Equilibrium Magnetic Properties in Magnetic Nanoscrews

Abstract

We investigate the equilibrium magnetization in ferromagnetic nanoscrews (NSw) using micromagnetic simulations. These systems consist of elongated three-dimensional magnetic membranes with helicoidal geometry, combining curvature, torsion ($\mathrm{w}$), and eccentricity ($ε$) along their length. We focus on the influence of these geometric parameters, together with membrane thickness and inner diameter, on remanent states and coercive fields. Our results, obtained over a broad range of eccentricities and torsions, reveal bistable magnetic behavior, with vortex-domain-wall propagation during magnetization reversal. We identify four degenerate configurations of a remarkably stable mixed remanent state. The coercive field is found to increase with eccentricity for structures with a major axis (larger inner diameter) approximately 30\% larger than the minor axis (smaller inner diameter), while remaining largely insensitive to variations in torsion. These findings are interpreted in terms of geometry-induced modifications of surface magnetostatic charges on the membrane mantle. Overall, our results demonstrate that nanoscrews exhibit robust bistability under systematic geometric deformation, together with enhanced coercivity, highlighting their potential for applications in three-dimensional nanomagnetism.

Figures (7)

• Figure 1: Illustration of a nanoscrew. (a) Elliptical cross-section of the nanoscrew formed by the larger and smaller inner diameters ($D_{a_\mathrm{int}}$ and $D_{b_{int}}$) with thickness $t$. (b) A nanoscrew of length $L$ and torsion ($\text{w}=L/\lambda$) related to its pitch $\lambda$. (c) System of cylindrical coordinates ($\rho, \varphi, z$) and the magnetization field oriented by the angles $\theta$ and $\alpha$
• Figure 2: Cross-section average of the magnetization field cylindrical components $\langle M_{z} \rangle$, $\langle M_{\rho} \rangle$ and $|\langle M_{\varphi} \rangle|$ at the nanoscrew ends, as function of the eccentricity $\epsilon$ and torsion $\text{w}$ for a nanoscrew with length $L=4 \mu$m, thickness $t=10$ nm and inner diameter $D_{a_\mathrm{int}}=40$ nm. (a)-(c) Averages at the top end. (d)-(f) Average at the bottom end.
• Figure 3: Cross-section average of the cylindrical magnetization components $\langle M_{z} \rangle$, $|\langle M_{\rho} \rangle|$ and $|\langle M_{\varphi} \rangle|$ at the nanoscrew ends as a function of the eccentricity for two representative torsion w$=1$ and $2.5$. Nanoscreo with an inner diameter $D_{a_\mathrm{int}}=40$ nm and thickness $t=10$ nm.
• Figure 4: Phase diagram of the equilibrium magnetization states of the nanoscrew in terms of its torsion (w) and eccentricity ($\epsilon$). A nanoscrew of length $L=4$$\mu$m and two thicknesses: (a)-(c) $t=10$nm and (d)-(e) $t=20$nm; and diameters: (a) and (d) $D_{a_\mathrm{int}}=40$nm, (b) and (e) $D_{a_\mathrm{int}}=60$nm, (c) and (f) $D_{a_\mathrm{int}}=80$nm. Illustration of the mixed magnetization state with the four configurations according the magnetic vorticity at the nanoscrew ends: black triangle (1,1); gray inverted triangle (-1,-1)); blue full square (1,-1); and blue empty square (-1,1) follow the notation $(a,b)$, where $a$($b$) denotes the magnetization vorticity at the top (bottom) NSw end with $a=\pm 1$ ($b=\pm 1$), where right(left) handed vorticity is denoted with +1(-1) as illustrated.
• Figure 5: Total magnetic energy of the mixed magnetic state in its four configurations at two eccentricities $\epsilon\in\{0.6,0.91\}$, and as a function of the torsion $\mathrm{w}$. A nanoscrew with $L=4$$\mu$m, $D_{a_\mathrm{int}}=40$nm and thickness $t=10$nm.