Topological Defect Mediated Helical Phase Reorientation by Uniaxial Stress

Abstract

Strain engineering enables precise, energy-efficient control of nanoscale magnetism. However, unlike well-studied strain-dislocation interactions in mechanical deformation, the spatial evolution of strain-induced spin rearrangement remains poorly understood. Using \emph{in situ} Lorentz transmission electron microscopy, we manipulate and observe helical domain reorientation under quantitatively applied uniaxial tensile stress. Our findings reveal striking similarity to plastic deformation in metals, where the critical stress for propagation vector (\emph{\textbf{Q}}) reorientation depends on its angle with the stress direction. Magnetic defects mediate reorientation via "break-and-reconnect" or "dislocation gliding-annihilation" processes. Simulations confirm that strain-induced anisotropic Dzyaloshinskii-Moriya interaction may play a key role. These insights advance strain-driven magnetism and offer a promising route for energy-efficient magnetic nanophase control in next-generation information technology.

Figures (4)

• Figure 1: Sequence of domain rotation during loading and unloading. (a) Initial LTEM image showing domain distribution before stress application. (b) In-plane magnetization map of helical phase revealing four domains from the dotted rectangle region in (a). The dashed lines indicate the domain wall position and the color represents the in-plane magnetic induction direction. (c)$-$(g) LTEM images illustrate the domain reorientation process. The sequence of reorientating domains is labeled as 1-5. Color inset and a yellow arrow in (c) illustrates the Q and stress ($\sigma$) directions and the quadruple junction, respectively. (h)$-$(i) LTEM images show recovering of the multidomain structure during the unloading process. (j) Temporal stress-strain curve obtained during experiment. Loading conditions in (c)$-$(i) are labeled accordingly. (k) Plot of critical stress $\sigma_{c}$ (left) or strain (right) vs. reorientation angle $\theta$, measured using domains 1$-$5 from (c)$-$(f).
• Figure 2: Mechanisms of domain reorientation under varying stress loads: (a)$-$(c) Break and reconnect: the helix breaks at small $\theta$ [as shown by white arrows in (b)] and then reconnects (c). (d)$-$(f) Dislocation gliding: as stress increases from 91 MPa to 143 MPa, an edge dislocation (yellow arrow, red sign) glides from left to right. A bridge feature emerges transiently during this process (e). (g)$-$(i) Dislocation annihilation: two oppositely signed edge dislocations [red signs in (g)] merge as stress rises from 159 to 248 MPa. Scale bar: 0.2 $\mathrm{\mu}$m.
• Figure 3: Physical origin of the $\mathbf{Q}$ and $\mathbf{\sigma}$ relationship. (a) The energy of the helix phase with edge dislocation ($E$ED) and without edge dislocation ($E$H) as functions of $\theta$ at different $\eta$. The dotted line shows the trajectory of the crossover point (TCP) at which $E$ED = $E$H, when continuously increasing $\eta$. The TCP reaches the upper limit at $\theta \approx 0.2\pi$ when $E$ED = $E$H approaches energy density of ferromagnetic state $E$FM=0. For the region above the TCP line, breaking the helix is favorable since $E$ED < $E$H, and this region lies in the small angle region ($\theta$<0.2$\pi$, the blue shaded region). For large $\theta$, strain promotes the annihilation of EDs since $E$ED> $E$H. The red and yellow arrows show the energy release of ED generation or annihilation, at small or large $\theta$, respectively. (b) and (c) Plot of energy distribution around an individual edge dislocation core in the absence of stress, or in the presence of stress ($\theta=13\pi/32\approx 73^\circ$, $\eta =0.6$), respectively. The imbalanced energy around the edge dislocation under external strain [as shown in (c)] causes it to glide.
• Figure 4: Micromagnetic simulations of the spin reorientation inside the helical phase under uniaxial stress. (a)$-$(c) domain structure evolution of a helix with $\theta = 0$ with increasing $\eta$ from 0 to 0.6. A single-$Q$ helix (a) breaks into a disorder mixture of short helices and skyrmions (b), followed by reconnection, forming a single-$Q$ helix with $\theta \approx \pi\text{/}2\ $in (c). (d)$-$(f) For large angle $\theta = 7\pi\text{/}16$, the reorientation follows the edge dislocation mediated process: the anisotropic DMI pushes the preexisting positive and negative dislocation gliding in opposite directions with increasing $\eta\ $from 0.4 to 0.8.