Anthony P. McFadden

Gating of high-mobility InAs metamorphic heterostructures

We investigate the performance of gate-defined devices fabricated on high mobility InAs metamorphic heterostructures. We find that heterostructures capped with In0.75Ga0.25As often show signs of parallel conduction due to proximity of their surface Fermi level to the conduction band minimum. Here, we introduce a technique that can be used to estimate the density of this surface charge that involves cool-downs from room temperature under gate bias. We have been able to remove the parallel conduction under high positive bias, but achieving full depletion has proven difficult. We find that by using In0.75Al0.25As as the barrier without an In0.75Ga0.25As capping, a drastic reduction in parallel conduction can be achieved. Our studies show that this does not change the transport properties of the quantum well significantly. We achieved full depletion in InAlAs capped heterostructures with non-hysteretic gating response suitable for fabrication of gate-defined mesoscopic devices.

Oxygen migration in epitaxial CoFe/MgO/Co2MnSi magnetic tunnel junctions

The effects of post-growth annealing in ultrahigh vacuum (UHV) on the temperature-dependent transport properties of single-crystal, full-Heusler CoFe/MgO/Co2MnSi magnetic tunnel junctions (MTJs) grown by molecular beam epitaxy have been correlated with in-situ X-ray photoelectron spectroscopy (XPS) studies of the MgO/Co2MnSi interface. CoFe and MgO layers were grown on single-crystal Co2MnSi at room temperature and annealed post growth. The structures were found to be epitaxial and single-crystal before and after annealing as assessed by in-situ reflection high-energy electron diffraction (RHEED). While annealing has little effect on RHEED patterns, post-growth annealing at temperatures as low as 200 °C has a dramatic effect on tunnel magnetoresistance and transport properties. XPS measurements conducted on MgO/Co2MnSi structures reveal the presence of interfacial Mn and Si oxides which form as a result of the e-beam deposition process used for MgO. Mn oxides are observed to be reduced upon UHV annealing ...

Valence-band offsets of CoTiSb/In0.53Ga0.47As and CoTiSb/In0.52Al0.48As heterojunctions

The valence-band offsets, ΔEv, between semiconducting half-Heusler compound CoTiSb and lattice-matched III-V In0.53Ga0.47As and In0.52Al0.48As heterojunction interfaces have been measured using X-ray photoemission spectroscopy (XPS). These interfaces were formed using molecular beam epitaxy and transferred in situ for XPS measurements. Valence-band offsets of 0.30 eV and 0.58 eV were measured for CoTiSb/In0.53Ga0.47As and CoTiSb/In0.52Al0.48As, respectively. By combining these measurements with previously reported XPS ΔEv (In0.53Ga0.47As/In0.52Al0.48As) data, the results suggest that band offset transitivity is satisfied. In addition, the film growth order of the interface between CoTiSb and In0.53Ga0.47As is explored and does not seem to affect the band offsets. Finally, the band alignments of CoTiSb with GaAs, AlAs, and InAs are calculated using the density function theory with the HSE06 hybrid functional and applied to predict the band alignment of CoTiSb with In0.53Ga0.47As and In0.52Al0.48As. Good ag...

Influence of the magnetic proximity effect on spin-orbit torque efficiencies in ferromagnet/platinum bilayers

Current-induced spin-orbit torques in Co2FeAl/Pt ultrathin bilayers are studied using a magnetoresistive harmonic response technique, which distinguishes the dampinglike and fieldlike contributions. The presence of a temperature-dependent magnetic proximity effect is observed through the anomalous Hall and anisotropic magnetoresistances, which are enhanced at low temperatures for thin platinum thicknesses. The fieldlike torque efficiency decreases steadily as the temperature is lowered for all Pt thicknesses studied, which we propose is related to the influence of the magnetic proximity effect on the fieldlike torque mechanism. 1 ar X iv :1 71 1. 07 96 9v 2 [ co nd -m at .m es -h al l] 2 2 N ov 2 01 7 Through the spin-orbit interaction (SOI), an electrical current je in a ferromagnet(F)/nonmagnetic metal(N) bilayer results in a torque on the magnetization M of F[1, 2]. This spin-orbit torque (SOT) may be decomposed into two perpendicular components – a component oriented along m̂ × (m̂ × σ̂) and a component along m̂ × σ̂, where σ̂ ≡ ĵe × n̂ denotes the orientation of the spin current created by the SOI and n̂ defines the unit vector normal to the plane formed by the F/N interface. These are referred to respectively as the dampinglike (DL) and fieldlike (FL) SOTs. Although the microscopic origins of the DL and FL SOTs remain unclear, the DL contribution has been widely interpreted using N bulk spin-Hall effect (SHE) diffusion models[2–5], and the FL contribution attributed to the F/N interfacial SOI[1, 4]. Amin and Stiles [6] have recently emphasized that this interpretation is overly simplistic, showing that the interfacial SOI and the SHE in the N layer may both produce FL and DL torques depending on the interface details. Unfortunately, the interfacial parameters used in spin diffusion models are not easily measured, and it remains an experimental challenge to separately identify the origins of the DL and FL torques. Also, in bilayers where interface scattering is dominant, a conventional normal-to-interface spin diffusion length becomes difficult to define. Furthermore, magnetic proximity effects (MPE) at F/N interfaces have been widely reported[7–10], yet how the MPE influences SOTs is unknown. In this article, we report a decrease in the FL and DL torques per unit current density (hereafter referred to as SOT efficiencies) at low temperature in Co2FeAl/Pt bilayers. In the same bilayers, a temperature-dependent MPE is revealed through magnetoresistance (MR) measurements. The FL SOT efficiency is suppressed by nearly a factor of 4 at 20 K with respect to room temperature for all Pt thicknesses studied, which we propose is related to the increasing influence of the MPE exchange field on the F/N interface Rashba spin accumulation. Meanwhile, the DL SOT efficiency monotonically increases with decreasing Pt thickness and closely tracks the Pt resistivity as temperature is varied. Within the Pt SHE diffusion model, the latter observation may be described by either the intrinsic SHE or spin backflow processes, between which we cannot differentiate. The F/N bilayers used in this study were grown on MgO(001) substrates by molecularbeam epitaxy (MBE). Prior to F growth, an in-situ MgO buffer was grown by e-beam evaporation on prepared MgO substrates in order to bury residual carbon and improve surface morphology. The F layer is the Heusler compound Co2FeAl (CFA) with thick-

