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.

Automated summary

This study systematically investigates the magnetic vortex structures in soft-magnetic NiFe and exchange coupled NiFe/IrMn and IrMn/NiFe/IrMn disk structures. It finds that the stability of the intermediate V-AV state is significantly enhanced in exchange coupled structures, with an increase in effective vortex core radius and distortion of FM vortex structure. The displacement of the vortex core in exchange coupled disks depends on the magnitude of exchange bias field and AFM grain characteristics.