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To enable suppression of two-magnon scattering (TMS) in nanometer-thick Co (ultrathin Co) layers and realize low-magnon damping in such layers, the magnon damping in ultrathin Co layers grown on various nonmagnetic seed layers with different surface magnetic anisotropy (SMA) energies are investigated. We verify the significantly weak magnon damping realized by varying the seeding layer species used. Although TMS is enhanced in ultrathin Co on Cu and Al seeding layers, the insertion of a Ti seeding layer below the ultrathin Co greatly suppresses the TMS, which is attributed to suppression of the SMA at the interface between Co and Ti. The Gilbert damping constant of the ultrathin Co layer on Ti (3 nm), 0.020, is comparable to the value for bulk Co, although the Co layer thickness here is only 2 nm. Realization of such weak magnon damping can open the door to tunable magnon excitation, thus enabling coupling of magnons with other quanta such as photons, given that the magnetization of ultrathin ferromagnets can be tuned using an external electric field.

We report the coherent-incoherent crossover of the spin Hall effect in Pd. By measuring spin-orbit torques generated by oxygen-incorporated Pd films, we find two distinct regimes of the spin Hall effect; one in which the spin Hall conductivity is insensitive to the electric conductivity, and the other in which the spin Hall conductivity decreases with decreasing electric conductivity. The crossover between the two regimes is consistent with the prediction of the coherent-incoherent crossover of the spin Hall effect, which is analogous to that of the anomalous Hall effect. Unlike the anomalous Hall effect, for which the crossover has been observed for a wide class of ferromagnets, the crossover of the spin Hall effect has been confirmed only for Pt and Pt-based alloys. The observation of the crossover of the spin Hall effect for Pd supports the generality of this phenomenon, illustrating an important correspondence between the spin Hall effect and the anomalous Hall effect.

Perpendicular magnetic anisotropy (PMA) in ferromagnet/oxide systems has attracted considerable attention owing to its significant potential in spintronic applications; however, an understanding of the causes of PMA enhancement in such systems remains inadequate. This study investigates the influence of epitaxial strain on PMA in the Fe/MgO system. The PMA energy increases monotonically as the in-plane lattice constant of Fe decreases, and increases further after annealing. We find that the interface component of the PMA energy is largely unaffected by the epitaxial strain and annealing process, and that changes in its bulk component play an important role. This study leads to a comprehensive understanding of PMA and voltage-controlled magnetic anisotropy effects in ultrathin magnetic films.

We investigate the DC voltage under an applied microwave current for platinum(Pt)/cobalt(Co), tantalum(Ta)/Co, and tungsten(W)/Co bilayers, where the microwave-induced alternating magnetic field (${B}_{\mathrm{rf}}$) is parallel to an external static magnetic field (${B}_{\mathrm{ext}}$). Whereas spin torque ferromagnetic resonance (ST FMR) signals do not appear because of the parallel configuration of ${B}_{\mathrm{rf}}$ and the magnetization of the Co layer, we recently found a clear hysteresis signal around ${B}_{\mathrm{ext}}=0\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ only when the microwave frequency (${f}_{\mathrm{MW}}$) is low, typically less than 10 GHz. This low-frequency ST FMR (LFST FMR) signal enables us to detect magnetization switching induced by spin-orbit torque (SOT) with high sensitivity. In this study, we found an additional background (BG) signal superimposed on the LSFT FMR signal. The additional BG signal also has hysteresis behavior and appears even at high ${f}_{\mathrm{MW}}$, where the LSFT FMR signal completely disappears. The sign of the BG signal is changed by changing the nonmagnetic material from Pt to Ta or W. By measuring ${f}_{\mathrm{MW}}$, the microwave current, the temperature, and the magnetic field angle dependences, we conclude that the BG signal is induced by spin-dependent unidirectional spin Hall magnetoresistance (SD USMR), which is generated by a spin current induced by the spin Hall effect of nonmagnetic metals and spin-dependent electron mobility in ferromagnetic metals. The SD USMR signal appears in wide ranges of ${f}_{\mathrm{MW}}$ and ${B}_{\mathrm{ext}}$ because the SD USMR originates from the nonlinearity of current-dependent resistance and is not related to magnetization dynamics. From the magnitude of the BG signal, the spin Hall angles of Pt and Ta are calculated to be 0.026 \ifmmode\pm\else\textpm\fi{} 0.006 and \ensuremath{-}0.042 \ifmmode\pm\else\textpm\fi{} 0.006, respectively. In addition, we demonstrate SOT magnetization switching using the BG signal.

