Suchergebnisse

[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.

[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.

[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.

[EN] The control of the magnetization at the microscale by pure optical means is fundamentally interesting and promises faster speeds for data storage devices. Although several experiments have shown that it is possible to locally reverse the magnetization of a ferromagnetic system by means of laser pulses, a completely theoretical description of these All Optical Switching processes is still lacking. Here, we develop an advanced micromagnetic solver that is applied to the numerical study of the All Optical Switching. The solver is based on the Landau-Lifshitz-Bloch equation that governs the dynamics of the magnetization coupled the microscopic three temperatures model, which describes the temporal evolution of the temperatures of the subsystems as caused by laser heating. The helicity-dependent magnetization switching is evaluated by a magneto-optical effective field caused by the Inverse Faraday Effect when a circularly polarized laser is applied to the sample. All the other usual terms of a full micromagnetic model are included (exchange, anisotropy, DMI…). As a test, the model is used to describe the local magnetization switching of thin film samples with high perpendicular anisotropy. The results are in good agreement with available experimental observations. ; MAT2014- 52477-C5-4-P, MAT2017-87072-C4-1-P, and MAT2017-90771-REDT from the Spanish government SA090U16 and SA299P18 from the Junta de Castilla y Leon.

[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.

[EN] Relying on both electromechanical and micromagnetic simulations, we propose a method to control the trajectory of current-driven skyrmions using an electric field in hybrid piezoelectric-magnetic systems. By applying a voltage between two lateral electrodes, a transverse strain gradient is created, as a result of the nonuniform electric field profile in the piezoelectric material. Due to magnetoelastic coupling, this transverse gradient leads to a lateral force on the skyrmions that can be used to suppress the skyrmion Hall angle for any given current density, if a proper voltage is applied. We show that this method works under realistic conditions, such as the presence of disorder in the ferromagnet, and that skyrmion trajectories can be controlled with moderate voltages. Moreover, our method allows the maximum current density that can be injected before the skyrmion is annihilated at the nanostrip edge to be increased, which leads to an increase in the maximum achievable velocities. ; European Union H2020 Program (MSCA MagnEFi ITN Grant No. 860060) Ministerio de Education y Ciencia (Project No. MAT2017-87072-C4-1-P) Ministerio de Ciencia e Innovacion (Project No. PID2020-117024GB-C41) Consejeria de Educación of Castilla y León (Projects No. SA114P20 and No.SA299P18).

[EN]Heating a ferromagnetic material is often perceived as detrimental for most applications. This is indeed the case for modern nano-scaled spintronic devices which are operated solely (at least ideally) by an electric current. Heat is a by-product of the current-driven operation and it deteriorates many functionalities of the device. A large scientific and technological effort is devoted these days to avoid heat in modern magnetic nano devices. Here we show that heat can be used to provide an additional and useful degree of freedom in the control of the local magnetization at the nanoscale. In a ferromagnetic nanowire, temperature is used to induce a magnetic switching through a perfectly deterministic mechanism. The nucleation of the magnetic domain walls that triggers the switching can be achieved at a field considerably smaller than the nucleation field and, importantly, the exact moment of the magnetic switching can be pre-determined with nanosecond precision by controlling the power delivered locally to the switching area. With the help of micromagnetic simulations and a theoretical model, we provide an accurate explanation of how this deterministic thermo-magnetic switching operates. The concepts described in this work may lead to an increased functionality in magnetic nano-devices based on magnetic domain walls. ; MAT2017-87072-C4-1-P,MAT2017-87072-C4-4-P and MAT2017-87072-C4-3-P from the Spanish government SA299P18 from the Junta de Castilla y Leon POCI-01-0145-FEDER-028676 from Portuguese FCT COMPETE 2020 (FEDER).

[EN] There is a lot of experimental evidence of All Optical Switching (AOS) by applying ultrashort laser pulses on ferromagnetic thin films with perpendicular magnetic anisotropy. However, the physical origin behind these processes remains under debate. In addition to the heating caused by the laser pulses, the Inverse Faraday Effect (IFE) and Magnetic Circular Dichroism (MCD) have been proposed as the most probable phenomena responsible for the observations of helicity-dependent AOS. Here, we review the influence of both phenomena by means of realistic micromagnetic simulations based on the Landau–Lifshitz–Bloch equation coupled to the heat transport caused by the laser heating. The analysis allows us to reveal the similarities and differences between both effects. While both mechanisms may lead to the local inversion of the initial magnetic state of a ferromagnetic sample submitted to a train of circularly polarized laser pulses, the Inverse Faraday Effect proves to be more efficient for nucleation and domain wall movement and it reproduces more accurately the different magnetic configurations that the experiments report for different values of the fluence of the laser beam. ; Project No. MAT2017-87072-C4-1-P from Ministerio de Economía y Competitividad of the Spanish Government Project No. SA299P18 from the Consejería de Educación of Junta de Castilla y León Project MagnEFi, Grant Agreement 860060 (H2020-MSCA-ITN-2019) funded by the European Commission.

[EN] Micromagnetic simulations are used to investigate the motion of magnetic skyrmions in an in-plane strain gradient. The skyrmion diameter and energy are found to depend on the strain, which leads to a force that moves the skyrmion toward regions with higher strain. An analytical expression for the skyrmion velocity as a function of the strain gradient is derived assuming a rigid profile for the skyrmion, and good agreement with simulations is obtained. Furthermore, electromechanical simulations of a hybrid ferromagnetic/piezoelectric device show that the in-plane strain gradients needed to move skyrmions can be achieved ; Project No. MAT2017-87072-C4-1-P from the (Ministerio de Economía y Competitividad) Spanish government Project No. SA299P18 from Consejería de Educacíon Junta de Castilla y León.

[EN] All-optical helicity-dependent switching in ferromagnetic layers has revealed an unprecedented route to manipulate magnetic configurations by circularly polarized femtosecond laser pulses. In this work, rare-earth free synthetic ferrimagnetic heterostructures made from two antiferromagnetically exchange coupled ferromagnetic layers are studied. Experimental results, supported by numerical simulations, show that the designed structures enable all-optical switching which is controlled, not only by light helicity, but also by the relative Curie temperature of each ferromagnetic layer. Indeed, through the antiferromagnetic exchange coupling, the layer with the larger Curie temperature determines the final orientation of the other layer and so the synthetic ferrimagnet. For similar Curie temperatures, helicity-independent back switching is observed and the final magnetic configuration is solely determined by the initial magnetic state. This demonstration of electrically-detected, optical control of engineered rare-earth free heterostructures opens a novel route toward practical opto-spintronics. ; Newton International Fellowship scheme NF150217 Deutsche Forschungsgemeinschaft tSFB/TRR 227 "Ultrafast Spin Dynamics," Project A08 UK EPSRC (Grant No. EP/P005713/1). Institut Carnot ICEEL for the project "Optic-switch" and Matelas and by the French PIA project "Lorraine Université d'Excellence," (Ref. No. ANR-151DEX-04-LUE) FEDER (EU), ANR, Région Grand Est, and Métropole Grand Nancy Project No. MAT2017-87072-C4-1-P from the (Ministerio de Economia y Competitividad) Spanish government SA299P18 from Consejeria de Educacion Junta de Castilla y Leon.