Abstract: A thermally written magnetic data storage medium retains desired domains of magnetization having field orientations representative of the data. A first magnetic layer retains one or more domains of desired magnetic field orientations. A second magnetic layer provides a biasing field for obtaining a predetermined magnetic field orientation in the first magnetic layer as a function of the temperature of the second magnetic layer. Disposed between the first and second layers is a thermal isolation layer which provides a thermal barrier for controlling the temperature of the second magnetic layer means. When the medium is heated for a short time, the fringe field from the first layer causes one direction of magnetic orientation to occur in a domain in the first layer. Upon cooling of the domain, the domain magnetization becomes stable with an orientation conforming to the orientation of the fringe field. The thermal isolation layer prevents significant heating of the second layer.