  Invisible cloaking is one of major outcomes of the metamaterial research, but
the practical potential, in particular for high frequencies (e.g., microwave to
visible light), is fatally challenged by the complex material properties they
usually demand. On the other hand, it will be advantageous and also
technologically instrumental to design cloaking devices for applications at low
frequencies where electromagnetic components are favorably uncoupled. In this
work, we vastly develop the bilayer approach to create a three-dimensional
magnetic cloak able to work in both static and dynamic fields. Under the
quasi-static approximation, we demonstrate a perfect magnetic cloaking device
with a large frequency band from zero to 250 kHz. The practical potential of
our device is experimentally verified by using a commercial metal detector,
which may lead us to having a real cloaking application where the dynamic
magnetic field can be manipulated in desired ways.
