doi: 10.1038/s41467-019-08327-6.
Dynamics of the Bloch point in an asymmetric permalloy disk
Affiliations
- PMID: 30723192
- PMCID: PMC6363748
- DOI: 10.1038/s41467-019-08327-6
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Dynamics of the Bloch point in an asymmetric permalloy disk
Nat Commun.
.
Abstract
A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes. With the difficulty of generating a stable BP in magnetic nanostructures, the intrinsic nature of a BP and its dynamic behaviour has not been verified experimentally. We report a realization of steady-state BPs embedded in deformed magnetic vortex cores in asymmetrically shaped Ni80Fe20 nanodisks. Time-resolved nanoscale magnetic X-ray imaging combined with micromagnetic simulation shows detailed dynamic character of BPs, revealing rigid and limited lateral movements under magnetic field pulses as well as its crucial role in vortex-core dynamics. Direct visualizations of magnetic structures disclose the unique dynamical feature of a BP as an atomic scale discrete spin texture and allude its influence on the neighbouring spin structures such as magnetic vortices.
Conflict of interest statement
The authors declare no competing interests.
Figures
Direct observation of stabilized Bloch point (BP) structures. a–c Schematic diagram of three possible configurations of a BP with skyrmion charge q = + 1, namely, hedgehog (a), circulating (b), and spiralling (c) configurations where the colour indicates the orientation of magnetization. d In-plane (IP) and out-of-plane (OOP) magnetic components observed by magnetic transmission soft X-ray microscopy (MTXM), and the corresponding magnetic structures determined by micromagnetic simulations. Zoomed images for magnetic configurations near vortex core structures are also inserted. The black and white contrasts in the images of the IP (OOP) magnetic component represent the magnetizations oriented in the left and right directions on the disk plane (perpendicular upward and downward to the disk plane), respectively. e Images of the distorted vortex core structures with no BP (left), a single BP (middle), and double BPs (right), and the configurations of the BPs embedded in the vortex cores. The OOP magnetic components with mz > + 0.8 (red) and mz < −0.8 (blue) were extracted and the BP configurations were obtained by interpolation from the simulated vortex core structures. The single BP is characterized by q = −1, while the double BPs are defined by q = −1 and +1, respectively. The single-BP core structure here is referred to as type I. Scale bars in (d, e) correspond to 500 nm and 50 nm, respectively
Different types of single-Bloch point (BP) core structures. a–d Single-BP core structures with different magnetic configurations, referred to as type I (a), type II (b), type III (c), and type IV (d). The zoomed-in magnetic transmission soft X-ray microscopy (MTXM) images for BP core structures together with simulated images are shown in the first and the second columns (scale bars: 150 nm), respectively. The boundaries between the black and white contrasts are outlined by the red, turquoise, and blue dotted lines. The simulated internal structures are shown in the third column (scale bar: 50 nm), and the configurations of the BPs contained in the vortex core structures are shown in the fourth column. The yellow line outline the central plane in the disk. The categorization is based on the polarizations of the cores on the top surface and bottom surface of the disk, volumes of cores, and the location of the BP
Quasi-static field-driven motions of Bloch points (BPs). a–c Series of images for BP motions driven by applying an external magnetic field from Hx = 0 to +300 Oe with a field step of 0.5 Oe/ns in the +x-direction observed in the non-BP (a), single-BP (b), double-BP (c) cores at Hx = 0, 150, and 300 Oe (scale bar: 500 nm). Damping constant was chosen as α = 0.5 to observe the quasi-static motion of the BP structure as the external magnetic field was slowly increased in the experiment to allow the system to maintain equilibrium. d Relative movements of the vortex cores on the top surface (ΔyTS, red line) and bottom surface (ΔyBS, blue line) of the disk, and of the BP (ΔyBP, black line) along the y-axis together with maximum rates of dm/dt (turquoise line) obtained by the cubic spline interpolation. e Perspective sketch of the quasi-static motions of the vortex cores and BP at the length scale of 0.2 nm
Time-resolved measurements for the dynamics of a Bloch point (BP). a, b Representative images of the vortex core structures with (a) and without (b) a BP (the first row) along with positions of core structures measured at t = 0, 2, 3.5, 4.5, 8, 9 ns during their dynamic processes triggered by the injection of a field pulse with an amplitude of 50 Oe and the width of 3 ns in the +x-direction. The blue and red dotted lines in zoomed images correspond to the initial position of the core structures with and without BP. The red and blue lines indicate perturbed positions (t > 0) of the single-BP core and non-BP core. Scale bars in (a, b) correspond to 500 nm (representative images) and 200 nm (zoomed images), respectively. c Quantitatively analysed differences between initial (t = 0, black line) and perturbed positions (t > 0) of the single-BP core (blue line) and non-BP core structure (red line). The experimental error bars are taken at one standard deviation. d Trajectory curves of the simulated dynamic motions of the BP and vortex cores with up (VC↑) and down (VC↓) polarizations in the single-BP and non-BP cores (scale bar: 50 nm). e, f The variations of normalized y-components of magnetizations, < my > = < My/Ms > (e) and total energy (f) during the dynamic relaxation processes of the single-BP and non-BP cores. Raw data of Fig. 4c are provided as the Source Data files Source Data
References
-
- Xu SY, et al. Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator. Nat. Phys. 2012;8:616–622. doi: 10.1038/nphys2351. - DOI
-
- Volovik, G. E. The Universe in a Helium Droplet (Oxford University Press, Oxford, 2003).
