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PublicationSeebeck effect and Joule heating in CoFeB/MgO/CoFeB-based perpendicular magnetic tunnel junctions with low resistance area product( 2022)
;Lv, H. ;Fidalgo, J. ; ;Langer, J. ;Wrona, J. ;Ocker, B. ;Freitas, P.P.Cardoso, S.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.
PublicationMulti‐Level Switching and Reversible Current Driven Domain‐Wall Motion in Single CoFeB/MgO/CoFeB‐Based Perpendicular Magnetic Tunnel Junctions( 2021)
;Lv, H. ;Fidalgo, J. ;Silva, A.V. ;Leitao, D.C. ;Kämpfe, T. ;Langer, J. ;Wrona, J. ;Ocker, B. ;Freitas, P.P.Cardoso, S.One of the critical issues in spintronics‐based technologies is to increase the data storage density. Current strategy is based on shrinking the devices size down to tens of nanometers, or several nanometers, which is reaching its limit. A new proposal is to use multi‐level cells (MLCs) to store more than two bits in each cell. In this work, the multi‐level switching is realized in CoFeB/MgO/CoFeB based nano‐scale single perpendicular magnetic tunnel junctions (p‐MTJs) with three or four stable resistance states. A large range of writing currents for each state is obtained, accompanying with a good repeatability of set‐reset operations between different states. The developed multi‐domain model perfectly matches the experimental results, reflecting the magnetic behaviors during multi‐level switching. Furthermore, current‐driven domain wall (DW) motion is revealed in the circular p‐MTJs, where the DW position can be reversibly manipulated by applied current. To design high‐performance multi‐level p‐MTJs, the parameter diagrams are calculated, suggesting various feasible strategies to improve the multi‐level switching through materials optimization and devices geometry. In summary, the demonstration of multi‐level switching in single p‐MTJ shows the high potential of realizing the new generation of p‐MTJ‐based multi‐level spintronic devices, such as multi‐level memories and spin‐neuron devices.