Abstract:Recent discoveries regarding current--induced spin--orbit torques produced by heavy--metal/ferromagnet and topological--insulator/ferromagnet bilayers provide the potential for dramatically--improved efficiency in the manipulation of magnetic devices. However, in experiments performed to date, spin--orbit torques have an important limitation -the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin--orbit torques can drive the most current--efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in--plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high--density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin--orbit torques in spin--source/ferromagnet bilayer devices by using a spin source material with low crystalline symmetry. We use WTe 2 , a transition--metal dichalcogenide whose surface crystal structure has only one mirror plane and no two--fold rotational invariance. Consistent with these symmetries, we generate an out--of--plane antidamping torque when current is applied along a low--symmetry axis of WTe 2 /Permalloy bilayers, but not when current is applied along a high--symmetry axis. Controlling S--O torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies.2 Current--induced torques generated by materials with strong spin--orbit (S--O) interactions are a promising approach for energy--efficient manipulation of nonvolatile magnetic memory and logic technologies 1 . However, S--O torques observed to date are limited by their symmetry so that they cannot efficiently switch the nanoscale magnets with perpendicular magnetic anisotropy (PMA) that are required for high--density applications 2 . S--O torques generated either in conventional heavy metal/ferromagnet thin--film bilayers 3--13 or in topological insulator/ferromagnet bilayers 14,15 are restricted by symmetry to have a particular form 16 : an "antidamping--like" component oriented in the sample plane that is even upon reversal of the magnetization direction, m , plus an "effective field" component that is odd in m . The fact that the antidamping torque lies in--plane means that the most efficient mechanism of S--O--torque--driven magnetic reversal for small devices (antidamping switching) 17,18 is available only for magnetic samples with in--plane magnetic anisotropy 8,9 , and not PMA samples. S--O torques can also arise from broken crystalline inversion symmetry, even within single layers of ferromagnets 19--22 or antiferromagnets 23 , but the antidamping torques that have been measured to date are still limited to lie in the sample plane 21,22,24 . Here we demonstrate that the allowed symmetries of S--O torques in spin source/ferromagnet bilayer samples can be changed by using a spin source material with reduced crystalli...

Interfacial polymerization with COF monomers and Sc(OTf) 3 afforded large-area (several cm 2) free-standing films with tunable thickness (2.5 nm to 100 mm). When the films were thick (100 mm), they exhibited X-ray diffraction corresponding to the expected crystalline structure. The films were integrated into the thin-film composite membranes for water nanofiltration, where they showed enhanced rejection of model pollutant Rhodamine WT.

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 singlemonolayer 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.arXiv:1707.03757v1 [cond-mat.mes-hall]

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 σ ≈ 10 (ℏ/2e)(Ωm) 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 m̂ × ẑ, where m̂ is the Permalloy magnetization direction and ẑ 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.

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