Magnetic switching behavior of each magnetic layer in perpendicular magnetic tunnel junctions

Abstract

Magnetization of the reference layer (RL) in a normal magnetic tunnel junction (MTJ) has to be fixed below the switching field of the free layer. However, in the fabrication of magnetic random access memory (MRAM) chip consisting of huge number of MTJs, there often exist a few poor MTJ devices which show an abnormal magnetoresistance (RH) behavior. To eliminate chip fails and reduce bit error rate, in this paper magnetic switching behavior of each layer in perpendicular MTJs was studied through detailed RH measurements as a function of maximum applied field (Hmax). Switching behavior of each layer is evaluated through detailed analysis on the magnetostatic interactions of all magnetic layers in MTJs. As a result, three types of abnormal magnetic behaviors of RL and their related physical mechanisms were explored: 1), magnetization of RL tilts in elevated fields due to weak perpendicular magnetic anisotropy (PMA) below the switching field of free layer; 2), RL switches at a field below switching field of the free layer due to weak exchange coupling field (Hex) in the synthetic antiferromagnetic (SAF) structure; 3), RL switch takes place due to so-called flip flop behavior in the SAF structure. Relevant methods to overcome these abnormal magnetic switching behaviors are suggested and used in our successful fabrication of MRAM chips.