Explain Curie point and hystereris?

An ordinary piece of iron generally has little or no net magnetic moment. Though, if it is placed in a strong enough external magnetic field, domains will re-orient in parallel with that field, and will remain re-oriented when field is turned off, Hence creating a "permanent" magnet. This magnetization as a function of external field is described by a hysteresis curve. However this state of aligned domains isn't a minimal-energy configuration, it's extremely stable and has been observed to persist for millions of years in seafloor magnetite aligned by Earth's magnetic field (whose poles can thereby be seen to flip at long intervals). Net magnetization can be destroyed by heating and then cooling (annealing) the material without an external field, however.