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Surface Acoustic Waves (SAW) have been used frequently in spintronic applications, mostly to decrease the magnetic field or the electric current required to move magnetic domain walls (DW). Because the SAW cannot achieve magnetic switching without the assistance of a magnetic field or a current, for a marginal improvement in the energy required for the magnetic switching, the device gains in complexity, making it impractical. In this work, we report a mechanism that allows a transfer of linear momentum from the acoustic wave to the magnetic domain wall. Experimentally we show that, using the appropriate dimensions of the magnetic strip, the SAW can move the DW in the same direction as the traveling SAW. With the help of micromagnetic simulations, we reveal a complex yet direct mechanism that allows the SAW to push the DW in the same direction of its travel, even without any external field or currents. The DW can reach velocities in the range of 100 m/s and at a very small energetic cost, equivalent to using a current density of $\sim 5 \cdot 10^4 A/cm^2$ if the movement was triggered by spin transfer. This new mechanism opens the door to designing innovative spintronic devices where the magnetization can be controlled exclusively by an acoustic wave., Comment: 11 pages, 5 figures. Submitted to Nature Communications

Magnetic skyrmions in thin films with perpendicular magnetic anisotropy are promising candidates for magnetic memory and logic devices, making the development of ways to transport skyrmions efficiently and precisely of significant interest. Here, we investigate the transport of skyrmions by surface acoustic waves (SAWs) via several modalities using micromagnetic simulations. We show skyrmion pinning sites created by standing SAWs at anti-nodes and skyrmion Hall-like motion without pinning driven by travelling SAWs. We also show how orthogonal SAWs formed by combining a longitudinal travelling SAW and a transverse standing SAW can be used for the 2D positioning of skyrmions. Our results also suggest SAWs offer a viable approach to the transport of multiple skyrmions along multichannel racetrack.

Ferromagnets generate an anomalous Hall effect even without the presence of a magnetic field, something that conventional antiferromagnets cannot replicate but noncollinear antiferromagnets can. The anomalous Hall effect governed by the resistivity tensor plays a crucial role in determining the presence of time reversal symmetry and the topology present in the system. In this work we reveal the complex origin of the anomalous Hall effect arising in noncollinear antiferromagnets by performing Hall measurements with fields applied in selected directions in space with respect to the crystalline axes. Our coplanar magnetic field geometry goes beyond the conventional perpendicular field geometry used for ferromagnets and allows us to suppress any magnetic dipole contribution. It allows us to map the in-plane anomalous Hall contribution and we demonstrate a 120$^\circ$ symmetry which we find to be governed by the octupole moment at high fields. At low fields we subsequently discover a surprising topological Hall-like signature and, from a combination of theoretical techniques, we show that the spins can be recast into dipole, emergent octupole and noncoplanar effective magnetic moments. These co-existing orders enable magnetization dynamics unachievable in either ferromagnetic or conventional collinear antiferromagnetic materials.

Disorder in ultra-thin magnetic films can significantly hinder domain wall motion. One of the main issues on the path towards efficient domain wall based devices remains the characterization of the pinning landscape at the nanoscale. In this paper, we study domain wall motion in W/CoFeB/MgO thin films with perpendicular magnetic anisotropy crystallized by annealing at 400$^{\circ}$C and a process based on He$^{+}$ irradiation combined with elevated temperatures. Magnetic properties are similar for the whole series of samples, while the magnetic domain wall mobility is critically improved in the irradiated samples. By using an analytical model to extract nanoscale pinning parameters, we reveal important variations in the disorder of the crystallized samples. This work offers a unique opportunity to selectively analyze the effects of disorder on the domain wall dynamics, without the contribution of changes in the magnetic properties. Our results highlight the importance of evaluating the nanoscale pinning parameters of the material when designing devices based on domain wall motion, which in return can be a powerful tool to probe the disorder in ultra-thin magnetic films., Comment: 22 pages, 12 figures. Currently under review

Ultrafast laser pulses provide unique tools to manipulate magnetization dynamics at femtosecond timescales, where the interaction of the electric field -- such as excitation of spin carriers to non-equilibrium states, generation of localized charge currents, demagnetization, or inverse Faraday effect -- dominates over the magnetic field. Recent proposals using structured laser beams have enlightened the possibility to generate intense femtosecond magnetic fields, spatially isolated from the electric field. Here we demonstrate the relevance of this novel scenario to femtomagnetism, unveiling the purely precessional, non-linear, chiral response of the magnetization when subjected to circularly polarized magnetic fields. This fundamental result not only opens an avenue in the study of laser-induced ultrafast magnetization dynamics, but also sustains technological implications as a route to promote all-optical non-thermal magnetization switching both at shorter timescales -- towards the attosecond regime -- and at THz frequencies.

