The strength of a permanent magnet is maintained by the alignment of its magnetic domains. In a permanent magnet, the magnetic domains, which are tiny regions of aligned magnetic moments within the material, are naturally aligned in the same direction, creating a strong magnetic field.
When a permanent magnet is magnetized by induction to form a closed loop, it means that an external magnetic field or another magnet has influenced the alignment of these domains to reinforce the existing magnetic field. The strength of the permanent magnet is maintained because the domains remain aligned even after the external influence is removed.
In simpler terms, the internal structure of the material in a permanent magnet is such that the magnetic moments are naturally aligned, creating a strong and lasting magnetic field. This alignment persists over time, allowing the magnet to maintain its strength.