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Register a new userMeasurement of spin mixing conductance in Ni$_{81}$Fe$_{19}$/$alpha$-W and Ni$_{81}$Fe$_{19}$/$beta$-W heterostrucutures via ferromagnetic resonance
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We present measurements of interfacial Gilbert damping due to the spin pumping effect in Ni$_{81}$Fe$_{19}$/W heterostructures. Measurements were compared for heterostructures in which the crystallographic phase of W, either $alpha$(bcc)-W or $beta$(A15)-W, was enriched through deposition conditions and characterized using X-ray diffraction (XRD) and high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Single phase Ni$_{81}$Fe$_{19}$/$alpha$-W heterostructures could be realized, but heterostructures with $beta$-W were realized as mixed $alpha$-$beta$ phase. The spin mixing conductances (SMC) for W at interfaces with Ni$_{81}$Fe$_{19}$ were found to be significantly lower than those for similarly heavy metals such as Pd and Pt, but comparable to those for Ta, and independent of enrichment in the $beta$ phase.
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Based on the spin-pumping theory and first-principles calculations, the spin-mixing conductance (SMC) is theoretically studied for Pt/Permalloy (Ni$_{81}$Fe$_{19}$, Py) junctions. We evaluate the SMC for ideally clean Pt/Py junctions and examine the effects of interface randomness. We find that the SMC is generally enhanced in the presence of interface roughness as compared to the ideally clean junctions. Our estimated SMC is in good quantitative agreement with the recent experiment for Pt/Py junctions. We propose possible routes to increase the SMC in Pt/Py junctions by depositing a foreign magnetic metal layer in Pt, offering guidelines for designing the future spintronic devices.
A recent theory by Chen and Zhang [Phys. Rev. Lett. 114, 126602 (2015)] predicts strongly anisotropic damping due to interfacial spin-orbit coupling in ultrathin magnetic films. Interfacial Gilbert-type relaxation, due to the spin pumping effect, is predicted to be significantly larger for magnetization oriented parallel to compared with perpendicular to the film plane. Here, we have measured the anisotropy in the Pt/Ni$_{81}$Fe$_{19}$/Pt system via variable-frequency, swept-field ferromagnetic resonance (FMR). We find a very small anisotropy of enhanced Gilbert damping with sign opposite to the prediction from the Rashba effect at the FM/Pt interface. The results are contrary to the predicted anisotropy and suggest that a mechanism separate from Rashba spin-orbit coupling causes the rapid onset of spin-current absorption in Pt.
An experimental study of the in-plane azimuthal behaviour and frequency dependence of the ferromagnetic resonance field and the resonance linewidth as a function of BiFeO$_3$ thickness is carried out in a polycrystalline exchange-biased BiFeO$_3$/Ni$_{81}$Fe$_{19}$ system. The magnetization decrease of the Pt/BiFeO$_3$/Ni$_{81}$Fe$_{19}$/Pt heterostructures with BiFeO$_3$ thickness deduced from static measurements has been confirmed by dynamic investigations. Ferromagnetic resonance measurements have shown lower gyromagnetic ratio in a perpendicular geometry compared with that of a parallel geometry. The monotonous decrease of gyromagnetic ratio in a perpendicular geometry as a function of the BiFeO$_3$ film thickness seems to be related to the spin-orbit interactions due to the neighbouring Pt film at its interface with Ni$_{81}$Fe$_{19}$ film. The in-plane azimuthal shape of the total linewidth of the uniform mode shows isotropic behaviour that increases with BiFeO$_3$ thickness. The study of the frequency dependence of the resonance linewidth in a broad band of 3 to 35 GHz has allowed the determination of intrinsic and extrinsic contributions to the relaxation as function of BiFeO$_3$ thickness in perpendicular geometries. In our system the magnetic relaxation is dominated by the spin-pumping mechanism due to the presence of Pt. The insertion of BiFeO$_3$ between Pt and Ni$_{81}$Fe$_{19}$ attenuates the spin-pumping damping at one interface.
We investigated the mechanism of the spin-reorientation transition (SRT) in the Ni/Fe/Ni/W(110) system using in situ low-energy electron microscopy, x-ray magnetic circular dichroism measurements, and first principles electronic structure calculations. We discovered that the growth of Fe on a flat Ni film on a W (110) crystal resulted in the formation of nanosized particles, instead of a uniform monolayer of Fe as commonly assumed. This interfacial nanostructure leads to a change of the systems dimensionality from two dimensional- to three dimensional-like, which simultaneously weakens the dipolar interaction and enhances the spin-orbit coupling in the system and drives the observed SRT.
We study spin pumping in a $mathrm{Y_3Fe_5O_{12}(YIG)/Pt/Ni_{81}Fe_{19}(Py)}$ trilayer film by means of the inverse spin Hall effect (ISHE). When the ferromagnets are not excited simultaneously by a microwave, ISHE-induced voltage is of the opposite sign at each ferromagnetic resonance (FMR). The opposite sign is consistent with spin pumping of bilayer films. On the other hand, the voltage is of the same sign at each FMR when both the ferromagnets are excited simultaneously. Futhermore, the voltage greatly increases in magnitude. The observed voltage is unconventional; neither its sign nor magnitude can be expected from spin pumping of bilayer films. Control experiments show that the unconventional voltage is dominantly induced by spin pumping at the Py/Pt interface. Interaction between YIG and Py layers is a possible origin of the unconventional voltage.
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