Ferromagnetics actually can lose their magnetic charge, they are not that pernament. If you have an HDD and you had it stored of several years without turning it on (let's say that it's a backup), then you might have trouble with reading some data. HDD controller periodically "refreshes" magnetical domain/memory cell so data won't fade out.

Source: https://en.wikipedia.org/wiki/Data_degradation#Data_degradation_in_storage