Bit selection scheme and dipolar interactions in high density precessional MRAM

Precessional switching of the magnetisation is a convenient way to ensure sub-ns, reliable, energy cost efficient writing of an MRAM cell. In this technique, a controlled pulse of magnetic field is applied perpendicular to the easy magnetisation axis of the MRAM cell free layer, which switches the magnetisation from one direction to its reverse. Bit selection and fast direct overwrite can be obtained by superimposing another field pulse along the easy magnetisation axis. It is proposed to change the cell shape while reducing its size to postpone the onset of superparamagnetism. Based on the Landau-Lifshitz-Gilbert equation an analytical theory of this precessional switching process has been developed. This allows the understanding of the minimal switching field and accountingly for the bit selection scheme. A study of how the writing parameter window is affected by a distribution of cell parameters in a array and by the intercell dipolar coupling has been made. A high-pass encoding, and a balanced encoding algorithm are proposed to reduce the maximum intercell dipolar field.