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Engineering Interfacial Magnetoelectric Effects in Ultrathin-Film Composite Nanostructures

  • Author(s): Fitzell, Kevin
  • Advisor(s): Chang, Jane P
  • et al.
Abstract

Key to the continued scaling of electronic devices is the efficient control of magnetism with electric fields. In this work, multiple routes of achieving this control are explored, including CoFeB/MgO-based magnetic tunnel junctions to be incorporated into next-generation magnetoelectric random-access memory arrays and FeGa/NiFe-based magnetic multilayers to be incorporated into microscale magnetoelectric antennae.

For the magnetic tunnel junction application, this work verifies that the VCMA effect is greatly affected by the quality of the CoFeB/MgO interface and suggests that both the interfacial oxidation and crystallinity of the CoFeB layer are key factors in optimizing the VCMA characteristics of CoFeB/MgO-based MeRAM material systems. Furthermore, the successful integration of CoFeB with single-crystal MgO substrates is a substantial step toward achieving a greater degree of crystallinity in magnetic memory technologies.

For microscale magnetoelectric antennae, magnetic multilayer composites were synthesized in an attempt to combine the complementary properties of FeGa and NiFe and create an optimal magnetoelastic material. These multilayers combine the low magnetic loss at high frequency of NiFe with the strong magnetoelastic coupling of FeGa, and the properties were optimized by varying the thin film deposition conditions, thickness, and composition, and the resulting properties are well-suited for high-frequency applications. The metallic nature of these films, representing a significant source of energy loss when they are subjected to high-frequency AC magnetic fields through the generation of eddy currents, was mitigated by the insertion of ultrathin insulating Al2O3 interlayers. Subsequent integration with thin-film piezoelectrics eliminated the reliance on bulk piezoelectric substrates and enables integration with Si-based electronics.

This work demonstrated the importance of interfacial engineering in the development of emerging magnetoelectric materials. A better understanding of the role that oxidation at the CoFeB/MgO interface plays in the electric-field control of magnetization in magnetic tunnel junctions has already contributed significantly to the field of spintronics. The insulated FeGa/NiFe multilayer composites serve as ideal candidates for incorporation into next-generation magnetoelectric antenna concepts.

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