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

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

Current-Driven Skyrmion Dynamics Along Curved Tracks

[EN] The current-driven skyrmion (Sk) motion along two exchange-coupled ferromagnetic (FM) layers with perpendicular magnetic anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single FM layer. Our results indicate that the two coupled Sks can be synchronously driven along each FM layer in the presence of a strong interlayer exchange coupling and that the velocity is significantly enhanced due to the antiferromagnetic (AF) exchange coupling as compared with the single-FM-layer case. The interfacial Dzyaloshinskii–Moriya interaction gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled Sks by a dragging mechanism, without depicting the unwanted Sk Hall effect. This observation is particularly relevant to drive Sks along curved strips, which are also evaluated here. Sks move with different velocities along single FM stacks with curved parts. On the contrary, the AF coupling between the FM layers mitigates the Sk Hall effect, which suggests these systems to achieve efficient and highly packed displacement of trains of Sks for spintronics devices. A study taking into account defects and thermal fluctuations analyzes the validity range of these claims.Spanish Governmentunder Project MAT2014-52477-C5-4-P, Project MAT2017- 87072-C4-1-P, and Project MAT2017-90771-REDT Junta de Castilla y Leon under Project SA090U16 and Project SA299P18

Current-driven domain wall motion along ferromagnetic strips with periodically-modulated perpendicular anisotropy

[EN]The dynamics of magnetic domain walls along ferromagnetic strips with spatially modulated perpendicular magnetic anisotropy are theoretically studied by means of micromagnetic simulations. Ferromagnetic layers with a periodic sawtooth profile of the anisotropy depict a well-defined set of energy minima where the walls are pinned in the absence of external stimuli, and favor the unidirectional propagation of domain walls. The performance of the current-driven domain wall motion along these ratchet-like systems is compared to the field-driven case. Our study indicates that the current-driven domain wall motion exhibits significant improvements with respect to the field-driven case in terms of bit shifting speed and storage density, and therefore, it is suggested for the development of novel devices. The feasibility of these current-driven ratchet devices is studied by means of realistic micromagnetic simulations and supported by a one-dimensional model updated to take into account the periodic sawthooth anisotropy profile. Finally, the current-driven domain wall motion is also evaluated in systems with a triangular modulation of the anisotropy designed to promote the bidirectional shifting of a series of walls, a functionality that cannot be achieved by magnetic fields.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

Current-driven domain wall motion based memory devices: Application to a ratchet ferromagnetic strip

[EN] Ratchet memories, where perpendicular magnetocristalline anisotropy is tailored so as to precisely control the magnetic transitions, has been recently proven to be a feasible device to store and manipulate data bits. For such devices, it has been shown that the current-driven regime of domain walls can improve their performances with respect to the field-driven one. However, the relaxing time required by the traveling domain walls constitutes a certain drawback if the former regime is considered, since it results in longer device latencies. In order to speed up the bit shifting procedure, it is demonstrated here that the application of a current of inverse polarity during the DW relaxing time may reduce such latencies. The reverse current must be sufficiently high as to drive the DW to the equilibrium position faster than the anisotropy slope itself, but with an amplitude sufficiently low as to avoid DW backward shifting. Alternatively, it is possible to use such a reverse current to increase the proper range of operation for a given relaxing time, i.e., the pair of values of the current amplitude and pulse time that ensures single DW jumps for a certain latency time.Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y Leon (SA090U16

Efficient and controlled domain wall nucleation for magnetic shift registers

[EN] Ultrathin ferromagnetic strips with high perpendicular anisotropy have been proposed for the development of memory devices where the information is coded in tiny domains separated by domain walls. The design of practical devices requires creating, manipulating and detecting domain walls in ferromagnetic strips. Recent observations have shown highly efficient current-driven domain wall dynamics in multilayers lacking structural symmetry, where the walls adopt a chiral structure and can be driven at high velocities. However, putting such a device into practice requires the continuous and synchronous injection of domain walls as the first step. Here, we propose and demonstrate an efficient and simple scheme for nucleating domain walls using the symmetry of the spin orbit torques. Trains of short sub-nanosecond current pulses are injected in a double bit line to generate a localized longitudinal Oersted field in the ferromagnetic strip. Simultaneously, other current pulses are injected through the heavy metal under the ferromagnetic strip. Notably, the Slonczewski-like spin orbit torque assisted by the Oersted field allows the controlled injection of a series of domain walls, giving rise to a controlled manner for writing binary information and, consequently, to the design of a simple and efficient domain wall shift register.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 ferromagnetic layers synthetically exchange-coupled by a spacer: A micromagnetic study

