University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Friday, January 25th 2019
1:15 pm:
Thesis Defense in PAN 110
Speaker: Mahendra DC, University of Minnesota
Subject: High charge-to-spin and spin-to-charge conversion enhanced by quantum confinement effect in sputtered topological insulator thin films"
This is the public portion of Mr. DC's thesis defense. His advisor is Jian-Ping Wang.

The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and ultra-high speed magnetic-based storage and computational devices. The SOT switching of magnetization can be used in SOT-memory and whereas the spin-to-charge conversion can be utilized for reading of magnetization state in spin-based logic device. Recent reports on topological insulators grown by MBE process show a relatively high SOT. However, this process is incompatible to modern semiconductor fabrication processes. An “unexpected” giant SOT effect was discovered in sputtered topological insulator through this research. This discovery led to and could be explained by our newly proposed concept: quantum-confinement in topological insulators. From the atomic force and transmission electron microscopy the magnetron-sputtered bismuth selenide thin films show unique nano-sized grains which are correlated with the thickness of the films. A systematic study on SOT effect in BixSe(1-x)/Co20Fe60B20 heterostructures was performed. Remarkably, the spin torque efficiency ( ) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08, using the dc planar Hall and spin-torque ferromagnetic resonance methods, respectively. Quantum transport simulations using a realistic sp3 tight binding model confirms that the giant SOT in sputtered BixSe(1-x) is due to the quantum confinement effect, whose charge-to-spin conversion efficiency enhances with reduced size and dimensionality. In addition to charge-to-spin conversion, I will also present results of spin-to-charge conversion in these sputtered bismuth selenide thin films. For the spin-pumping experiment, a conducting ferromagnet (CoFeB) and insulating ferrimagnet (YIG) were used to pump spins into the bismuth selenide layer. The spin-to-charge conversion voltage is affected by thermal effects such as the Nernst effect, anomalous Nernst effect, and spin Seebeck effect. The thermal contribution in the spin-to-charge conversion voltage was studied by using an insulating barrier. An additional spin-momentum locking was created by inserting a thin Ag layer in between bismuth selenide and CoFeB, which enhanced the spin-to-charge conversion figure of merit quite significantly.

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