The calculation of the transfer matrix for a large non-periodic multilayered system may become unstable in the presence of absorption. We discuss the origin of this instability and we explore two methods to overcome it: the use of a total matrix to solve for all the fields at all the interfaces simultaneously and an expansion in the Bloch-like modes of a periodic artificially repeated system. We apply both methods to obtain the reflectance spectra of multilayered chirped structures composed of nanostructured porous silicon (PS). Both methods yield reliable and numerically stable results. The former allows an analysis of the field within all layers while the latter is much more efficient computationally, allowing the design of novel structures and the optimization of their parameters. We compare numerical and experimental results across a wide spectral range from the infrared to the ultraviolet., Comment: 20 pages, 7 figures

Spin waves are perturbations in the relative orientation of magnetic moments in a continuous magnetic system, which have been proposed as a new kind of information carrier for spin-based low power applications. For this purpose, a major obstacle to overcome is the energy-efficient excitation of coherent short wavelength spin waves and alternatives to excitation via the Oersted field of an alternating current need to be explored. Here we show, by means of micromagnetic simulations, how, in a perpendicularly magnetized thin strip, a domain wall pinned at a geometrical constriction emits spin waves when forced to rotate by the application of a low direct current flowing along the strip. Spin waves propagate only in the direction of the electron's flow at the first odd harmonic of the domain wall rotation frequency for which propagation is allowed. Excitation is due to in-plane dipolar stray field of the rotating domain wall and that the resulting unidirectionality is a consequence of the domain wall displacement at the constriction. On the other hand, the application of an external field opposing domain wall depinning breaks the symmetry for spin wave propagation in the two domains, allowing emission in both directions but at different frequencies. The results presented define a new approach to produce tunable high frequency spin wave emitters of easy fabrication and low power consumption.

Magnetic domain wall (DW) motion induced by a localized Gaussian temperature profile is studied in a Permalloy nanostrip within the framework of the stochastic Landau-Lifshitz-Bloch equation. The different contributions to thermally induced DW motion, entropic torque and magnonic spin transfer torque, are isolated and compared. The analysis of magnonic spin transfer torque includes a description of thermally excited magnons in the sample. A third driving force due to a thermally induced dipolar field is found and described. Finally, thermally induced DW motion is studied under realistic conditions by taking into account the edge roughness. The results give quantitative insights into the different mechanisms responsible for domain wall motion in temperature gradients and allow for comparison with experimental results., Comment: 12 pages, 12 figures

Spin waves can transport both energy and angular momentum over long distances as they propagate. However, due to damping, their amplitude decreases exponentially as they move away from the source, leaving little capability for manipulating how much energy and angular momentum is to be delivered where. Here we show that a suitable local reduction of the effective field can lead to a large accumulation of spin wave energy far from the source. Moreover, both the location and the amount of energy to be delivered can be controlled accurately with geometry and externalm magnetic fields. Thus, we put forward a general, robust and flexible approach to convey both heat and spin in ferromagnets, which can be directly used in spintronic devices.

We present a micromagnetic study of linear and nonlinear spin-wave modes excited in an extended permalloy thin film by a microwave driven nanocontact. We show that the linear mode having the frequency equal to the excitation frequency (f) is driven by the ac Oersted field component perpendicular to the static external field (applied in-plane of the sample). The nonlinear mode with the frequency f /2 is excited as an independent eigenmode within a parametric longitudinal pumping process (due ac Oersted field component parallel to the bias field). Spectral positions of those modes are determined both in the space and phase domain. The results are important for the transfer of information coded into spin-waves between nanocontacts, and for synchronization of spin transfer torque nano-oscillators., Comment: 5 pages, 4 figures