Curved magnetic structures often exhibit unique magnetic behavior both as a result of their curvilinear boundary conditions as well as curvature-induced energy effects, such as the curvature-induced Dzyaloshinskii-Moriya (DM) interaction and curvature induced anisotropy. These structures are also of considerable interest for technological applications. For example, magnetic systems have been proposed for a number of wearable devices, such as wearable magnetoreceptors [1] or wearable touchless sensors [2], and the development of such devices requires an understanding of the effects of curvature in magnetic systems. Such effects are best studied in 3D structures, where the curvature can have a significant spatial extent. Nanowires are relatively simple 3D curved structures capable of exhibiting unique magnetization configurations, especially during magnetization reversal [3], and are well suited for applications requiring integration with electrical contacts. Here we have simulated the magnetization reversal in cylindrical NiFe nanowires and supported the results of these simulations with in situ Lorentz TEM images. We found that the reversal of these nanowires falls into two different regimes, depending on the diameter of the nanowire. For nanowires with a diameter less than or equal to 100 nm, magnetization reversal occurred via coherent rotation of the spins. For nanowires with a diameter greater than or equal to 150 nm, reversal occurred through a winding of the magnetization at the edges that eventually resulted in a double-helix configuration at the zero-field state, shown in Figure 1. The reversal completed through the gradual unwinding of the double helix state. In both regimes, domain wall propagation was not observed to occur during reversal.

Magnetization Reversal in Cylindrical NiFe Nanowires

Discovery of the Dzyaloshinskii-Moriya Interaction (DMI) in magnetic thin films has launched an intense research effort into its effects on the structure of magnetic bubbles and domain walls including the formation of topological excitations like skyrmions. While it is well established that the internal structure of a DW transitions from a Bloch to a Néel domain wall when there is sufficient DMI, less attention has been paid to excitations contained within the domain wall. For example, recently, a new feature known as a domain wall (DW) skyrmion has been theoretically predicted and observed experimentally [1,2]. These are 360 degree rotations of the internal magnetization of a DW, which are the post-DMI analogue of a vertical Bloch line (VBL), where there is a 180 degree rotation. Moving forward, it will be important to probe excitations like these in situ to understand their dynamic behavior. Here we demonstrate the manipulation of VBLs via in-plane magnetic fields observed in situ using Lorentz transmission electron microscopy (LTEM). In this work, (Co/Ni)M multi-layers are deposited via magnetron sputtering directly onto custom-sized Si3N4 TEM membranes. We chose to examine an M=10 sample as we have previously observed an abundance of 1-π and 2-π VBLs along its DWs [3]. We note that 2-π VBLs are topologically equivalent to DW skyrmions. Fresnel mode Lorentz TEM was performed on an FEI Tecnai F20 in Lorentz mode (objective lens turned off). In-plane magnetic fields are applied in situ through the use of a Hummingbird Scientific magnetic biasing holder; the specific holder used here has a field range of ±300 Oe. In the as-prepared state, many VBLs are observed along Bloch DWs in Fresnel-mode micrographs (Fig. 1). Upon application of in-plane magnetic fields of increasing strength, we observe the movement of VBLs along DWs. When the field direction is reversed, we observe the same VBLs to move in the opposite direction. Additionally, we observe the annihilation of VBLs following the impingement of their movement by VBLs of the same winding direction. The winding of these VBLs can be reasoned to be the same sign as one another because those with opposite winding would have a zero-energy barrier to annihilation as seen in Fig. 2. Details regarding challenges to imaging, observations in high-DMI samples, and other implications of this technique will be further discussed in this presentation [4].

**References:**

 [1] R. Cheng, M. Li, A. Sapkota, A. Rai, et al., Phys. Rev. B, Vol. 99, p. 184412 (2019). [2] M. Li, A. Sapkota, A. Rai, A. Pokhrel, et al., arXiv: 2004.07888, (2020). [3] M. Li, D. Lau, M. De Graef, and V. Sokalski, Phys. Rev. Mater., Vol. 3, p. 064409, (2019). [4] This research is supported by the Defense Advanced Research Project Agency (DARPA) program on Topological Excitations in Electronics (TEE) under grant number D18AP00011. The authors also acknowledge use of the Materials Characterization Facility at Carnegie Mellon University supported by grant MCF-677785.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9dd928d7fad895994fe561ade50c29b7.jpg)
Micrographs of magnetic domain walls in [Co/Ni]10 samples with magnetic fields applied in the (left) ``positive" and (right) ``negative" directions. Arrows highlight positions of vertical Bloch lines whereas dotted circles indicate locations of 2-π vertical Bloch lines that have annihilated.

![](https://assets.underline.io/uploads/markdown_image/1/image/4e2179244bbc8d33a73434151e51b02a.jpg)
Micrographs of two vertical Bloch lines ``unwinding" as magnetic field strength is increased. Insets depict magnified view of circled regions with potential domain wall configuration labeled with arrows. Scale bar is 1 μm.

Direct observation of vertical Bloch line manipulation via in-plane magnetic fields

MOA: Nanocomposite Magnetic Materials

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Magnetization Reversal in Cylindrical NiFe Nanowires

Direct observation of vertical Bloch line manipulation via in-plane magnetic fields

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