Magnetic memories are based on the magnetization of metal compounds whose hysteresis ensures stable states. There are two groups of magnetic memories: permanent, non-volatile memory and volatile memory.
Permanent memory is based on ferrite and iron oxides, as used in magnetic tapes, magnetic tape cartridges, magnetic disks, magnetic disks, magnetic cards, magnetic drums and also magnetic labels. The data words are applied by induction and can be overwritten. In the case of permanent memory, the data is retained even when the memory is disconnected from the supply voltage.
The second group, volatile memories, are based on magnetic fields that must be maintained constantly. When the supply voltage is switched off, the volatile memories lose the stored content. Bubble memories belong to this group.
According to the principle, in magnetic storage the magnetizable storage elements, the ferrites or iron oxide particles, are magnetically influenced by induction. This induction can take place, as in the case of core memory, by means of read/write wires which are passed through minute ferrite beads, or by means of a read/write head, as in the case of floppy disks, magnetic tapes, hard disks, etc. During magnetization, the magnetic material is guided past the read/write heads at a defined speed. During writing, the data is recorded in predetermined tracks according to specific recording procedures. During reading, the induction takes place in the opposite direction: the magnetized particles generate a small induction voltage in the read heads, which is amplified and processed in the sense amplifier.