Y. T. Fanchiang

High-quality single-crystal thulium iron garnet films with perpendicular magnetic anisotropy by off-axis sputtering

High-quality single-crystal thulium iron garnet (TmIG) films of 10-30 nm thick were grown by off-axis sputtering at room temperature (RT) followed by post-annealing. X-ray photoelectron spectroscopy (XPS) was employed to determine the TmIG film composition to optimize the growth conditions, along with the aid of x-ray diffraction (XRD) structural analysis and atomic force microscope (AFM) for surface morphology. The optimized films exhibited perpendicular magnetic anisotropy (PMA) and the saturation magnetization at RT was ∼99 emu/cm3, close to the RT bulk value ∼110 emu/cm3 with a very low coercive field of ∼2.4 Oe. We extracted the H⊥ of 1734 Oe and the peak-to-peak linewidth ΔH of ferromagnetic resonance are only about 99 Oe, significantly lower than that of PLD grown TmIG film and bulk single crystals. The high-quality sputtered single-crystal TmIG films show great potential to be integrated with topological insulators or heavy metals with strong spin-orbit coupling for spintronic applications.

Strongly enhanced spin current in topological insulator/ferromagnetic metal heterostructures by spin pumping

Spin pumping effect in Bi2Se3/Fe3Si and Fe/Bi2Te3 heterostructures was studied. High quality films of Bi2Se3(001) on ferromagnetic Fe3Si(111) layer and Fe(111) films on Bi2Te3(001) layer were grown epitaxially by molecular beam epitaxy. Using a microwave cavity source, large voltages due to the Inverse Spin Hall Effect (VISHE) were detected in Bi2Se3(001)/Fe3Si(111) bi-layer at room temperature. VISHE of up to 63.4 ± 4.0 μV at 100 mW microwave power (PMW) was observed. In addition, Fe(111)/Bi2Te3(001) bi-layer also showed a large VISHE of 3.0 ± 0.1 μV at PMW of 25 mW. VISHE of both structures showed microwave linear power dependence in accordance with the theoretical model of spin pumping. The spin Hall angle was calculated to be 0.0053 ± 0.002 in Bi2Se3 and was estimated to be 0.0068 ± 0.003 in Bi2Te3. The charge current density (Jc) of Bi2Se3/Fe3Si and Fe/Bi2Te3 structures are comparable and are about 2–5 times higher than the Fe3Si/normal metal and Fe3Si/GaAs results. The significant enhancement of spi...

Observation of strongly enhanced inverse spin Hall voltage in Fe3Si/GaAs structures

We performed spin pumping experiment on high quality, epitaxial Fe3Si/GaAs structures grown by molecular beam epitaxy. By tailoring the thickness and doping (n, p) level of the conducting GaAs epi-layer, thermal heating common of ferromagnetic metal/semiconductor heterostructure was removed effectively. A large inverse spin Hall Effect (ISHE) voltage up to 49.2 μV was observed for Fe3Si/p-GaAs. Smaller ISHE voltage (VISHE) by a factor of ∼0.4 was obtained for Fe3Si/n-GaAs, as scaled with its resistivity. By taking into account of the “self-induced” ISHE apparently observed in our samples, the minimum value of spin Hall angle θISHE for n-GaAs and p-GaAs was estimated to be 1.9 × 10−4 and 2.8 × 10−5, respectively.

Demonstration of large field effect in topological insulator films via a high-κ back gate

The spintronics applications long anticipated for topological insulators (TIs) has been hampered due to the presence of high density intrinsic defects in the bulk states. In this work we demonstrate the back-gating effect on TIs by integrating Bi2Se3 films 6–10 quintuple layer (QL) thick with amorphous high-κ oxides of Al2O3 and Y2O3. Large gating effect of tuning the Fermi level EF to very close to the band gap was observed, with an applied bias of an order of magnitude smaller than those of the SiO2 back gate, and the modulation of film resistance can reach as high as 1200%. The dependence of the gating effect on the TI film thickness was investigated, and ΔN2D/ΔVg varies with TI film thickness as ∼t−0.75. To enhance the gating effect, a Y2O3 layer thickness 4 nm was inserted into Al2O3 gate stack to increase the total κ value to 13.2. A 1.4 times stronger gating effect is observed, and the increment of induced carrier numbers is in good agreement with additional charges accumulated in the higher κ oxides. Moreover, we have reduced the intrinsic carrier concentration in the TI film by dopingTe to Bi2Se3 to form Bi2TexSe1−x. The observation of a mixed state of ambipolar field that both electrons and holes are present indicates that we have tuned the EF very close to the Dirac Point. These results have demonstrated that our capability of gating TIs with high-κ back gate to pave the way to spin devices of tunable EF for dissipationless spintronics based on well-established semiconductor technology.

Reconstruction at the interface of one cycle of trimethylaluminum and water on GaAs(111)A-2×2 from atomic layer deposition

During the initial stage of atomic layer deposition, the exposure of a GaAs(111)A-2×2 surface to trimethylaluminum (TMA) leads to occupying a Ga-vacancy site on the surface by a chemisorbed As-bonded aluminum with the loss of all methyl ligands. The water purge disrupts physisorbed TMA and initiates the growth of Al2O3. The subsequent growth of Al2O3 does not result in the oxidation of As, Ga, or Al in the Ga vacancy. The reconstructed Ga atoms retain the bulk-terminated positions resulting in a dangling bond, and all As atoms become four-fold coordinated. The correlation of an interfacial electronic structure with an electric performance is discussed for Al2O3 on GaAs(001)-2×4 and GaAs(001)-4×6.