Advanced Electron Microscopy of Novel Ferromagnetic Materials and Ferromagnet / Oxide Interfaces in Magnetic Tunnel Junctions

electron microscopy;spintronics;magnetic tunnel junctions

Shi, Fengyuan

University of Wisconsin-Madison

关键词: electron microscopy;    spintronics;    magnetic tunnel junctions;   

Others  :  https://minds.wisconsin.edu/bitstream/handle/1793/79219/Fengyuan%27s%20PHD%20thesis%20FINAL.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: University of Wisconsin

【 摘 要 】

We have studied novel ferromagnetic (FM) materials and FM electrode/tunnel barrier interfaces in magnetic tunnel junctions (MTJs) by advanced electron microscopy including scanning transmission electron microscopy (HRSTEM) and electron energy loss spectroscopy (EELS). MTJs are one of the prototypical spintronic devices, with applications in magnetic random access memory, sensors and read heads. The performance of MTJs depends on several factors, including the FM electrodes and the FM/tunnel barrier interfaces. Therefore, to realize the high performance of MTJs, we first need high quality ferromagnetic electrodes with high spin polarization. High-quality Fe3O4 and Fe4N electrodes with theoretically predicted -100% spin polarization were fabricated by various methods and investigated by HRSTEM and STEM EELS. The Fe3O4 and Fe4N thin films indeed have low defect density and good crystallinity, but when integrated as electrodes in a MTJ, problems emerged. In a Fe4N/AlOx/Fe MTJ, the magnetoresistance was negative, but relatively small, due to a defective Fe3O4 reaction layer formed at the Fe4N/tunnel barrier interface revealed by HRSTEM and EELS. The interfacial reaction layer was thin and discontinuous which made the direct imaging difficult. Therefore, STEM EELS was used to map out the reaction layer. A Fe3O4 reaction layer was also found in a nominally symmetric CoFe/AlOx/CoFe MTJs after annealing, which also exhibited inverse TMR and a non-symmetric bias dependence. We also investigated the MTJs with the Heusler alloy Co2MnSi as one or both electrode and crystalline MgO as the tunnel barrier, which exhibit quite high TMR due to coherent tunneling. We showed that the Co2MnSi/MgO interface in these junctions is dominated by a configuration of a pure Mn plane bonded across the interface to O. This was the first observation of that interface termination. The MnMn/O interface termination has 100% spin polarization which explains in part the high TMR in the junctions, especially at room temperature. HRSTEM images also show that the fraction of MnMn/O interface termination increases with increasing Mn concentration in the CMS electrode.