The recently discovered unidirectional spin-Hall magnetoresistance in nonmagnet-ferromagnet (NM-FM) bilayer structures is believed the only way to electrically sense the in-plane 180-degree magnetization rotation in such NM-FM systems without adding additional terminals or structures. On the other front, spintronic memory and logic devices involving topological insulators (TIs) are being studied intensively due to TI’s exciting potential of efficiently generate spins. In our work, we observed the unidirectional spin-Hall and Rashba−Edelstein magnetoresistance (USRMR) in topological insulator-ferromagnet (TI-FM) layer heterostructures for the first time. TI films (Bi2Se3 or (Bi1−xSbx)2Te3) were first grown by molecular beam epitaxy and then transferred to a magnetron sputtering chamber where the CoFeB and capping MgO were deposited. Then the stacks were patterned into Hall bars and tested with harmonic measurements in a Quantum Design PPMS. The measured longitudinal second harmonic resistance contains the USRMR signal plus contributions from other thermoelectric effects. Then series of transverse second harmonic measurements with various external magnetic field strengths were carried out to carefully determine the contributions of these effects. Finally, the data was analyzed, and results show non-zero USRMR. By varying the temperature, TI thickness and TI material and comparing the USRMR performances, we have also concluded that the topological surface states of TI played important roles in generating USRMR signal. The transport properties and conditions seem to govern heavily on the USRMR performance. And the largest USRMR was observed in 6 quintuple layer thick Bi2Se3 sample at 150K and being more than twice as large as the best reported USMR in Ta/Co samples.

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Yang Lv, James Kally, Delin Zhang, Joon Sue Lee, Mahdi Jamali, Nitin Samarth, and Jian-Ping Wang, "Unidirectional spin Hall and Rashba-Edelstein magnetoresistance in topological insulator-ferromagnet layer heterostructures (Conference Presentation)," Proc. SPIE 10732, Spintronics XI, 1073211 (Presented at SPIE Nanoscience + Engineering: August 20, 2018; Published: 18 September 2018); https://doi.org/10.1117/12.2323329.5836437164001.