  Magnetic topological materials have attracted much attention due to the
correlation between topology and magnetism. Recent studies suggest that
EuCd$_2$As$_2$ is an antiferromagnetic topological material. Here by carrying
out thorough magnetic, electrical and thermodynamic property measurements, we
discover a long time relaxation of the magnetic susceptibility in
EuCd$_2$As$_2$. The (001) in-plane magnetic susceptibility at 5 K is found to
continuously increase up to $\sim$10% over the time of $\sim$14 hours. The
magnetic relaxation is anisotropic and strongly depends on the temperature and
the applied magnetic field. These results will stimulate further theoretical
and experimental studies to understand the origin of the relaxation process and
its effect on the electronic structure and physical properties of the magnetic
topological materials.
