Spin-orbit torques and photocurrents in 2D materials

While conventional electronics rely on the electron charge as information carrier, using another intrinsic property of the electron, its spin, offers promising ways to further improve information storage technologies. However, the key hurdle lies in gaining precise control over the electron spin. Currently, both electrical and optical methods are being explored to achieve this control.This thesis delves into the realm of spintronics and optoelectronics, focusing on the effects observed in layered two-dimensional (2D) materials called transition metal dichalcogenides (TMDs). These materials are particularly well-suited for this purpose due to their direct bandgap in atomically thin layers and strong spin-orbit coupling, which is advantageous for spintronic and optospintronic effects.The initial section of the thesis addresses spintronic effects, specifically the spin-orbit torque (SOT) in TMD/ferromagnetic bilayers. Notably, our study on WSe2/permalloy devices reveals a lack of clear dependence on WSe2 thickness for SOTs, suggesting an interfacial origin. Additionally, we observe the presence of SOTs in a device with a single ferromagnetic layer, highlighting the importance of studying reference samples for accurate determination of the SOT strength.Turning to the optoelectronic aspect of TMDs, our exploration uncovers that the Schottky barrier at the MoSe2-metallic contacts interface induces additional polarization-dependent photocurrents. Furthermore, we demonstrate that modifying the crystal structure of MoTe2 locally enhances the optoelectronic performance of TMDs based devices.This thesis provides important steps for the integration of 2D materials in future spintronic and optoelectronic devices

Spin-orbit torques and photocurrents in 2D materials

While conventional electronics rely on the electron charge as information carrier, using another intrinsic property of the electron, its spin, offers promising ways to further improve information storage technologies. However, the key hurdle lies in gaining precise control over the electron spin. Currently, both electrical and optical methods are being explored to achieve this control.This thesis delves into the realm of spintronics and optoelectronics, focusing on the effects observed in layered two-dimensional (2D) materials called transition metal dichalcogenides (TMDs). These materials are particularly well-suited for this purpose due to their direct bandgap in atomically thin layers and strong spin-orbit coupling, which is advantageous for spintronic and optospintronic effects.The initial section of the thesis addresses spintronic effects, specifically the spin-orbit torque (SOT) in TMD/ferromagnetic bilayers. Notably, our study on WSe2/permalloy devices reveals a lack of clear dependence on WSe2 thickness for SOTs, suggesting an interfacial origin. Additionally, we observe the presence of SOTs in a device with a single ferromagnetic layer, highlighting the importance of studying reference samples for accurate determination of the SOT strength.Turning to the optoelectronic aspect of TMDs, our exploration uncovers that the Schottky barrier at the MoSe2-metallic contacts interface induces additional polarization-dependent photocurrents. Furthermore, we demonstrate that modifying the crystal structure of MoTe2 locally enhances the optoelectronic performance of TMDs based devices.This thesis provides important steps for the integration of 2D materials in future spintronic and optoelectronic devices

Spin-orbit torques and photocurrents in 2D materials

While conventional electronics rely on the electron charge as information carrier, using another intrinsic property of the electron, its spin, offers promising ways to further improve information storage technologies. However, the key hurdle lies in gaining precise control over the electron spin. Currently, both electrical and optical methods are being explored to achieve this control.This thesis delves into the realm of spintronics and optoelectronics, focusing on the effects observed in layered two-dimensional (2D) materials called transition metal dichalcogenides (TMDs). These materials are particularly well-suited for this purpose due to their direct bandgap in atomically thin layers and strong spin-orbit coupling, which is advantageous for spintronic and optospintronic effects.The initial section of the thesis addresses spintronic effects, specifically the spin-orbit torque (SOT) in TMD/ferromagnetic bilayers. Notably, our study on WSe2/permalloy devices reveals a lack of clear dependence on WSe2 thickness for SOTs, suggesting an interfacial origin. Additionally, we observe the presence of SOTs in a device with a single ferromagnetic layer, highlighting the importance of studying reference samples for accurate determination of the SOT strength.Turning to the optoelectronic aspect of TMDs, our exploration uncovers that the Schottky barrier at the MoSe2-metallic contacts interface induces additional polarization-dependent photocurrents. Furthermore, we demonstrate that modifying the crystal structure of MoTe2 locally enhances the optoelectronic performance of TMDs based devices.This thesis provides important steps for the integration of 2D materials in future spintronic and optoelectronic devices

Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures

In recent years, there has been a growing interest in spin-orbit torques (SOTs) for manipulating the magnetization in nonvolatile magnetic memory devices. SOTs rely on the spin-orbit coupling of a nonmagnetic material coupled to a ferromagnetic layer to convert an applied charge current into a torque on the magnetization of the ferromagnet (FM). Transition metal dichalcogenides (TMDs) are promising candidates for generating these torques with both high charge-to-spin conversion ratios, and symmetries and directions which are efficient for magnetization manipulation. Moreover, TMDs offer a wide range of attractive properties, such as large spin-orbit coupling, high crystalline quality and diverse crystalline symmetries. Although numerous studies were published on SOTs using TMD/FM heterostructures, we lack clear understanding of the observed SOT symmetries, directions, and strengths. In order to shine some light on the differences and similarities among the works in literature, in this mini-review we compare the results for various TMD/FM devices, highlighting the experimental techniques used to fabricate the devices and to quantify the SOTs, discussing their potential effect on the interface quality and resulting SOTs. This enables us to both identify the impact of particular fabrication steps on the observed SOT symmetries and directions, and give suggestions for their underlying microscopic mechanisms. Furthermore, we highlight recent progress of the theoretical work on SOTs using TMD heterostructures and propose future research directions.Comment: 14 pages, 1 figure, 1 tabl

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

The role of device asymmetries and Schottky barriers on the helicity-dependent photoresponse of 2D phototransistors

Circular photocurrents (CPC), namely circular photogalvanic (CPGE) and photon drag effects, have recently been reported both in monolayer and multilayer transition metal dichalcogenide (TMD) phototransistors. However, the underlying physics for the emergence of these effects are not yet fully understood. In particular, the emergence of CPGE is not compatible with the D3h crystal symmetry of two-dimensional TMDs, and should only be possible if the symmetry of the electronic states is reduced by influences such as an external electric field or mechanical strain. Schottky contacts, nearly ubiquitous in TMD-based transistors, can provide the high electric fields causing a symmetry breaking in the devices. Here, we investigate the effect of these Schottky contacts on the CPC by characterizing the helicity-dependent photoresponse of monolayer MoSe2 devices both with direct metal-MoSe2 Schottky contacts and with h-BN tunnel barriers at the contacts. We find that, when Schottky barriers are present in the device, additional contributions to CPC become allowed, resulting in emergence of CPC for illumination at normal incidence

Interfacial Spin-Orbit Torques and Magnetic Anisotropy in WSe2_{2}/Permalloy Bilayers

Transition metal dichalcogenides (TMDs) are promising materials for efficient generation of current-induced spin-orbit torques 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 spin-orbit torques observed in these TMD/ferromagnet bilayers. To shine light on the microscopic mechanisms at play, here we perform thickness dependent spin-orbit torque measurements on the semiconducting WSe$_{2}$/permalloy bilayer with various WSe$_{2}$ layer thickness, down to the monolayer limit. We observe a large out-of-plane field-like torque with spin-torque conductivities up to $1\times10^4 ({\hbar}/2e) ({\Omega}m)^{-1}$. For some devices, we also observe a smaller in-plane antidamping-like torque, with spin-torque conductivities up to $4\times10^{3} ({\hbar}/2e) ({\Omega}m)^{-1}$, comparable to other TMD-based systems. Both torques show no clear dependence on the WSe$_{2}$ thickness, as expected for a Rashba system. Unexpectedly, we observe a strong in-plane magnetic anisotropy - up to about $6.6\times10^{4} erg/cm^{3}$ - induced in permalloy by the underlying hexagonal WSe$_{2}$ crystal. Using scanning transmission electron microscopy, we confirm that the easy axis of the magnetic anisotropy is aligned to the armchair direction of the WSe$_{2}$. Our results indicate a strong interplay between the ferromagnet and TMD, and unveil the nature of the spin-orbit torques 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.Comment: 19 pages, 3 figure

Interfacial Spin-Orbit Torques and Magnetic Anisotropy in WSe2_{2}/Permalloy Bilayers

Transition metal dichalcogenides (TMDs) are promising materials for efficient generation of current-induced spin-orbit torques 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 spin-orbit torques observed in these TMD/ferromagnet bilayers. To shine light on the microscopic mechanisms at play, here we perform thickness dependent spin-orbit torque measurements on the semiconducting WSe$_{2}$/permalloy bilayer with various WSe$_{2}$ layer thickness, down to the monolayer limit. We observe a large out-of-plane field-like torque with spin-torque conductivities up to $1\times10^4 ({\hbar}/2e) ({\Omega}m)^{-1}$. For some devices, we also observe a smaller in-plane antidamping-like torque, with spin-torque conductivities up to $4\times10^{3} ({\hbar}/2e) ({\Omega}m)^{-1}$, comparable to other TMD-based systems. Both torques show no clear dependence on the WSe$_{2}$ thickness, as expected for a Rashba system. Unexpectedly, we observe a strong in-plane magnetic anisotropy - up to about $6.6\times10^{4} erg/cm^{3}$ - induced in permalloy by the underlying hexagonal WSe$_{2}$ crystal. Using scanning transmission electron microscopy, we confirm that the easy axis of the magnetic anisotropy is aligned to the armchair direction of the WSe$_{2}$. Our results indicate a strong interplay between the ferromagnet and TMD, and unveil the nature of the spin-orbit torques 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