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VIDEO DOI: https://doi.org/10.48448/bqga-mq12

technical paper

MMM 2022

November 07, 2022

Minneapolis, United States

Growth, characterization and spin orbit torque of BiSb topological insulator/MnGa perpendicular ferromagnet systems

Recently, topological insulator(TI)/ferromagnetic(FM) systems, such as Bi2Se3/FM 1,2, Bi1-xSbx/FM 3,4 have attracted much attention with their promising properties for emerging memory technologies, such as magnetic random-access memories(MRAM). Indeed, these systems may drastically reduce the writing current using spin-orbit torque(SOT) switching. In this case SOT is generated by injecting a charge current into the TI topologically protected surface states(SS), then converted into a spin current in the FM layer. The charge current may shunt the TI SS through the metallic FMs which conductivity is often higher and may not contribute efficiently to the SOT5. Thus, FMs with lower conductivities are required and may help to better understand the SOT effect. Also, for applications, FMs with high Curie temperature and perpendicular magnetic anisotropy(PMA) are desirable.

In this work, we focused on the molecular beam epitaxy (MBE) growth conditions of Bi1-xSbx(top)/MnxGa1-x(bottom) heterostructure on the semi-insulating GaAs(001) substrate. A high epitaxial quality was confirmed by the high-resolution scanning transmission electron microscopy (STEM) as shown in Fig.1. The Bi0.85Sb0.15 top surface states have been characterized by performing ARPES measurements. Electronic band structure with topological surface states was observed. Mn0.45Ga0.55 exhibit a PMA with a magnetization of 315 kA/m.

To investigate the charge-to-spin current conversion of Bi0.9 Sb0.1(9 nm)/Mn0.45Ga0.55(7 nm) we first performed anomalous Hall effect (AHE) measurements at various temperatures. A large value of ΔRAHE = 0.4Ω was obtained at room temperature. We then perform second harmonic Hall voltage measurements in order to quantify the damping-like HDL(Fig.2.) and field-like SOT HFL by applying an ac current and in-plane external magnetic field. For this relatively large MnGa thickness (7nm), a value of HDL=2.6±1.3mT for a current density of 1010 A/m2 was estimated. Part of future work will focus on controlling the growth of a thinner layer.

References 1 Mellnik, A. et al. Spin-transfer torque generated by a topological insulator. Nature 511, 449–451 (2014) 2 Wang, Y., Zhu, D., Wu, Y. et al. Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques. Nat Commun 8, 1364 (2017). 3 Fan, T., Khang, N.H.D., Nakano, S. et al. Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers. Sci Rep 12, 2998 (2022) 4 Tuo Fan, Nguyen Huynh Duy Khang, Takanori Shirokura, Ho Hoang Huy, and Pham Nam Hai , Low power spin–orbit torque switching in sputtered BiSb topological insulator/perpendicularly magnetized CoPt/MgO multilayers on oxidized Si substrate, Appl. Phys. Lett. 119, 082403 (2021) 5 Liu, L. et al. Spin-torque switching with the giant spin Hall efect of tantalum. Science 336, 555–558 (2012)

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