The impact of kiloelectronvolt ${\mathrm{He}}^{+}$ and kiloelectronvolt ${\mathrm{Ne}}^{+}$ ion irradiation on the magnetic properties of rare earth-3$d$ transition metal amorphous alloys was investigated. The studies were performed for 20-nm-thick ${\mathrm{Gd}}_x{\mathrm{Fe}}_{100-x}$ and ${\mathrm{Tb}}_x{\mathrm{Fe}}_{100-x}$ films with perpendicular magnetic anisotropy, irradiated with doses ranging from $5\times {10}^{13}$ to $5\times {10}^{15}\mathrm{ions}\mathrm{per}\mathrm{c}{\mathrm{m}}^2$. We demonstrate that this approach can be used to engineer magnetic properties such as magnetization, magnetic anisotropy, and compensation temperature in both alloys and to control the spin-flop reorientation transition in GdFe. We find that fine-tuning of the compensation point is possible without losing perpendicular magnetic anisotropy by appropriate selection of ion energy and irradiation dose. The experiments were supported by atomistic simulations revealing that the observed changes can be attributed to selective oxidation of rare earth atoms. Oxidation of 1% of atoms in the lattice gives reasonable agreement to the experimental results for low and medium ion doses.

The microscopic origin of the topological magnon band gap in ${\mathrm{CrI}}_3$ ferromagnets has been a subject of controversy for years since two main models with distinct characteristics, i.e., Dzyaloshinskii-Moriya (DM) and Kitaev, provided possible explanations with different outcome implications. Here, we investigate the angular magnetic field dependence of the magnon gap of ${\mathrm{CrI}}_3$ by elucidating what main contributions play a major role in its generation. We implement stochastic atomistic spin-dynamics simulations to compare the impact of these two spin interactions on the magnon spectra. We observe three distinct magnetic field dependencies between these two gap opening mechanisms. First, we demonstrate that the Kitaev-induced magnon gap is influenced by both the direction and amplitude of the applied magnetic field, while the DM-induced gap is solely affected by the magnetic field direction. Second, the position of the Dirac cones within the Kitaev-induced magnon gap shifts in response to changes in the magnetic field direction, whereas they remain unaffected by the magnetic field direction in the DM-induced gap scenario. Third, we find a direct-indirect magnon band gap transition in the Kitaev model by varying the applied magnetic field direction. These differences may distinguish the origin of topological magnon gaps in ${\mathrm{CrI}}_3$ and other van der Waals magnetic layers. Our findings pave the way for exploration and engineering topological gaps in van der Waals materials.

A highly versatile and scalable path to obtain buried magnetic nanostructures within alloy thin films, while maintaining a flat topography, is described. A magnetic pattern of nanoscale periodicity is generated over ∼cm² areas by employing a B2 → A2 structural transition in the prototype Fe₆₀Al₄₀ thin alloy films. The phase transition was induced in the confined regions via ion-irradiation through self-assembled nanosphere masks. In this way, large area patterns of a hexagonal symmetry of ferromagnetic nanostructures embedded within a paramagnetic Fe₆₀Al₄₀ thin film are realized. The depth and lateral distribution of the induced magnetization was investigated by magnetometry and microscopy methods. Magnetic contrast imaging as well as simulations shows that the obtained magnetic structures are well defined, with the magnetic behavior tunable via the mask geometry.

Defects can significantly affect performance of nanopatterned magnetic devices, therefore their influence on the material properties has to be understood well before the material is used in technological applications. However, this is experimentally challenging due to the inability of the control of defect characteristics in a reproducible manner. Here, we construct a micromagnetic model, which accounts for intrinsic and extrinsic defects associated with the polycrystalline nature of the material and with corrugated edges of nanostructures. The predictions of the model are corroborated by the measurements obtained for highly ordered arrays of circular Co/Pd antidots with perpendicular magnetic anisotropy. We found that magnetic properties, magnetic reversal and the evolution of the domain pattern are strongly determined by density of defects, heterogeneity of nanostructures, and edge corrugations. In particular, an increase in the Néel domain walls, as compared to Bloch walls, was observed with a increase of the antidot diameters, suggesting that a neck between two antidots can behave like a nanowire with a width determined by the array period and antidot size. Furthermore, the presence of edge corrugations can lead to the formation of a network of magnetic bubbles, which are unstable in non-patterned flat films.