hysteresis loss and other considerations...
The idea of using Mu-metal to create a magnetic perpetual motion device is as silly as the notion of a flat Earth.
Never can such a device produce even the slightest amount of energy gain... it is always losing energy.
As far as energy production, all energy on the Earth comes from stars... our nearest star, Sol is a gigantic hydrogen-fusion source. If we can engineer a safe fusion reactor we will have virtually limitless energy. Fusion is the best energy production we can conceive of at this time... but even fusion power is not something-for-nothing, it is NOT a perpetual motion system.