Topological Hall plateau in quasi-2D kagome magnet YMn$_6$Sn$_6$
We examine the impact of the Dzyaloshinskii--Moriya interaction (DMI) in kagome magnets and show that a predominantly planar DMI together with ferromagnetic exchange stabilizes a disordered skyrmion phase in quasi-two-dimensional (2D) YMn$_6$Sn$_6$. Within an {\it{ab initio}} framework combining density functional theory and spin-dynamics simulations, we generate realistic spin textures of disordered skyrmion and find that this phase persists for $B_{\rm ext} \leq 0.5$~T, with a decreasing skyrmion size as magnetic field increases. We demonstrate the emergence of topological Hall plateau in the range $-0.5 \leq B_{\mathrm{ext}} \leq 0.5$~T, driven by nearly uniform scalar spin chirality and the resulting constant real-space Berry curvature. This response is anti-symmetric with magnetic field while magnitude and sign of these plateau are determined by a complex interplay between Hund's coupling strength and chemical potential signifying the role of Dirac points and van Hove singularities. In addition, we reveal topological magnon excitations in the disordered skyrmion phase of quasi-2D YMn$_6$Sn$_6$.