118 research outputs found
Disorder is not always bad for charge-to-spin conversion in WTe2
The Wang group at Stanford University demonstrates disordered WTex films for efficient charge-to-spin conversion phenomena. The deposition of these films by sputtering and the charge-to-spin conversion resilience against disorder make them attractive for applications in new magnetic memory devices
Thickness dependence of spin-orbit torques generated by WTe2
We study current-induced torques in WTe2/permalloy bilayers as a function of
WTe2 thickness. We measure the torques using both second-harmonic Hall and
spin-torque ferromagnetic resonance measurements for samples with WTe2
thicknesses that span from 16 nm down to a single monolayer. We confirm the
existence of an out-of-plane antidamping torque, and show directly that the
sign of this torque component is reversed across a monolayer step in the WTe2.
The magnitude of the out-of-plane antidamping torque depends only weakly on
WTe2 thickness, such that even a single-monolayer WTe2 device provides a strong
torque that is comparable to much thicker samples. In contrast, the
out-of-plane field-like torque has a significant dependence on the WTe2
thickness. We demonstrate that this field-like component originates
predominantly from the Oersted field, thereby correcting a previous inference
drawn by our group based on a more limited set of samples.Comment: 8 pages, 8 figure
Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor
Electric control of magnetization dynamics in two-dimensional (2D) magnetic
materials is an essential step for the development of novel spintronic
nanodevices. Electrostatic gating has been shown to greatly affect the static
magnetic properties of some van der Waals magnets, but the control over their
magnetization dynamics is still largely unexplored. Here we show that the
optically-induced magnetization dynamics in the van der Waals ferromagnet
CrGeTe can be effectively controlled by electrostatic gates, with a
one order of magnitude change in the precession amplitude and over 10% change
in the internal effective field. In contrast to the purely thermally-induced
mechanisms previously reported for 2D magnets, we find that coherent
opto-magnetic phenomena play a major role in the excitation of magnetization
dynamics in CrGeTe. Our work sets the first steps towards electric
control over the magnetization dynamics in 2D ferromagnetic semiconductors,
demonstrating their potential for applications in ultrafast opto-magnonic
devices
Spin-orbit torques in NbSe/Permalloy bilayers
We present measurements of current-induced spin-orbit torques generated by
NbSe, a fully-metallic transition-metal dichalcogenide material, made using
the spin-torque ferromagnetic resonance (ST-FMR) technique with
NbSe/Permalloy bilayers. In addition to the out-of-plane Oersted torque
expected from current flow in the metallic NbSe layer, we also observe an
in-plane antidamping torque with torque conductivity (m) and indications of a weak field-like
contribution to the out-of-plane torque oriented opposite to the Oersted
torque. Furthermore, in some samples we also measure an in-plane field-like
torque with the form , where is the Permalloy
magnetization direction and is perpendicular to the sample plane. The
size of this component varies strongly between samples and is not correlated
with the NbSe thickness. A torque of this form is not allowed by the bulk
symmetries of NbSe, but is consistent with symmetry breaking by a
uniaxial strain that might result during device fabrication.Comment: 14 pages, 3 figure
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
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