Absence of Spin‐Orbit Torque and Discovery of Anisotropic Planar Nernst Effect in CoFe Single Crystal

Abstract

Exploration of exotic spin polarizations in single crystals is of increasing interest. A current of longitudinal spins, the so‐called “Dresselhaus‐like” spin current, which is forbidden in materials lacking certain inversion asymmetries, is implied to be generated by a charge current at the interface of single‐crystal CoFe. This work reports unambiguous evidence that there is no indication of spin current of any spin polarizations from the interface or bulk of single‐crystalline CoFe and that the sin2φ second harmonic Hall voltage, which is previously assumed to signify Dresselhaus‐like spin current, is not related to any spin currents but rather a planar Nernst voltage induced by a longitudinal temperature gradient within the sample. Such sin2φ signal is independent of large applied magnetic fields and interfacial spin‐orbit coupling, inversely correlated to the heat capacity of the substrates and overlayers, quadratic in charge current, and appears also in polycrystalline ferromagnets. Strikingly, the planar Nernst effect (PNE) in the CoFe single crystal has a strong fourfold anisotropy and varies with the crystalline orientation. Such strong, anisotropic PNE has widespread impacts on the analyses of a variety of spintronic experiments and opens a new avenue for the development of PNE‐based thermoelectric battery and sensor applications.

Giant, Linearly Increasing Spin–Orbit Torque Efficiency in Symmetry-Broken Spin–Orbit Torque Superlattices

“…where S3), H DL is the dampinglike SOT field, H k is the effective perpendicular anisotropy field, V ANE is the anomalous Nernst voltage induced by the vertical thermal gradient, V PNE is the planar Nernst voltage induced by the longitudinal thermal gradient, 28 V PHE is the planar Hall voltage, H FL is the fieldlike SOT field, and H Oe is the transverse Oersted field exerted on the Co layers by the in-plane charge current in other layers. The values of V AHE and H k are extracted from the dependence of V 1ω on the swept out-of-plane magnetic field (H z ) under zero H xy (Figure 2b).…”

“…We used an H xy of ≥0.5 kOe to rotate the sample magnetization as a rigid macrospin during the HHV measurements (see Figure S1). Under the macrospin approximation, the second HHV is given by V 2 ω = V DL+ANE cos φ + V FL + Oe cos φ cos 2 φ + V P N E sin 2 φ where V DL+ANE = V AHE H DL /2( H xy + | H k |) + V ANE , V FL+Oe = − V PHE ( H FL + H Oe )/2 H xy , V AHE is the anomalous Hall voltage (Figure S3), H DL is the dampinglike SOT field, H k is the effective perpendicular anisotropy field, V ANE is the anomalous Nernst voltage induced by the vertical thermal gradient, V PNE is the planar Nernst voltage induced by the longitudinal thermal gradient, V PHE is the planar Hall voltage, H FL is the fieldlike SOT field, and H Oe is the transverse Oersted field exerted on the Co layers by the in-plane charge current in other layers. The values of V AHE and H k are extracted from the dependence of V 1ω on the swept out-of-plane magnetic field ( H z ) under zero H xy (Figure b).…”

Enhanced Spin–Orbit Torques in Graphene by Pt Adatoms Decoration

“…Regarding the source of the antisymmetric sin(2𝜙) signal, it has been reported in the literature that it may come from a planar Nernst signal induced by the ∇𝑇𝑥 within the sample. [42] The solid blue line in Fig. 2(b) represents the fitting results by Eq.…”

“…It should be mentioned that the 𝑅∇𝑇 𝑥 does not depend on 𝐻ext. [42] Due to the same symmetry of 𝐻DL and the ∇𝑇𝑧 induced contributions, they appear in pairs in Eq. ( 2).…”

“…2(c), and it does not change with 𝐻ext. [42] (b) (a) To evaluate the influence of Pt adatoms on the SOT in Gr/Pt(𝑡Pt)/FeNi(5), we extract 𝐻DL and 𝐻FL of SOTs. 𝐻DL and 𝐻FL can be obtained from the coefficient of the value of 𝑅DL+∇𝑇 𝑧 and 𝑅FL+Oe, respectively, according to Eq.…”