Direct imaging of distorted vortex structures and magnetic vortex annihilation processes in ferromagnetic/antiferromagnetic disk structures

Chiral spin textures, such as skyrmions, merons, and vortices in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures are actively explored for utilization in future data storage and signal processing devices. Here, we present a systematic study of the magnetic vortex structures in soft-magnetic NiFe and exchange coupled NiFe/IrMn and IrMn/NiFe/IrMn disk structures. The annihilation of the magnetic vortex state is mediated by the creation and subsequent annihilation of the intermediate vortex-antivortex (V-AV) pairs. Using the combination of high-resolution in-field magnetic force microscopy (MFM) and magneto-optic Kerr effect magnetometry, we show a considerable enhancement in the stability of the intermediate V-AV state in exchange coupled NiFe/IrMn and IrMn/NiFe/IrMn disk structures. Analysis of the remanent high-resolution MFM images shows a significant increase in the effective vortex core radius and an additional distortion of the FM vortex structure in exchange coupled disks, most likely caused by the randomly distributed uncompensated spins at the surface of the AFM layer. We further suggest that the displacement of the vortex core from the center of the exchange coupled disks depend on the magnitude of the exchange bias field and AFM grain characteristics. Additionally, we present a summary of crucial magnetic vortex parameters and properties, such as effective core radius, core displacement, handedness, nucleation field, V-AV annihilation field, and interfacial exchange energy in exchange coupled disk structures.

Figure 1

The magnetic vortex annihilation mediated by the creation and annihilation of the V-AV pairs in a static magnetic field, obtained from the micromagnetic simulation for a disk of $2\text{−}\mu \mathrm{m}$ diameter with handedness $cp=+1$. (a) In-plane (IP) spin structure of the vortex with an out-of-plane (OOP) core polarization $p=+1$. (b), (c) Vortex core deflection in the positive $y$ direction as a consequence of the applied magnetic field in the positive $x$ direction. (d) Subsequent creation of the V-AV pairs, followed by an edge vortex state (e) and saturation state (f). (g) Detailed spin structure of the V-AV pairs overlaid on the buckled IP spin texture from the region indicated by an arrow in (d). (h) Schematic for the injection and propagation mechanism of a BP through the film thickness [57]. The original vortex $V_1\left(p=+1\right),$ and the newly nucleated vortex $V_2$ with opposite core polarization ($p=-1)$ [27, 28, 37, 39, 57]. An antivortex with polarization $p=-1$ is created to separate the two vortex cores. A spiraling BP structure for $q=+1$ is shown on the right [30, 42].

Figure 2

Disk structures and the corresponding hysteresis loops, and in-field HR-MFM image sequences. (a)–(c) Disk structures with $4\mu \mathrm{m}$ diameter investigated in this study consisting of a simple FM, AFM/FM, and AFM/FM/AFM, respectively. Scanning electron microscopy (SEM) images of the patterned disk arrays are shown in Supplemental Material Fig. S6 [48]. (d)–(f) Longitudinal MOKE hysteresis loops obtained at 300 K for the FM, AFM/FM, and AFM/FM/AFM disk structures after FC and ZFC procedure, as indicated. The full hysteresis loops are shown as an inset. The brown dashed lines indicate the magnetic field region where the V-AV state can be stabilized. Black arrowheads indicate the magnetic field values at which the in-field MFM images were obtained. In-field MFM image sequences for the simple FM case (g)(i)–(g)(vi), the FM/AFM case for disk 1 (h)(i)–(h)(vi), and disk 2 (i)(i)–(i)(vi), and the AFM/FM/AFM case (j)(i)–(j)(vi) are shown.

Figure 3

Line profile extracted from the MFM images for the vortex state at remanence (left column) and the in-field buckled (V-AV) state in FM, FM/AFM and AFM/FM/AFM disks (right column). (a) and (b) Line profile for the vortex core and the buckled (V-AV) spin structure for the FM disk structure. The line profile is extracted from the position shown in the inset MFM images. (c),(d) The line profile for the FM/AFM case, for disk 1 $\left(cp=-1\right)$, extracted from the MFM images shown in Figs. 2 and 2. (e),(f) Similarly, for disk 2 ($cp=+1$), extracted from the MFM images shown in Figs. 2 and 2. (g), (h) The line profile for the AFM/FM/AFM disk structures, extracted from Figs. 2 and 2. Black triangles indicate the vortex core position on the $4\text{−}\mu \mathrm{m}$ disk. The dashed rectangles indicate the magnetic contrast variation corresponding to the buckled (V-AV) spin texture. The inset in (b) shows a guide to the eye for extracting the number of V-AV pairs from the line profile of the buckled spin texture.