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 writing device (21). The writing device provides at least one beam of radiation (26) for heating the electro-magnetic material at the bit locations to a programming temperature. The magnetic state of the bit locations is programmed by applying a magnetic field during said heating of selected bit locations via the beams of radiation. Hence the memory device provides a magnetic read-only memory (MROM) that cannot be (re-)programmed without the proper writing device.