  Weyl semimetals (WSM) have been extensively studied due to their exotic
properties such as topological surface states and anomalous transport
phenomena. Their band structure topology is usually predetermined by material
parameters and can hardly be manipulated once the material is formed. Their
unique transport properties appear usually at very low temperature, which sets
challenges for practical device applications. In this work, we demonstrate a
way to modify the band topology via a weak magnetic field in a ferromagnetic
topological semimetal, Co2MnAl, at room temperature. We observe a tunable,
giant anomalous Hall effect, which is induced by the transition between Weyl
points and nodal rings as rotating the magnetization axis. The anomalous Hall
conductivity is as large as that of a 3D quantum anomalous Hall effect (QAHE),
with the Hall angle reaching a record value (21%) at the room temperature among
magnetic conductors. Furthermore, we propose a material recipe to generate the
giant anomalous Hall effect by gaping nodal rings without requiring the
existence of Weyl points. Our work reveals an ideal intrinsically magnetic
platform to explore the interplay between magnetic dynamics and topological
physics for the development of a new generation of spintronic devices.
