Sputtered topoloical insulator/ferromagnet heterostructures and interfaces for energy efficent spintronic device applications

Abstract

TIs in presence of TRS breaking magnetic order can exhibit QAH or Axion insulator edge states. In this work, first creation of a novel topological AFM phase is presented at the interface of a sputtered, c-axis-oriented, TI/FM heterostructure - Bi2Te3/Py -due to interfacial diffusion of Ni in Bi2Te3 (Ni-Bi2Te3). Structural, chemical, and magnetic properties of the interfacial AFM Ni-Bi2Te3 layer in parallel with first-principles calculations revealed a solid-state chemical reaction leading to formation of Ni-Te bonds and the presence of a topological AFM compound NiBi2Te4 in the Ni-Bi2Te3 interface layer. Next, the effect of crystalline structural disorder of TI on interfacial and magnetic properties of sputter-deposited TI/FM, Bi2Te3/Py, heterostructures is reported. For highly crystalline c-axis oriented Bi2Te3 films coupled to a FM, a giant enhancement in Gilbert damping is observed. This is accompanied by a noticeable change in magnetic anisotropy and enhanced damping-like spin-orbit torque (DL-SOT), possibly due to the strong TSS of Bi2Te3. An antiferromagnetic topological interfacial layer was formed due to TSS promoted solid-state reactions, even in highly disordered Bi2Te3 heterostructured with Py. This shows the resilience of TSSs even in highly disordered TIs. However, for increasing disorder of Bi2Te3, a significant weakening of exchange interaction in the AFM interfacial layer is found suggesting weakening of topological properties due to disorder. Using an interface engineering technique using TiOx, low-temperature spin-spray growth of MI, NZFO was achieved. Oxidized TI surface without any capping layer was found to be chemically and magnetically decoupled from a FM layer grown on top. A thin layer of TiOx was found to protect the TI surface from oxidation maintaining its topological properties. A TiOx thin film of thickness > 2nm was found to act as an excellent barrier against interfacial diffusion without significantly reducing spin-pumping from a FM thin film. An ultrathin TiOx barrier was found to result in an AFM interface because of solid-state reaction. Further, increase in TiOx was found to effectively prevent any interfacial diffusion and reaction. A significant exchange interaction between TSS electrons and the MI was found to result in deviation of values of gyromagnetic ratio which showed a gradual reduction with increase in TiOx barrier. The results presented in this dissertation will stimulate further exploration of heterostructures of TIs and MIs for topologically nontrivial interfaces. Fundamental understanding of topologically nontrivial interfaces in TI/FM heterostructures will lead to realization of energy efficient spintronic devices.--Author's abstract