Title: Chiral magnetic textures in Ir/Fe/Co/Pt multilayers: Evolution and topological Hall signature,
Abstract: Skyrmions are topologically protected, two-dimensional, localized hedgehogs
and whorls of spin. Originally invented as a concept in field theory for
nuclear interactions, skyrmions are central to a wide range of phenomena in
condensed matter. Their realization at room temperature (RT) in magnetic
multilayers has generated considerable interest, fueled by technological
prospects and the access granted to fundamental questions. The interaction of
skyrmions with charge carriers gives rise to exotic electrodynamics, such as
the topological Hall effect (THE), the Hall response to an emergent magnetic
field, a manifestation of the skyrmion Berry-phase. The proposal that THE can
be used to detect skyrmions needs to be tested quantitatively. For that it is
imperative to develop comprehensive understanding of skyrmions and other chiral
textures, and their electrical fingerprint. Here, using Hall transport and
magnetic imaging, we track the evolution of magnetic textures and their THE
signature in a technologically viable multilayer film as a function of
temperature ($T$) and out-of-plane applied magnetic field ($H$). We show that
topological Hall resistivity ($\rho_\mathrm{TH}$) scales with the density of
isolated skyrmions ($n_\mathrm{sk}$) over a wide range of $T$, confirming the
impact of the skyrmion Berry-phase on electronic transport. We find that at
higher $n_\mathrm{sk}$ skyrmions cluster into worms which carry considerable
topological charge, unlike topologically-trivial spin spirals. While we
establish a qualitative agreement between $\rho_\mathrm{TH}(H,T)$ and areal
density of topological charge $n_\mathrm{T}(H,T)$, our detailed quantitative
analysis shows a much larger $\rho_\mathrm{TH}$ than the prevailing theory
predicts for observed $n_\mathrm{T}$.