All optical writing and current-driven shifting of bits in ferrimagnetic strips: A micromagnetic study

[EN]Nucleation of domains and domain walls by means of ultrashort laser pulses, and their current-driven shifting along a ferrimagnetic strip with high perpendicular magnetic anisotropy on top of a heavy metal, are both explored here by means of advanced micromagnetic modeling. Our results indicate that these systems are ideal candidates to develop high-density and high-efficient domain wall-based memory devices where the information is coded in series of bits in the form of perpendicular up and down domains flanked by chiral domain walls.This work was supported by Grant MAT2017-87072-C4-1-P funded by Ministerio de Ciencia e Innovacion and No. PID20 20117024GB-C41 funded by MCIN/AEI/10.13039/501100011033, both from the Spanish government, Projects No. SA299P18 and No. SA114P20 from Consejeria de Educacion of Junta de Castilla y León, and project MagnEFi, Grant Agreement No. 860060, (H2020-MSCA-ITN-2019) funded by the European Commission

Collective coordinate descriptions of magnetic domain wall motion in perpendicularly magnetized nanostructures under the application of in-plane fields

Manipulation of magnetic domain walls in nanostructures can be used to improve the capabilities of the next generation of memory and sensing devices. Materials of interest for such devices include heterostructures of ultrathin ferromagnets sandwiched between a heavy metal and an oxide, where spin-orbit coupling and broken inversion symmetry give rise to the Dzyaloshinskii-Moriya interaction (DMI), stabilizing chiral domain walls. The efficiency of the motion of these chiral domain walls may be controlled using in-plane magnetic fields. This property has been used both for measurement of DMI strength, and for improved performance in applications. While micromagnetic simulations are able to accurately predict domain wall motion under in-plane fields in these materials, collective coordinate models such as the q-phi and q-phi-chi models fail to reproduce the micromagnetic results. In this theoretical work, we present a set of extended collective coordinate models including canting in the domains, which better reproduce micromagnetic results, and improve our understanding of the effect of in-plane fields on magnetic domain walls. These models are used in conjunction with micromagnetic simulations to develop simpler descriptions of DW motion under specific conditions. Our new models and results help in the development of future domain wall based devices based on perpendicularly magnetized materials.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P, MAT2017-87072-C4-1-P) Junta de Castilla y Leon (SA090U16, SA282U14

Pinned domain wall oscillator as a tuneable direct current spin wave emitter

[EN] Local perturbations in the relative orientation of the magnetic moments in a continuous magnetic system can propagate in the form of waves. These so-called spin waves represent a promising candidate as an information carrier for spin-based low-power applications. A localized, energy-efficient excitation of coherent and short-wavelength spin waves is a crucial technological requirement, and alternatives to excitation via the Oersted field of an alternating current must be explored. Here, we show how a domain wall pinned at a geometrical constriction in a perpendicularly magnetized thin nanowire emits spin waves when forced to rotate by the application of a low direct current flowing along the wire. Spin waves are excited by the in-plane stray field of the rotating domain wall and propagate at an odd harmonic of the domain wall rotation frequency in the direction of the electron's flow. The application of an external field, opposing domain wall depinning induced by the current, 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 manufacture tuneable high-frequency spin wave emitters of easy fabrication and low power consumption.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y Leon (SA282U14, SA090U16

Current-driven domain wall dynamics in ferrimagnets: Micromagnetic approach and collective coordinates model

[EN] Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic alloys and, by extension, other antiferromagnetically coupled systems as some multilayers, are here presented. The analysis has been made by means of micromagnetic simulations that consider these systems as constituted by two subsystems coupled in terms of an additional exchange interlacing them. Both subsystems differ in their respective gyromagnetic ratios and temperature dependence. Other interactions, as for example anisotropic exchange or spin-orbit torques, can be accounted for differently within each subsystem according to the physical structure. Micromagnetic simulations are also endorsed by means of a collective coordinates model which, in contrast with some previous approaches to these antiferromagnetically coupled systems, based on effective parameters, also considers them as formed by two coupled subsystems with experimentally definite parameters. Both simulations and the collective model reinforce the angular moment compensation argument as accountable for the linear increase with current of domain wall velocities in these alloys at a certain temperature or composition. Importantly, the proposed approach by means of two coupled subsystems permits to infer relevant results in the development of future experimental setups that are unattainable by means of effective models.MAT2017-87072-C4-1-P from the Spanish government SA299P18 from the Junta de Castillay León

Novel interpretation of recent experiments on the dynamics of domain walls along ferrimagnetic strips

[EN] Domain wall motion along ferrimagnets is evaluated using micromagnetic simulations and a collective-coordinates model, both considering two sublattices with independent parameters. Analytical expressions are derived for strips on top of either a heavy metal or a substrate with negligible interfacial Dzyaloshinskii-Moriya Interaction. The work focuses its ndings in this latter case, with a eld-driven domain wall motion depicting precessional dynamics which become rigid at the angular momentum compensation temperature, and a current-driven dynamics presenting more complex behavior, depending on the polarization factors for each sublattice. Importantly, our analyses provide also novel interpretation of recent evidence on current-driven domain wall motion, where walls move either along or against the current depending on temperature. Besides, our approach is able to substantiate the large non-adiabatic efective parameters found for these systems.Project No. MAT2017-87072-C4-1-P from the (Ministerio de Economía y Competitividad) Spanish Government Project No. SA299P18 from the (Consejería de Educación of) Junta de Castilla y León