We report the robustness of the intrinsic spin Hall effect in Pt against a change in the lattice constant. We measure spin-torque ferromagnetic resonance for $\mathrm{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ bilayers, where the lattice constant and carrier scattering time of the Pt layer are varied by incorporating the nitrogen atoms as interstitial impurities. We find that the spin Hall angle is enhanced by a factor of 3 as the resistivity of the Pt layer is increased by the nitrogen incorporation. The variation of the spin Hall conductivity is consistent with the prediction of the crossover of the intrinsic spin Hall effect from the moderately dirty regime to the dirty-metal regime. This result demonstrates the variation of the intrinsic spin Hall conductivity in Pt is dominated by the change in the scattering time, rather than the change in the band structure, under a few percent change in the lattice constant.

Recent experimental observation of an unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet pointed to a possibly prominent role of the orbital Rashba effect (ORE) in this system. Here, we use first principles methods to investigate the ORE in a system of oxygen monolayer deposited on top of a Cu(111) film. We show that surface oxidization of the Cu film leads to a gigantic enhancement of the ORE near the Fermi energy. The resulting chiral orbital texture in the momentum space is exceptionally strong, reaching as much as $\ensuremath{\sim}0.5\ensuremath{\hbar}$ in magnitude. We find that resonant hybridization between O $p$ states and Cu $d$ states is responsible for the emergence of the ORE at the interface. We also present a minimal model that captures the emergence of the ORE through the $pd$ hybridization mechanism. We demonstrate that an application of an external electric field to the system generates colossal orbital Hall currents which are an order of magnitude larger than the spin Hall currents found in heavy metals. This implies that the ``orbital torque'' mechanism may be significant in surface oxidized Cu/ferromagnet structures. Our results encourage an experimental verification of the rich orbital physics in surface oxidized Cu films through optical measurements such as angle-resolved photoemission spectroscopy and momentum microscopy.

Semiconductor/ferromagnet hybrid systems are attractive platforms for investigation of spin conversion physics, such as the (inverse) spin Hall effect. However, the superimposed rectification currents originating from anisotropic magnetoresistance have been a serious problem preventing unambiguous detection of dc spin Hall electric signals in semiconductors. In this study, we applied a microwave frequency inductive technique immune to such rectification effects to investigate the spin to charge conversion in heterostructures based on Si, one of the primitive semiconductors. The Si doping dependence of the spin-orbit torque conductivity was obtained for the Si/Cu/NiFe trilayer system. A monotonous modulation of the spin-orbit torque conductivity by doping and relative sign change of spin to charge conversion between the degenerate $n$- and $p$-type Si samples were observed. These results unveil spin to charge conversion mechanisms in semiconductor/metal heterostructures and show a pathway for further exploration of spin-conversion physics in metal/semiconductor heterostructures.

Weyl semimetals are playing a major role in condensed matter physics due to exotic topological properties, and their coexistence with ferromagnetism may lead to enhanced spin-related phenomena. Here, the inverse spin Hall effect (ISHE) in the ferromagnetic Weyl-semimetal Heusler alloy Co$_2$MnGa was investigated at room temperature by means of electrical spin injection in lateral spin valve structures. Spin transport properties such as spin polarization and spin diffusion length in this material were precisely extracted in order to estimate the spin Hall angle $\theta_{\textrm{SH}}$, which was found to be $-0.19\pm0.04$ and is among the highest reported for a ferromagnet. Although this value is on the same order of magnitude of known heavy metals, the significantly higher resistivity of Co$_2$MnGa implies an improvement on the magnitude of detection voltages, while its ferromagnetic nature allows controlling the intensity of SHE through the magnetization direction. It was also shown that Onsager's reciprocity does not hold for this system, which is in part attributable to a different spin-dependent Hall conductivity for spin-up and spin-down carriers., Comment: 14 pages with 13 figures including supplemental materials

