Skyrmionics and Meronics Based on Inversion Symmetric Layered Ferromagnets

Here, we will discuss the role of topological spin textures on the electrical and thermal transport properties of layered ferromagnets like Fe_3-xGeTe₂ and Fe_5-xGeTe₂. In Fe_3-xGeTe₂, a layered ferromagnetic displaying a Curie temperature <i>T</i>_c ≈ 220 K, we observe[1] anomalous transport variables for magnetic fields m₀<i>H</i> applied along the gradient of the chemical potential generated by thermal gradients or electrical currents, a configuration that should not lead to their observation due to the absence of Lorentz force. These anomalous planar quantities are found to not scale with the component of the planar magnetization (<i>M</i>_||), showing instead a sharp decrease beyond m₀<i>H</i>_|| = 4 T which is the field required to align the magnetic moments along m₀<i>H</i>||. We argue that locally chiral spin structures, such as skyrmions, and probably hybrid domain walls, lead to a field dependent spin-chirality. This in turn generates a novel type of topological transport in the absence of interaction between the magnetic field and electrical or thermal currents. For Fe_5-xGeTe₂ (<i>T</i>_c ≈ 310-330 K) this unconventional topological transport is observed at, and beyond room temperature, correlating with the observation of merons through Lorentz transmission electron microscopy (LTEM)[2]. LTEM in Fe_5-xGeTe₂ also reveals the coexistence of merons with skyrmions upon approaching its magnetostructural transition at <i>T</i>_s ≈ 110 K, due to a progressive rotation of the magneto-crystalline axial anisotropy towards the c-axis. The presence of these spin textures and their coexistence is exposed by thermal transport measurements as a function of the temperature, which unveils, for example, an anomalously large Nernst effect. Therefore, in Fe_5-xGeTe₂ the formation and stabilization of distinct thermally driven topological spin textures can be chosen on demand by just adjusting the temperature. Given that Fe_5-xGeTe₂ can be grown in large area substrates and its Ni and Co doped crystals display <i>T</i>_cs well above room temperature, we suggest that this compound along with its doped variants, can, not only open a new era in skyrmionics, but also open a new field of research, that of meronics.<br/><br/>[1] J. Macy <i>et al</i>., Appl. Phys. Rev. <b>8</b>, 041401 (2021).<br/>[2] B. Casas <i>et al</i>., (under review).