order on the Magnetization Dynamics of Amorphous Cobalt-Iron Nanodisks

TL;DR

Using a multiscale MD/SD framework, the study links local icosahedral short-range order to magnetization dynamics in amorphous Co_xFe_{1-x} nanodisks through a distance-dependent exchange J_{ij}(r_{ij}) and a Curie-temperature calibration $T_C^{MF} = \frac{2 \overline{J_0}}{3 k_B}$. The results show cobalt-rich icosahedral motifs create a stiff structural backbone that preserves exchange connectivity and promotes high saturation, while iron-rich regions introduce local disorder that increases damping. These atomistic insights explain ferromagnetic stability in Co–Fe metallic glasses and suggest stoichiometric control as a route to tune damping in amorphous spintronic devices. The approach provides a pathway to design amorphous magnetic nanostructures with tailored dynamic properties for magnonic and spintronic applications.

Abstract

The microscopic origin of soft magnetic properties in amorphous alloys is fundamentally linked to the interplay between local topological disorder and magnetic exchange interactions. In this work, we employ a multiscale Spin-Lattice Dynamics (SLD) approach to investigate the magnetostructural correlations in amorphous Co$_{x}$Fe$_{1-x}$ nanodisks ($x=35, 50, 65$). By integrating classical molecular dynamics with a generalized magnetic Hamiltonian, we capture the dynamic feedback loop between lattice vibrations and spin precession. Topological analysis via Voronoi tessellation reveals a persistent species-dependent structural heterogeneity: Cobalt atoms preferentially adopt "solid-like" icosahedral packing, forming a rigid structural backbone, whereas Iron atoms exhibit a higher propensity for "liquid-like" disordered environments. We demonstrate that this topological disparity dictates the macroscopic magnetic response. The Cobalt-driven structural stiffness preserves a robust exchange network that maximizes saturation magnetization, while the local disorder inherent to Iron-rich regions introduces exchange fluctuations that act as an intrinsic damping mechanism, delaying magnetic relaxation. These findings provide an atomistic explanation for the stability of ferromagnetic order in Co-Fe metallic glasses and offer a pathway for tuning damping parameters in amorphous spintronic devices through stoichiometric control.

Figures (5)

• Figure 1: Structural characterization of the amorphous $Co_{x}Fe_{1-x}$ nanodisk. (a) Atomistic 3D representation of the disk for the $Co_{65}Fe_{35}$ stoichiometry. (b) Partial radial distribution function, $g(r)$, where the sharp first peak and subsequent dampened oscillations confirm the stable amorphous phase.
• Figure 2: Topological analysis of the local atomic environment for the Co$_{50}$Fe$_{50}$ nanodisk. (a) Voronoi polyhedra classification by atomic species, showing the distribution of "solid-like", "transition", and "liquid-like" coordination environments. (b) Frequency distribution of specific Voronoi indices $\langle n_3, n_4, n_5, n_6 \rangle$, highlighting the dominance of icosahedral-like packing motifs such as $\langle 0, 1, 10, 2 \rangle$ and $\langle 0, 0, 12, 0 \rangle$.
• Figure 3: Structural characterization of the iron-rich Co$_{35}$Fe$_{65}$ stoichiometry. (a) Species-dependent Voronoi classification, illustrating the persistent higher degree of local order around Cobalt atoms compared to Iron. (b) Distribution of coordination polyhedra indices, indicating a high prevalence of dense packing units that stabilize the amorphous matrix despite the increased Iron concentration.
• Figure 4: Structural characterization of the Co$_{65}$Fe$_{35}$ stoichiometry. (a) Voronoi polyhedra classification illustrating the dominance of transition and "solid-like" environments. (b) Frequency distribution of Voronoi indices, highlighting the prevalence of icosahedral packing motifs that stabilize the amorphous phase.
• Figure 5: Longitudinal magnetization relaxation $M_z$ as a function of time for Co$_{35}$Fe$_{65}$, Co$_{50}$Fe$_{50}$, and Co$_{65}$Fe$_{35}$ amorphous nanodisks. The curves demonstrate the impact of chemical composition on the saturation magnetization and relaxation kinetics.