Role of self-torques in transition metal dichalcogenide/ferromagnet bilayers

In recent years, transition metal dichalcogenides (TMDs) have been extensively studied for their efficient spin-orbit torque generation in TMD/ferromagnetic bilayers, owing to their large spin-orbit coupling, large variety of crystal symmetries, and pristine interfaces. Although the TMD layer was considered essential for the generation of the observed spin-orbit torques (SOTs), recent reports show the presence of a self-torque in single-layer ferromagnetic devices with magnitudes comparable to TMD/ferromagnetic devices. Here, we perform second-harmonic Hall SOT measurements on metal-organic chemical vapor deposition (MOCVD) grown MoS2/permalloy/Al2O3 devices and compare them to a single-layer permalloy/Al2O3 device to accurately disentangle the role of self-torques, arising from the ferromagnetic layer, from contributions from the TMD layer in these bilayers. We report a fieldlike spin-torque conductivity of σFL=(-2.8±0.3)×103ℏ2e(ωm)-1 in a single-layer permalloy/Al2O3 device, which is comparable to our MoS2/permalloy/Al2O3 devices and previous reports on similar TMD/ferromagnetic bilayers, indicating only a minor role of the MoS2 layer. In addition, we observe a comparatively weak dampinglike torque in our devices, with a strong device-to-device variation. Finally, we find a linear dependence of the SOT conductivity on the Hall bar arm/channel width ratio of our devices, indicating that the Hall bar dimensions are of significant importance for the reported SOT strength. Our results accentuate the importance of delicate details, like device asymmetry, Hall bar dimensions, and self-torque generation, for the correct disentanglement of the microscopic origins underlying the SOTs, essential for future energy-efficient spintronic applications.</p

Photoluminescence and charge transfer in the prototypical 2D/3D semiconductor heterostructure MoS₂/GaAs

The new generation of two-dimensional (2D) materials has shown a broad range of applications for optical and electronic devices. Understanding the properties of these materials when integrated with the more traditional three-dimensional (3D) semiconductors is an important challenge for the implementation of ultra-thin electronic devices. Recent observations have shown that by combining MoS$_2$ with GaAs it is possible to develop high quality photodetectors and solar cells. Here, we present a study of the effects of intrinsic GaAs, p-doped GaAs, and n-doped GaAs substrates on the photoluminescence of monolayer MoS$_2$. We observe a decrease of an order of magnitude in the emission intensity of MoS$_2$ in all MoS$_2$/GaAs heterojunctions, when compared to a control sample consisting of a MoS$_2$ monolayer isolated from GaAs by a few layers of hexagonal boron nitride. We also see a dependence of the trion to A-exciton emission ratio in the photoluminescence spectra on the type of substrate, a dependence that we relate to the static charge exchange between MoS$_2$ and the substrates when the junction is formed. Scanning Kelvin probe microscopy measurements of the heterojunctions suggest type-I band alignments, so that excitons generated on the MoS$_2$ monolayer will be transferred to the GaAs substrate. Our results shed light on the charge exchange leading to band offsets in 2D/3D heterojunctions which play a central role in the understanding and further improvement of electronic devices.Comment: Accepted in Applied Physics Letter

Interfacial spin-orbit torques and magnetic anisotropy in WSe₂/permalloy bilayers

Transition metal dichalcogenides (TMDs) are promising materials for efficient generation of current-induced spin-orbit torques (SOTs) on an adjacent ferromagnetic layer. Numerous effects, both interfacial and bulk, have been put forward to explain the different torques previously observed. Thus far, however, there is no clear consensus on the microscopic origin underlying the SOTs observed in these TMD/ferromagnet bilayers. To shine light on the microscopic mechanisms at play, here we perform thickness dependent SOT measurements on the semiconducting WSe2/permalloy bilayer with various WSe2 layer thickness, down to the monolayer limit. We observe a large out-of-plane field-like torque with spin-torque conductivities up to 1 × 104 (ℏ/2e) (Ωm)−1. For some devices, we also observe a smaller in-plane antidamping-like torque, with spin-torque conductivities up to 4 × 103 (ℏ/2e) (Ωm)−1, comparable to other TMD-based systems. Both torques show no clear dependence on the WSe2 thickness, as expected for a Rashba system. Unexpectedly, we observe a strong in-plane magnetic anisotropy—up to about 6.6 × 104 erg cm−3—induced in permalloy by the underlying hexagonal WSe2 crystal. Using scanning transmission electron microscopy, we confirm that the easy axis of the magnetic anisotropy is aligned to the armchair direction of the WSe2. Our results indicate a strong interplay between the ferromagnet and TMD, and unveil the nature of the SOTs in TMD-based devices. These findings open new avenues for possible methods for optimizing the torques and the interaction with interfaced magnets, important for future non-volatile magnetic devices for data processing and storage

Erratum: Interfacial spin-orbit torques and magnetic anisotropy in WSe2/permalloy bilayers

Upon further analysis of the data, we find that the field dependence of the B-component in figure 2(b) is more likely caused by a significant unidirectional magnetoresistance (UMR), hindering the accurate determination of a damping-like torque for our devices. We would like to stress that all the main conclusions of our work remain the same

Anisotropic laser-pulse-induced magnetization dynamics in van der Waals magnet Fe3GeTe2

Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe3GeTe2 (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modeling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin-orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics

dataset of 'Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor'

This .zip archive contains all raw data used for the paper 'Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor'. In addition, it contains (partially) processed data, and the data that is plotted in the figures of this paper