Danielle Reifsnyder Hickey

Surface-State-Dominated Spin-Charge Current Conversion in Topological-Insulator-Ferromagnetic-Insulator Heterostructures.

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films-Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3}-deposited by molecular beam epitaxy on Y_{3}Fe_{5}O_{12} thin films. By systematically varying the Bi_{2}Se_{3} film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (λ_{IREE}), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator-ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of Y_{3}Fe_{5}O_{12}/(Bi,Sb)_{2}Te_{3} heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and λ_{IREE}.

Mapping the chemical potential dependence of current-induced spin polarization in a topological insulator

We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disentangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/Al2O3) as a spin detector on a back-gated (Bi,Sb)2Te 3 channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or antiparallel to the spin polarization of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied dc bias current in the (Bi,Sb)2Te 3 channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e., the Dirac point).

Room-temperature high spin–orbit torque due to quantum confinement in sputtered Bi x Se (1–x) films

The spin–orbit torque (SOT) that arises from materials with large spin–orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1–x) thin films in BixSe(1–x)/Co20Fe60B20 heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θS) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the BixSe(1–x) was observed at room temperature with a low critical magnetization switching current density of 4.3 × 105 A cm–2. Quantum transport simulations using a realistic sp3 tight-binding model suggests that the high SOT in sputtered BixSe(1–x) is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θS, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1–x) films provide an avenue for the use of BixSe(1–x) as a spin density generator in SOT-based memory and logic devices.Sputtered BixSe(1–x) thin films can generate very large current-induced spin–orbit torque, capable to switch both in-plane and out-of-plane magnetized CoFeB-based structures deposited on top, at room temperature.Room-temperature perpendicular magnetization switching through giant spin-orbit torque from sputtered BixSe(1-x) topological insulator material Mahendra DC1, Mahdi Jamali2, Jun-Yang Chen2, Danielle Reifsnyder Hickey3, Delin Zhang2, Zhengyang Zhao2, Hongshi Li3, P. Quarterman2, Yang Lv2, Mo Li2, K. Andre Mkhoyan3 and Jian-Ping Wang2,1,3,* 1School of Physics and Astronomy, University of Minnesota, MN 55455 2Department of Electrical and Computer Engineering, University of Minnesota, MN 55455 3Department of Chemical Engineering and Material Science, University of Minnesota, MN 55455

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L10-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in re...

Cross-sectional STEM Imaging and Spectroscopy of Devices with Embedded 2D Materials

Two-dimensional (2D) systems have been demonstrated to be excellent materials for charge [1] and spin transport [2] in devices. Their exceptional performance in these devices is afforded by their unique electronic structure in their singleor few-layer states. As such, it would be advantageous to characterize how their atomic and electronic structures change while embedded in actual devices, as compared to in their free-standing states. Although simulations have modeled 2D materials embedded within a solar cell or field effect transistor in order to predict the material’s performance [3], experimental results to corroborate these theoretical predictions that show the structure of the 2D material or the interface it shares with the substrate or contacts in the device remain scarce. This region, together with its interface, is as thick as the embedded 2D material in the device, which is often only a few atomic layers, making access inherently difficult.

Probing Two-dimensional (Bi,Sb)2Te3/h-BN Heterostructures Using Complementary S/TEM and Simulation Techniques

To revolutionize electronics, materials must be developed, characterized, and engineered that can outperform conventional, silicon-based technology. Because of their momentum-locked surface states, topological insulators (TIs) are emerging as materials that could provide important advances for magnetoelectronic technologies. Already, spin-transfer torque [1], current-induced spin polarization [2,3], and room-temperature spin injection [4] have been demonstrated as phenomena of interest in TI-based devices.

Sputtering growth of Y3Fe5O12/Pt bilayers and spin transfer at Y3Fe5O12/Pt interfaces

For the majority of previous work on Y3Fe5O12 (YIG)/normal metal (NM) bi-layered structures, the YIG layers were grown on Gd3Ga5O12 first and were then capped by an NM layer. This work demonstrates the sputtering growth of a Pt/YIG structure where the Pt layer was grown first and the YIG layer was then deposited on the top. The YIG layer shows well-oriented (111) texture, a surface roughness of 0.15 nm, and an effective Gilbert damping constant less than 4.7 × 10−4, and the YIG/Pt interface allows for efficient spin transfers. This demonstration indicates the feasibility of fabricating high-quality NM/YIG/NM tri-layered structures for new physics studies.

S/TEM Investigation of the Structure of (Bi,Sb) 2 Te 3 /h-BN Heterostructures Grown by Molecular Beam Epitaxy

Topological insulators are promising materials for magnetoelectronic applications. Therefore, bismuth chalcogenides have emerged as materials of interest due to their strong spin–orbit coupling, which results in spin–momentum locking. This property has enabled the demonstration of properties such as spin-transfer torque [1], current-induced spin polarization [2,3], and room-temperature spin injection [4].

Observation of Moiré-like Fringes in HAADF-STEM Images of Heterostructures of Two-dimensional Materials

Moiré patterns occur in conventional transmission electron microscopy (CTEM) and bright-field scanning transmission electron microscopy (BF-STEM) images due to the interference resulting from two sets of periodic features [1]. These Moiré fringes can provide useful insights into a number of materials’ properties, such as the relative orientation of crystal lattices, the existence of strain at interfaces, and the presence of defects such as dislocations.

Investigation of Layer Composition and Morphology in Perpendicular Magnetic Tunnel Junctions

As traditional complementary metal–oxide semiconductor (CMOS) technology approaches its limit, alternative technologies such as magnetic tunnel junctions (MTJs) are being explored to replace CMOSbased devices for memory and logic applications. MTJs have advantages such as nonvolatility, low power consumption, and high densities [1]. These features have enabled application in technologies such as magnetic random access memory (MRAM), static random access memory (SRAM), and spin-transfer torque MTJs (STT-MTJs).