[EN] The current-driven domain wall motion along two exchange-coupled ferromagnetic layers with perpendicular anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single ferromagnetic layer. Our results, where only the lower ferromagnetic layer is subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin Hall effect, indicate that the domain walls can be synchronously driven in the presence of a strong interlayer exchange coupling, and that the velocity is significantly enhanced due to the antiferromagnetic exchange coupling as compared with the single-layer case. On the contrary, when the coupling is of ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic characterization of the current-driven motion in these multilayers, both in the absence and in the presence of longitudinal fields, and the results are explained based on a one-dimensional model. The interfacial Dzyaloshinskii-Moriya interaction, only necessary in this lower layer, gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled walls by a dragging mechanism, without significant tilting of the domain wall plane. Finally, the domain wall dynamics along curved strips is also evaluated. These results indicate that the antiferromagnetic coupling between the ferromagnetic layers mitigates the tilting of the walls, which suggest these systems to achieve efficient and highly packed displacement of trains of walls for spintronics devices. A study, taking into account defects and thermal fluctuations, allows to analyze the validity range of these claimsComisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y Leon (SA282U14, SA090U16

Elaboración de material informático para la enseñanza de las interacciones fundamentales en electromagnetismo

Material no publicadoEste proyecto ha sido realizado por cinco profesores del Área de Electromagnetismo del Departamento de Electricidad y Electrónica en la Universidad de Valladolid. Los objetivos son la elaboración de una serie de unidades didácticas con las bases teóricas de las interacciones fundamentales en electromagnetismo, complementadas con ejemplos prácticos y ejercicios para el alumno. Por otra parte se han desarrollado varios programas informáticos para simular el comportamiento de los sistemas eléctricos y magnéticos, con posibilidad de diseñar diferentes problemas, obtener resultados numéricos y variar distintos parámetros de modo interactivo. Estos programas disponen de ayuda en pantalla, así como de guías de usuario impresas para facilitar su uso. Con este material se pretende favorecer el aprendizaje del alumno aprovechando las posiblidades que ofrece el empleo del ordenador como recurso docente, tanto en lo que respecta al almacenamiento de información de las unidades didácticas como en la simulación de experimentos prácticos de complicada reproductividad en un laboratorio de alumnos, y en la visualización de magnitudes de complicada representación en el aula. Se espera obtener una primera evaluación en el curso 1999-2000.Junta de Castilla y León. Dirección General de Universidades e InvestigaciónCastilla y LeónES

Elaboración de material para demostraciones prácticas en el ámbito del electromagnetismo

Realizado en la Facultad de Ciencias de Valladolid, por 5 profesores del centro, para las asignaturas de Física, en general y de Electromagnetismo. El objetivo era elaborar un material multimedia que resultara de apoyo para la enseñanza y ejecución de prácticas de laboratorio en el ámbito del electromagnetismo. Se confeccionó una colección de prácticas cuya realización sólo necesitaba material usual en la mayor parte de los laboratorios o, si no, fácil de construir. Esta colección se dirige tanto a alumnos como a profesores de diferentes niveles educativos. También, se han realizado vídeos que sirvan de apoyo a las prácticas. El trabajo final se compone del manual en soporte papel y de soporte informático en formato DVD en el que se incluye: el manual en PDF con hipervínculos para buscar rápidamente la información que se busca, los vídeos en formato MPG2 y las aplicaciones necesarias para el correcto visionado del curso. A fin de evaluar la utilidad de los contenidos propuestos se ha usado el material para realizar diferentes demostraciones prácticas con los alumnos de la Licenciatura en Ciencias Físicas y con alumnos de centros de Enseñanza Media que han visitado el laboratorio. Esta actividad ha permitido constatar que este tipo de enfoque docente tanto para obtener una mejor calidad de enseñanza y despertar en el alumno una curiosidad por temas afines.Junta de Castilla y León. Consejería de Educación y CulturaCastilla y LeónJunta de Castilla y León. Consejería de Educación y Cultura; Monasterio de Nuestra Señora de Prado. Autovía Puente Colgante, s. n.; 47071 Valladolid; Tel. +34983411881; Fax +34983411939; [email protected]