Novel Phase Formation and Magnetism at the Sb₂Te₃/Ni₈₀Fe₂₀ Interface

Bilayer topological insulator/ferromagnet heterostructures are promising for spintronic memory applications due to their low switching energy and therefore power efficiency.¹ Topological insulators have been grown with molecular beam epitaxy (oriented, epitaxial films)^2,3 and RF magnetron sputtering (amorphous to crystalline oriented films)^4-6and have demonstrated large spin-to-charge conversion efficiencies. However, the reactivity of topological insulators with ferromagnetic films is often overlooked in the spin-orbit-torque literature, even though there are reports that it is energetically favorable for topological insulators to react with metals and form interfacial layers.^7-11 Previously, we have investigated the interface of Bi₂Te₃/Ni₈₀Fe₂₀ and discovered that a novel topological antiferromagnetic phase forms at the bilayer interface due to selective Ni diffusion, which is possibly catalyzed by the topological surface states.¹² In our previous work the Bi₂Te₃ was grown via sputtering on thermal SiO₂/Si, and while it showed <i>c</i>-axis orientation, we suspect mosaicity to be present, which may limit the diffusion depth of the Ni into the Bi₂Te₃ film, thus limiting the thickness and uniformity of the interfacial antiferromagnetic phase. In our current work, we have grown Sb₂Te₃ with molecular beam epitaxy to ensure a highly ordered and epitaxial film, with the expectation that the diffusion depth of the Ni into the Sb₂Te₃ may be much deeper, providing a smaller gradient resulting in a more homogeneous antiferromagnetic interlayer. However, we find this interface to be highly complex with multiple phase formations, including a novel antiferromagnetic phase evident by the presence of large, negative exchange bias. We support our findings with temperature dependent magnetometry, high-angle annular dark-field scanning transmission electron microscopy, and theoretical calculations. Despite the complex nature of the Sb₂Te₃/Ni₈₀Fe₂₀ interface, we still observe significant enhancement of the Gilbert damping and reduction of effective in-plane magnetization, which is indicative of spin pumping in this heterostructure – Sb₂Te₃ is a spin sink due to its large spin-orbit-coupling.¹³ This work highlights the role of interfacial chemistry in topological insulator/ferromagnet heterostructures.<br/> <br/>1. Y. Cao et al, <i>iScience</i>, <b>23</b>, 101614, (2020).<br/>2. S. Hsuan Su et al, <i>ACS Appl. Electron. Mater.</i>, <b>3</b>, 2988-2994, (2021).<br/>3. N.H.D. Kang et al, <i>Nat. Mater.,</i> <b>17</b>, 808-813, (2018).<br/>4. M. DC et al, <i>Nat. Mater.,</i><b>17</b>, 800-807, (2018).<br/>5. W. Jie Wang et al, <i>Sci. Rep.</i>, <b>6</b>, 25291, (2015).<br/>6. T. Fan et al, <i>Sci. Rep.</i>, <b>12</b>, 2998, (2022).<br/>7. C.D Spataru et al, <i>Phys. Rev. B.,</i> <b>90</b>, 085115 (2014).<br/>8. W. Ye et al, <i>arX</i>iv<i>, (2015).</i><br/>9. L.A. Walsh et al., <i>J. Phys. Chem. C</i>, <b>121</b>, 23551 (2017).<br/>10. S.J. Chang et al., <i>RSC Adv</i>., <b>8</b>, 7785–7791, (2018).<br/>11. G. Li and C. Felser, <i>Appl. Phys. Lett.,</i><b> 116</b>, 070501, (2020).<br/>12. N. Bhattacharjee et al., <i>Adv. Mater.</i>, 2108790, (2022).<br/>13. A.A. Baker et al., <i>Sci. Rep.,</i> <b>5</b>, 7907 (2015).