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Harmonic oscillation of spin structures is a physical phenomenon that offers great potential for applications in nanotechnologies such as nano-oscillators and bio-inspired computing. The effective tuning of oscillations over wide frequency ranges within a single ferromagnetic nanoelement is a prerequisite to realize oscillation-based nanodevices, but it has not been addressed experimentally or theoretically. Here, utilizing a vortex core structure, one of spin structures, we report a drastic change of oscillation modes over the frequency range from MHz to sub-GHz in a 100 nm-thick permalloy circular disk. Oscillation mode was found to considerably depend on the shape and dimension of the vortex core structure and various oscillation modes over a wide range of frequencies appeared with dimensional change in the vortex core structure. This work demonstrates that oscillation modes of the vortex core structure can be effectively tuned and opens a way to apply spin structures to oscillation-based technology.

Author(s): Han, HS; Lee, S; Je, SG; Kang, M; Ok, HJ; Kim, N; Chao, W; Im, MY; Lee, KS | Abstract: Stochasticity in magnetic nanodevices is an essential characteristic for harnessing these devices to computing based on population coding or the building blocks of probabilistic computing, p-bits. A magnetic tunneling junction (MTJ) consisting of a patterned magnetic element is considered a promising computing unit in the concept of artificial neurons and p-bits. A comprehensive understanding of the stochasticity in the switching of patterned magnetic elements is crucial for realizing MTJ-based probabilistic computing technology. In the present work, the stochastic behavior in the switching process of a perpendicularly magnetized Co/Pt disk within an array was directly observed utilizing full-field soft X-ray microscopy. Within 50 repeated hysteretic cycles, the stochastic magnetization switching of individual Co/Pt disks within disk arrays is identified. We found that the stochasticity in the magnetization switching of disks considerably depends on the disk size. The stochasticity initially decreases as the disk radius gets bigger from 125 to 375 nm (region I), then increases with further enlarging the disk size to 625 nm (region II). The variance of thermal fluctuation relevant to the disk size and the multilevel switching within a disk are severely involved in the observed size-dependent stochasticity. This work provides the way for controlling the stochasticity in the switching of nanopatterned elements, which is a key aspect of MTJ-based probabilistic computing.

Magnetic bubbles are circular magnetic domains that may occur in thin magnetic films with perpendicular magnetic anisotropy (PMA). Because they can form with high topological stability and can be manipulated by external driving forces, magnetic bubbles have been considered as prominent information carriers, which are set to 1 or 0, corresponding to the presence or absence. For practical applications, such information carriers must be written and deleted in a specific area of the magnetic thin film. Herein, we report that the magnetic bubbles can be written and deleted using local magnetic fields. By applying a localized magnetic field from the magnetic tip of a magnetic force microscopy to the stripe domain structures of the PMA multilayer, bubbles can be written at room temperature via the transformation from stripe domains to magnetic bubbles. The deleting of the bubbles in the targeted area demonstrated by the local magnetic field accompanied by a uniform external field. Our findings can provide a key for manipulating information carriers in the spintronic device based on topological magnetic structures such as magnetic skyrmions and bubbles.

We realize a magnetic skyrmion diode operated by a unidirectional skyrmion transport that flows in only one direction, which is highly significant for information processing in spintronic and nanoelectronic devices. We easily control the skyrmion transport by engineering asymmetric shapes of geometric structures. Here, we present a simple method to describe the underlying mechanism behind the unidirectional skyrmion transport by characterizing the topography of potential energy surfaces from a purely geometric perspective. Our approach enables a deeper physical insight into skyrmion transport manipulation and efficient design of skyrmion-based devices in geometric structures.

Chirality of the magnetic vortex plays an essential role in dynamic excitations of the magnetic vortex structure. In a harmonic excitation of the vortex gyrotropic motion, it has been known that the chirality determines its phase to the driving force. From our micromagnetic simulations, we find an additional role of chirality in the harmonic excitation of the vortex gyration. The shear deformation of the three-dimensional structure of the vortex core is determined by the chirality of the vortex. We confirm that this is due to the gyrotropic field. For the same vortex core motion with the same polarization but with opposite chirality, it turns out that the opposite gyrotropic field is formed at the spiral magnetization in the vicinity of the vortex core structure.

This study is concerned with the estimation of vibration-field of a cylindrical structure by modal expansion method(MEM). MEM is a technique that identifies modal participation factors using some of vibration signals and natural modes of the structure: The selection of sensor locations has a big influence on predicted vibration results. Therefore, this paper deals with four optimal sensor placement(OSP) methods, EFI, EFI-DPR, EVP, AutoMAC, for the estimation of vibration field. It also finds optimal sensor locations of the cylindrical structure by each OSP method and then performs MEMs. Predicted vibration results compared with reference ones obtained by forced response analysis. The standard deviations of errors between reference and predicted results were also calculated. It is utilized to select the most suitable OSP method for estimation of vibration field of the cylindrical structure.