SPIN

Spin pumping and spin-orbit torques play important roles in advancing spintronic device technologies. Our curated special issue focuses on these phenomena in magnetic heterostructures, which are critical for various applications. The high-quality papers^,,,,,, delve into spin pumping and spin-orbit torques in diverse heterostructures, addressing topics such as insulating/metallic interfaces tailoring spin-to-charge conversion, the influence of HfO_x capping layers on perpendicular magnetic anisotropy, anisotropy-controlled spin current generation, sputter pressure-dependent spin Hall angle, the effect of oxygen ion irradiation on spin-orbit torques, and a review paper of emerging two-dimensional/ferromagnetic interfaces. This Editorial discusses the reported articles and the future perspective of these topics. With this special issue, we anticipate significant contributions to both academic research and industrial developments in this direction.

We perform a comparative study of spin pumping in NiO/Ni${}_{80}$Fe${}_{20}$ (Py) by altering the crystallinity of NiO. We simultaneously grow high-quality NiO/Py heterostructures on Si/SiO₂ and Al₂O₃(0001) substrates. Although the growth on Si/SiO₂ substrate produced polycrystalline NiO (Poly-NiO), the growth on Al₂O₃(0001) substrate produced epitaxial NiO (Epi-NiO). Our ferromagnetic resonance measurements reveal the presence of two-magnon scattering (TMS) in the measured linewidth for the Epi-NiO/Py system. By carefully considering the contribution of TMS, we found the effective spin mixing conductance, g${}_{\mathrm{\text{eff}}}^{\uparrow \downarrow }$ to be comparable in both Poly-NiO/Py and Epi-NiO/Py, despite the different crystallinity of NiO. Our findings show that g${}_{\mathrm{\text{eff}}}^{\uparrow \downarrow }$ of an optimized polycrystalline-NiO/Py system can reach the value for an epitaxial-NiO/Py system.

Large perpendicular magnetic anisotropy (PMA) and spin-orbit torques are two essential requirements for high-density and energy-efficient spintronic devices. Recent advances in magneto-electric and magneto-ionic control of PMA and spin-orbit torques have opened a pathway to explore different oxide materials with large dielectric constant and high ion mobility. Motivated by interesting memristive properties of HfO_x, here, using broadband ferromagnetic resonance measurements, we study the spin pumping in-plane to perpendicularly magnetized W(5nm)/CoFeB(t_CFB) system capped with 4nm HfO_x thin films. We find a large spin pumping and significant PMA strength for HfO_x-based thin films. Using ferromagnetic thickness dependence, we also calculate the spin mixing conductance in the trilayer system which is found to be similar to MgO-based thin film stacks. These results will be highly useful for various spintronic applications with electro-static, ionic and memristive gating.

Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of the CoFeB thin film by tilting the magnetization from in-plane to out-of plane direction due to interfacial anisotropy from higher thickness of CoFeB thin film. The ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 1.10MA/m² to 2.40MA/m², with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices.

The generation of spin current in materials with high spin–orbit coupling is one of the primary topics for future spintronic applications. In this context, the high conversion efficiency of spin current to charge current is desired. Ferromagnetic resonance (FMR)-based inverse spin Hall effect (ISHE) is a quite popular method to study such spin-to-charge conversion. Here microwave frequency plays a crucial role in generating spin current which further gets converted to charge current due to spin–orbit interaction of the given material. In this work, we show the effect of microwave frequency on spin-to-charge current conversion efficiency in La${}_{0.67}$Sr${}_{0.33}$MnO₃/Pt heterostructure prepared by oxygen-assisted molecular beam epitaxy (OMBE). From the ISHE analysis the maximum spin pumping voltage of ∼98 μV has been observed. Further spin Hall angle has been calculated for different frequencies and the maximum calculated value (∼0.06) for this system has been obtained at 14GHz.

