demagnetize

verb

to disperse solids in a dense medium that have flocculated magnetically
cause to lose magnetic properties

Wiktionary

verb

Etymology: Etymology tree Proto-Indo-European *de Proto-Indo-European *-h₁ Proto-Indo-European *déh₁ Proto-Italic *dē Latin dē Latin dē-der. English de- Old French magnetebor. Pre-Greekbor.? Ancient Greek Μάγνης (Mágnēs) Ancient Greek μᾰ́γνης (mắgnēs)bor. Latin magnēsbor. Middle English magnete English magnet Proto-Indo-European *-id- Proto-Indo-European *-yéti Proto-Indo-European *-idyéti Proto-Hellenic *-íďďō Ancient Greek -ῐ́ζω (-ĭ́zō)bor. Late Latin -izōder. Middle French -iserbor. Middle English -isen English -ize English magnetize English demagnetize From de- + magnetize.

To make something nonmagnetic by removing its magnetic properties.
To erase the contents of a magnetic storage device.