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We present an experimental, theoretical, and computational study of the spreading of a millimetric silicon oil drop under the excitation of a surface acoustic wave (SAW), traveling in the underlying solid substrate. Our theoretical model, formulated within the long wave approach, dynamically captures the interplay between fluid dynamics and acoustic driving forces, emphasizing Eckart streaming as the dominant mechanism over Schlichting and Rayleigh streaming due to drop thickness. Following an initial deformation phase to accommodate acoustic stress, the drop accelerates to a sustained translation at nearly constant speed, leaving behind a thin wetting layer. This speed is sustained by the quasi-steady shape of the drop's main body, influenced by SAW attenuation. Notably, SAW attenuation becomes significant once the wave reaches the main body of the drop, with weaker effects observed under the air and the micron-thick liquid trail left behind the drop. The translating drop's speed is found to be dependent on the drop size, as well as on the SAW intensity. The drop's steady shape and speed are further informed by analytical consideration of a simplified traveling wave type model, that clarifies contributions from different physical effects. The theoretical results show qualitative and sometimes even quantitative agreement with the experiments, particularly regarding drop height and traveling speed.

A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal ($fct$) films, with a reduced number of stacking faults as the only structural defect observable by high resolution scanning transmission electron microscopy (HR-STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4~nm (20~ML) and 2~nm (10~ML) for Pt and Ir substrates, respectively, while X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anistropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co $fct$ films on HM buffers do not sustain PMAs beyond around 6~MLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local $hcp$ stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt(Ir)., Comment: 25 pages, 4 figures, 1 table

We study the equilibrium shape of a liquid drop resting on top of a liquid surface, i.e., a floating lens. We consider the surface tension forces in non--wetting situations (negative spreading factor), as well as the effects of gravity. We obtain analytical expressions for the drop shape when gravity can be neglected. Perhaps surprisingly, when including gravity in the analysis, we find two different families of equilibrium solutions for the same set of physical parameters. These solutions correspond to drops whose center of mass is above or below the level of the external liquid surface. By means of energetic considerations we determine the family that has the smallest energy, and therefore is the most probable to be found in nature. A detailed explanation of the geometrical differences between the both types of solutions is provided., Comment: 15 pages, 14 figures

Spin polarized currents are employed to efficiently manipulate the magnetization of ferromagnetic ultrathin films by exerting a torque on it. If the spin currents are generated by means of the spin-orbit interaction between a ferromagnetic and a non-magnetic layer, the effect is known as spin-orbit torque (SOT), and is quantified by measuring the effective fields produced by a charge current injected into the device. In this work, we present a new experimental technique to quantify directly the SOT based on the measurement of non-linearities of the dc current-voltage (IV) characteristics in Hall bar devices employing a simple instrumentation. Through the analysis of the IV curves, the technique provides directly the linearity of the effective fields with current, the detection of the current range in which the thermal effects can be relevant, the appearance of misalignments artefacts when the symmetry relations of SOT are not fulfilled, and the conditions for the validity of the single domain approximations, which are not considered in switching current and second harmonic generation state-of-the-art experiments. We have studied the SOT induced antidamping and field-like torques in Ta/Co/Pt asymmetric stacks with perpendicular magnetic anisotropy.

Spin currents can modify the magnetic state of ferromagnetic ultrathin films through spin-orbit torque. They may be generated by means of spin-orbit interaction by either bulk or interfacial phenomena. Electrical transport measurements reveal a six-fold increase of the spin-orbit torque accompanied by a drastic reduction of the spin Hall magnetoresistance upon the introduction of a Cu interlayer in a Pt/Cu/Co/Pt structure with perpendicular magnetic anisotropy. We analyze the dependence of the spin Hall magnetoresistance with the thickness of the interlayer in the frame of a drift diffusion model that provides information on the expected spin currents and spin accumulations in the system. The results demonstrate that the major responsible of both effects is spin memory loss at the interface. The enhancement of the spin-orbit torque when introducing an interlayer opens the possibility to design more effient spintronic devices based on materials that are cheap and abundant such as copper., Comment: 12 pages, 5 figures

The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM), we achieve a nanoscale control of thermal activated intercalation of homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited in turn onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters, and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in-situ high quality Gr-FM/HM heterostructures enabling the realization of novel spin-orbitronic devices that exploits the extraordinary properties of Gr., Comment: 13 pages, 6 figures

We consider here the effects of inertia on the instability of a flat liquid film under the effects of capillary and intermolecular forces (van der Waals interaction). Firstly, we perform the linear stability analysis within the long wave approximation, which shows that the inclusion of inertia does not produce new regions of instability other than the one previously known from the usual lubrication case. The wavelength, $\lambda_m$, corresponding to he maximum growth, $\omega_m$, and the critical (marginal) wavelength do not change at all. The most affected feature of the instability under an increase of the Laplace number is the noticeable decrease of the growth rates of the unstable modes. In order to put in evidence the effects of the bidimensional aspects of the flow (neglected in the long wave approximation), we also calculate the dispersion relation of the instability from the linearized version of the complete Navier-Stokes (N-S) equation. Unlike the long wave approximation, the bidimensional model shows that $\lambda_m$ can vary significantly with inertia when the aspect ratio of the film is not sufficiently small. We also perform numerical simulations of the nonlinear N-S equations and analyze to which extent the linear predictions can be applied depending on both the amount of inertia involved and the aspect ratio of the film.