Spin-to-charge conversion and vice versa due to spin-orbit coupling in ferromagnet-heavy metal heterostructure is of paramount interest for developing energy-efficient spintronic devices. Here, we have systematically investigated the effect of Ar deposition pressure ($P_{\mathrm{\text{Ar}}})$ on the tungsten (W) crystalline phase and extracted spin-dependent transport parameters. X-ray diffraction results show that 10nm-thick W films exhibit a structural phase transition from a mixed phase of $(\alpha +\beta )$-W to a single phase of $\beta$-W as a function of $P_{\mathrm{\text{Ar}}}$. The observed phase transition is due to a decrease in adatom’s energy and surface mobility. Interestingly, only the $(\alpha +\beta )$-W phase is found to stabilize when W sputtered on a seed Ni${}_{80}$Fe${}_{20}$ (Permalloy or Py) film. The growth of $(\alpha +\beta )$-W on the seed Py layer could be due to the strain that facilitates the mixed phase. W deposited on the Py layer is shown to be dependent on $P_{\mathrm{\text{Ar}}}$, in which the $\beta$-W relative phase fraction is relative. A ferromagnetic resonance (FMR)-based spin pumping method was employed for spin current injection. The FMR linewidth ($\mathrm{\Delta }H)$ is enhanced for Py/W compared to the bare Py layer due to the spin current transport across the interface. The spin-mixing conductance ($g_{\uparrow \downarrow })$ is found to be a function of the relative phase fraction of W. The extracted $g_{\uparrow \downarrow }$ is $4.90\times 10^{18}$m${}^{-2}$ for $P_{\mathrm{\text{Ar}}}=5$mTorr and $4.05\times 10^{18}$m${}^{-2}$ for $P_{\mathrm{\text{Ar}}}=10$mTorr. From the inverse spin Hall effect (ISHE) measurements, the effective spin Hall angle ($({\theta }_{\mathrm{\text{SH}}})$ is estimated to be $-0.17$ for $\alpha$-W rich mixed phase of $(\alpha +\beta )$-W, whereas it is $-0.10$ for $\beta$-W rich $(\alpha +\beta )$-W. Our systematic study demonstrates the relatively large effective spin Hall angle via low-longitudinal resistivity by controlling the relative phase fraction of W and helps in developing energy-efficient spintronic devices.

The impurity incorporation in host high spin-orbit coupling materials like platinum (Pt) has shown improved charge-to-spin conversion efficiency by modifying the up-spin and down-spin electrons trajectories via bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiated the Pt thin films with pulsed hot dense oxygen plasma in non-focus mode operation and analyzed the spin Hall angle of the oxygen plasma irradiated Pt films using spin torque ferromagnetic resonance (ST-FMR). Our results demonstrate a 2.4 times enhancement in the spin Hall angle after transient oxygen plasma treatment of Pt as compared to pristine Pt. This improvement might be because of the introduction of disorder and defects in the Pt lattice due to transient oxygen plasma processing, which enhanced the spin-orbit coupling and leads to more efficient charge-to-spin conversion without breaking the spin-orbit torque symmetries. Our findings offer a new method of dense plasma focus device-based modification of material for the development of advanced spintronic devices based on Pt and other heavy metals.

Spin-orbit torque (SOT) plays an efficient and versatile role in electrical manipulation in spintronic devices at the nanoscale, which shows great promise for ultrafast and energy-efficient magnetic random-access memory (MRAM). To get high-performance SOT devices, their charge-to-spin conversion ratio must be sufficiently high and low current consumption is the desired one. Two-dimensional van der Waals (2D-vdW) materials possess strong tunability and spin-orbit coupling compared to conventional metals, which can efficiently achieve both things. This review covers a generalized introduction to SOT and its origin, their measurement techniques, SOTs observed in various 2D material-based heterostructures made of topological insulators (TIs), transition metal dichalcogenides (TMDs), and van der Waals (vdW) materials as they have excellent electronic properties down to their monolayer limit and ease of integration. Further, it covers the recent progress of SOT devices in each category, highlighting their potential for achieving high-performance and energy-efficient spintronic devices and their potential applications.