A memory device has an information plane (32) for storing data bits in a magnetic state of an electro-magnetic material at an array of bit locations (31). The device further has an array of electro-magnetic sensor elements (51) that are aligned with the bit locations. The information plane (32) is programmable or programmed via a separate magnetic writing device (21). In particular a read-only sensor element (60) is described for a read-only magnetic memory.