  Topological quantum materials can exhibit unconventional surface states and
anomalous transport properties. Still, their applications in spintronic devices
are restricted as they require the growth of high-quality thin films with
bulk-like properties. Here, we study 10--30 nm thick epitaxial ferromagnetic
Co$_{\rm 2}$MnGa films with high structural order and very high values of the
anomalous Hall conductivity, $\sigma_{\rm xy}=1.35\times10^{5}$ $\Omega^{-1}
m^{-1}$ and the anomalous Hall angle, $\theta_{\rm H}=15.8\%$, both comparable
to bulk values. We observe a dramatic crystalline orientation dependence of the
Gilbert damping constant of a factor of two and a giant intrinsic spin Hall
conductivity, $\mathit{\sigma_{\rm SHC}}=(6.08\pm 0.02)\times 10^{5}$
($\hbar/2e$) $\Omega^{-1} m^{-1}$, an order of magnitude higher than literature
values of multilayer Co$_{\rm 2}$MnGa stacks [1-3] and single-layer Ni, Co, Fe
[4], and Ni$_{\rm 80}$Fe$_{\rm 20}$~[4,5]. As a consequence, spin-orbit-torque
driven auto-oscillations of a 30 nm thick magnetic film are observed for the
first time, at an ultralow threshold current density of
$J_{th}=6.2\times10^{11}$ $Am^{-2}$. Theoretical calculations of the intrinsic
spin Hall conductivity, originating from a strong Berry curvature, corroborate
the results and yield values comparable to the experiment. Our results open up
for the design of spintronic devices based on single layers of magnetic
topological quantum materials.
