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Helical domain walls in constricted cylindrical NiFe nanowires
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Image of FIG. 1.
FIG. 1.

(a) Simulated magnetizationconfiguration of non-constricted cylindrical NiFe nanowires with diameters 150 nm, 250 nm, and 350 nm at remanent state. Three-dimensional vortex magnetization with clockwise or anticlockwise orientation is present at the ends of the nanowire, while most of the inner part isdominated with magnetization that aligns parallel to the nanowire long axis. When the diameter of the cylindrical nanowire increases, the two vortices gradually extends towards the center of the nanowire and eventually they are connected via a helical domain wall. (b)Simulated magnetization configuration of constricted cylindrical NiFe nanowires with a diameter of 350 nm at remanent state; the constriction dimensions are 250 nm and 150 nm.

Image of FIG. 2.
FIG. 2.

The calculated magnetic volume pole as a function of the position along the long axis of the nanowire of 350 nm diameter. The magnetic volume pole value is obtained by taking the divergence of the magnetization. It is a representation of the magnetic charge present along the nanowire, and the pole sign indicates the chirality of the vortices formed. The transition region between the two peaks represents the helical domain wall. Also shown are the snapshot images of cross-sectional magnetization at various positions from left edge to right edge along the long axis of the nanwoire. The blue circle is the guideline for the vortex cores. The cross-sectional images are tagged with symbols (a)–(e) to represent the corresponding magnetic charge variations in the plot.

Image of FIG. 3.
FIG. 3.

(a) SEM image of constricted Ni95Fe5 (l = 125 nm, Ø = 350 nm)/Ni87Fe13 (l = 50 nm, Ø = 250 nm) nanowires. (b) A free standing individual strand of constricted Ni95Fe5/Ni87Fe13 nanowire with different dimension of constriction. (c), (d) EDX measurements of the NixFe1−x nanowire deposited at applied potentials of −1.4 V and −1.0 V. The composition is determined to be Ni95Fe5 and Ni87Fe13. Inset shows EDX elemental line scanning of Ni and Fe elements (Ni red, Fe blue) along the multilayered Ni95Fe5 and Ni87Fe13nanowires. (e) TEM image of a constricted Ni95Fe5/Ni87Fe13 nanowire. (f) Surface analysis morphology at the tip of the nanowire. The results indicate that the nanowire is polycrystalline with an alternate segment of Ni95Fe5 and Ni87Fe13.

Image of FIG. 4.
FIG. 4.

Hysteresis loop measurements of the constricted Ni95Fe5/Ni87Fe13 nanowires with a magnetic field applied parallel, H (//) and perpendicular, H () to the nanowire axis.

Image of FIG. 5.
FIG. 5.

(a) MFM image of the non-constricted cylindrical NiFe nanowire. Dark and bright magnetic contrasts shown at the two ends of the nanowire indicate the presence of two magnetic vortices with different chiralities. Also shown is the magnetic charge calculation along a non-constricted cylindrical nanowire. As the strength of the magnetic charges is proportional to the stray magnetic fields generated by the vortex magnetization, the magnetic charge property shown in the plot gives clear understanding of the magnetic contrast observation in the MFM image. (b)MFM image of the constricted cylindrical NiFe nanowire. The periodical bright and dark contrast spots represent the two different chirality vortices formed along the nanowire. A close-up view of the MFM image and a simulated magnetization configuration are shown for comparison. The boundary between the dark and bright spots indicates the helical DWs present in the constricted cylindrical nanowire. Shown below is the plot of magnetic charge variation at a single constriction section of along the constricted cylindrical nanowire. Helical domain walls are present at location 1 and 2, and C is the constricted region.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Helical domain walls in constricted cylindrical NiFe nanowires