Low magnetic damping and large negative anisotropic magnetoresistance in half-metallic Co2−xMn1+xSi Heusler alloy films grown by molecular beam epitaxy

Co2−xMn1+xSi films with various composition x were epitaxially grown using molecular beam epitaxy (MBE). High crystallinity and atomic ordering in the prepared Co2−xMn1+xSi films were observed, and their magnetic damping and anisotropic magnetoresistance (AMR) effect were systematically investigated. An ultra-low magnetic damping constant of 0.0007 was obtained in the Co2−xMn1+xSi film with a valence electron number (NV) of about 29.0. Additionally, a relatively large negative AMR effect was observed in the Co2−xMn1+xSi films that had a NV of about 29.0. This low damping and the large negative AMR effect indicate that epitaxial Co2−xMn1+xSi films with high atomic ordering grown by MBE possess a high-spin polarization.Co2−xMn1+xSi films with various composition x were epitaxially grown using molecular beam epitaxy (MBE). High crystallinity and atomic ordering in the prepared Co2−xMn1+xSi films were observed, and their magnetic damping and anisotropic magnetoresistance (AMR) effect were systematically investigated. An ultra-low magnetic damping constant of 0.0007 was obtained in the Co2−xMn1+xSi film with a valence electron number (NV) of about 29.0. Additionally, a relatively large negative AMR effect was observed in the Co2−xMn1+xSi films that had a NV of about 29.0. This low damping and the large negative AMR effect indicate that epitaxial Co2−xMn1+xSi films with high atomic ordering grown by MBE possess a high-spin polarization.

Growth, structural, and magnetic properties of single-crystal full-Heusler Co2TiGe thin films

This work presents the growth, structural characterization, and measurement of magnetic properties of Co2TiGe thin films grown by molecular beam epitaxy on insulating MgO (001) substrates and conductive lattice matched InAlAs/InGaAs/InAlAs epitaxial layers grown on n-InP (001) substrates. A GdAs diffusion barrier was used to minimize interfacial reactions during Co2TiGe growth on InAlAs. The surface morphology, structural quality, and magnetic behavior were examined by reflection high-energy electron diffraction, scanning tunneling microscopy, X-ray diffraction, and superconducting quantum interference device magnetometry. The results reveal high quality Co2TiGe thin films with a saturation magnetization of ∼1.8 μB/formula unit and a Curie temperature of ∼375 K. The magnetic easy axis was found to lie in the [110] direction but magnetometry also reveals that there is only a small difference in energy between the [110] and [010] magnetization directions.

Studies of scattering mechanisms in gate tunable InAs/(Al,Ga)Sb two dimensional electron gases

A study of scattering mechanisms in gate tunable two dimensional electron gases confined to InAs/(Al,Ga)Sb heterostructures with varying interface roughness and dislocation density is presented. By integrating an insulated gate structure the evolution of the low temperature electron mobility and single-particle lifetime was determined for a previously unexplored density regime, 1011–1012 cm−2, in this system. Existing theoretical models were used to analyze the density dependence of the electron mobility and single particle lifetime in InAs quantum wells. Scattering was found to be dominated by charged dislocations and interface roughness. It was demonstrated that the growth of InAs quantum wells on nearly lattice matched GaSb substrate results in fewer dislocations, lower interface roughness, and improved low temperature transport properties compared to growth on lattice mismatched GaAs substrates.