Seebeck effect and Joule heating in CoFeB/MgO/CoFeB-based perpendicular magnetic tunnel junctions with low resistance area product

Abstract

Perpendicular magnetic tunnel junctions (p-MTJs) have attracted great interest due to their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). Here, the resistance states can be manipulated by an applied current in the order of 109–1010 A m−2, yet the appearance of a heating influence must be understood. In this work, we systematically study the Seebeck effect in nano scale p-MTJs induced due to Joule heating by the tunneling current. The CoFeB/MgO/CoFeB-based p-MTJs were nanofabricated and the current-induced switching was characterized. We find a sign change of the thermovoltage (ΔV) between AP (positive) and P (negative) states, indicating a significant dependence of the Seebeck effect on the magnetic state of the p-MTJ. The temperature distribution in the stack was simulated, by which the Seebeck coefficient (S) and the tunnel magneto-Seebeck ratio were calculated. Our further study indicates that the thermal STT can reduce the switching currents, showing the possibility to re-use this dissipative heating energy. To improve the efficiency of the energy re-use, a method is proposed through the materials optimization of the non-magnetic layers but still retaining high tunneling magnetoresistance effect. Our study shows that the magneto-Seebeck effect plays an important role in the p-MTJs, which can be crucial and must be considered in the design of the high performance p-STT-MRAMs and thermal-assisted MRAMs.