In-plane magnetization reversal of a permalloy/platinum bilayer was detected using the spin rectification effect. Using a subgigahertz microwave frequency to excite spin-torque ferromagnetic resonance (ST-FMR) in the bilayer induces two discrete DC voltages around an external static magnetic field of 0 mT. These discrete voltages depend on the magnetization directions of the permalloy and enable detection of the in-plane magnetization reversal. The threshold current density for the magnetization reversal is around $10--20\phantom{\rule{0.16em}{0ex}}\mathrm{MA}/\mathrm{c}{\mathrm{m}}^{2}$, the same order as for known spin-orbit torque switching with in-plane magnetization materials. The magnitude of the signal is the same or larger than that of the typical ST-FMR signal; that is, detection of magnetization switching is highly sensitive in spite of deviation from the optimal ST-FMR condition. The proposed method is applicable to a simple device structure even for a small ferromagnetic electrode with a width of 100 nm.

Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrathin Co film can change the electron occupations according to the orbitals, combining x-ray absorption and x-ray magnetic circularly dichroism spectroscopies with the sum rules. Under a positive electric field, the ${d}_{xz(yz)}$-occupied states decrease mainly via the modification of orbital hybridizations, while the ${d}_{{z}^{2}}$-occupied state increases. The changes in the ${d}_{xz}$ and ${d}_{yz}$ states dominantly alter the Curie temperature.

We report that current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnetic-metal bilayers are strongly altered by the oxidation of the ferromagnetic layer near the interface. We measured damping-like (DL) and field-like (FL) SOTs for Pt/Co and $\mathrm{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (Pt/Py) films using spin-torque ferromagnetic resonance. In the Pt/Co film, we found that the oxidation of the Co layer near the interface enhances both DL and FL SOTs in spite of the insulating nature of the ${\mathrm{CoO}}_{x}$ layer. The enhancement of the SOTs disappears by inserting a thin Ti layer at the $\mathrm{Pt}/{\mathrm{CoO}}_{x}$ interface, indicating that the dominant source of the SOTs in the $\mathrm{Pt}/{\mathrm{CoO}}_{x}/\mathrm{Co}$ film is the spin-orbit coupling at the $\mathrm{Pt}/{\mathrm{CoO}}_{x}$ interface. In contrast to the $\mathrm{Pt}/{\mathrm{CoO}}_{x}/\mathrm{Co}$ film, the SOTs in the $\mathrm{Pt}/{\mathrm{PyO}}_{x}/\mathrm{Py}$ film are dominated by the bulk spin-orbit coupling. Our result shows that the interfacial oxidation of the Pt/Py film suppresses the DL-SOT and reverses the sign of the FL-SOT. The change of the SOTs can be attributed to the change of the real and imaginary parts of the spin mixing conductance induced by the insertion of the insulating ${\mathrm{PyO}}_{x}$ layer. These results show that the interfacial oxidation provides an effective way to manipulate the strength and sign of the SOTs.

We report the observation of spin transport through antiferromagnetic insulators using spin pumping driven by short-wavelength magnons. The short-wavelength magnons are excited by parametric pumping in a $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ trilayer. The parametrically excited magnons in the ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ layer inject a spin current into the NiO layer, which is detected electrically using the inverse spin Hall effect in the Pt layer. We found that the spin-pumping efficiency of the parametrically excited magnons in the $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film is nearly an order of magnitude smaller than that in the $\mathrm{Pt}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film. The suppression of the parametric spin-pumping efficiency induced by inserting the NiO layer is comparable to that of the FMR spin-pumping efficiency. We further found that the suppression of the spin-pumping efficiency due to the NiO insertion for the dipole-exchange magnons with the wave number of $k\ensuremath{\sim}{10}^{3}\ensuremath{-}{10}^{4}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is more significant than that for the exchange magnons with $k\ensuremath{\sim}{10}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This difference is attributed to two-magnon scattering and spin pinning due to antiferromagnetic grains in the NiO layer. These results provide insight into the understanding of the spin pumping into antiferromagnetic insulators.