Current driven domain wall dynamics in ferrimagnetic strips explained by means of a two interacting sublattices model

[EN] The current-driven domain wall dynamics along ferrimagnetic elements are here theoretically analyzed as a function of temperature by means of micromagnetic simulations and a one dimensional model. Contrarily to conventional effective approaches, our model takes into account the two coupled ferromagnetic sublattices forming the ferrimagnetic element. Although the model is suitable for elements with asymmetric exchange interaction and spin-orbit coupling effects due to adjacent heavy metal layers, we here focus our attention on the case of single-layer ferrimagnetic strips where domain walls adopt achiral Bloch configurations at rest. Such domain walls can be driven by either out-of-plane fields or spin transfer torques upon bulk current injection. Our results indicate that the domain wall velocity is optimized at the angular compensation temperature for both field-driven and current-driven cases. Our advanced models allow us to infer that the precession of the internal domain wall moments is suppressed at such compensation temperature, and they will be useful to interpret state-of-the art experiments on these elements.MAT2017- 87072-C4-1-P from the (Ministerio de Economía y Competitividad) Spanish Government SA299P18 from the (Consejería de Educación) of Junta de Castilla y León

Diseño de una instalación solar térmica de agua caliente sanitaria para una residencia de estudiantes en La Habana

El proyecto se enmarca dentro de una pasantía en el Centro de Estudio de Tecnologías Energéticas Renovables (CETER) dentro el Campus universitario José Antonio Echevarría (CUJAE), facultad de ingeniería de la ciudad de La Habana en Cuba. En dicha pasantía se realizó un curso sobre energía solar térmica a modo de preparatorio para la realización del proyecto. En el presente trabajo se realiza el diseño preliminar de una instalación solar térmica para el suministro de agua caliente sanitaria a una residencia de estudiantes en la facultad de ingeniería de la CUJAE, en la ciudad de La Habana (Cuba). En primer lugar se estudian los elementos principales que componen un sistema de calentamiento de agua mediante la utilización de la energía solar. Para el cálculo de la instalación se utiliza un método estático, para después analizar la instalación diseñada mediante un método dinámico, a fin de poder comparar los resultados obtenidos mediante ambos métodos. Posteriormente se realiza el análisis de la viabilidad económico-medioambiental de la instalación, enmarcado dentro del desarrollo de la tecnología solar en Cuba. Para ello se compara el sistema de energía solar con un sistema de energía convencional. Finalmente se presentan las conclusiones obtenidas a partir de los diversos análisis realizados en el proyecto y se plantea el trabajo futuro al que da pie el proyecto

Domain wall motion by localized temperature gradients

[EN] 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 ofmagnonic spin transfer torque includes a description of thermally excitedmagnons in the sample. A third driving force due to a thermally induced dipolar field is found and described. Finally, thermally induced DWmotion is studied under realistic conditions by taking into account the edge roughness. The results give quantitative insights into the differentmechanisms responsible for domain wall motion in temperature gradients and allow for comparison with experimental results.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y León (SA282U14 y SA090U16

Domain wall motion by localized temperature gradients

[EN] 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 ofmagnonic spin transfer torque includes a description of thermally excitedmagnons in the sample. A third driving force due to a thermally induced dipolar field is found and described. Finally, thermally induced DWmotion is studied under realistic conditions by taking into account the edge roughness. The results give quantitative insights into the differentmechanisms responsible for domain wall motion in temperature gradients and allow for comparison with experimental results.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y León (SA282U14 y SA090U16

Realistic micromagnetic description of all-optical ultrafast switching processes in ferrimagnetic alloys

[EN]Both helicity-independent and helicity-dependent all-optical switching processes driven by single ultrashort laser pulse have been experimentally demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been previously reproduced by atomistic simulations, the lack of a robust micromagnetic framework for ferrimagnets limits the predictions to small nanosystems, whereas the experiments are usually performed with lasers and samples of tens of micrometers. Here we develop a micromagnetic model based on the extended Landau-Lifshitz-Bloch equation, which is firstly validated by directly reproducing atomistic results for small samples and uniform laser heating. After that, the model is used to study ultrafast single shot all-optical switching in ferrimagnetic alloys under realistic conditions.We find that the helicity-independent switching under a linearly polarized laser pulse is a pure thermal phenomenon, in which the size of inverted area directly correlates with the maximum electron temperature in the sample. On the other hand, the analysis of the helicity-dependent processes under circular polarized pulses in ferrimagnetic alloys with different composition indicates qualitative differences between the results predicted by the magnetic circular dichroism and the ones from inverse Faraday effect. Based on these predictions, we propose experiments that would allow one to resolve the controversy over the physical phenomenon that underlies these helicity-dependent all optical processes.This work was supported by Projects No. MAT2017- 87072-C4-1-P funded by Ministerio de Educacion y Ciencia and No. PID2020117024GB-C41 funded by Ministerio de Ciencia e Innovacion, both from the Spanish government, Projects No. SA299P18 and No. SA114P20 from Consejeria de Educacion of Junta de Castilla y León, and project MagnEFi, Grant Agreement No. 860060, (H2020-MSCAITN- 2019) funded by the European Commission. U.A. would like to acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project- ID 328545488—TRR 227, Project No. A08