Micro and nanostrips in spintronics: How to keep them cool

[EN] This tutorial explores the problem of Joule heating on metallic micro or nanostrips, still one of the most popular geometries in modern spintronics. Many of the effects that result from the interaction of a spin polarized current and the local magnetization, require of a sizeable current density. This implies, quite often, an unneglectable Joule heating. Despite the few articles devoted to some aspects of Joule heating, there is still disinformation and many misconceptions in this topic, which is key for the correct interpretation of the scientific results. In this tutorial, we highlight the material parameters that are important to keep the temperature of the strip under control and those that give only a marginal advantage. In the vast majority of papers, at least one of these parameters is missing. We also focus on some misconceptions, such as the belief that performing the measurement on a cryostat, rules Joule heating out. In fact, for a fixed current density, measuring in a cryostat decreases the temperature but not enough to justify the use of such a costly measuring set-up. At the practical level, we put forward a 1D model to calculate, in few seconds, if Joule heating is present and if it should be taken into account when interpreting the results. Finally, and importantly, we describe a simple fabrication route to enhance the dissipation of heat in the strip considerably. This fabrication strategy is more effective at keeping the temperature under control than performing the experiment at cryogenic temperatures.Project MAT2017-87072-C4-3-P and MAT2017-87072-C4-1-P from the Spanish government Project No. SA299P18 from Consejería de Educación Junta de Castilla y Leó

Micromagnetic Modeling of All Optical Switching of Ferromagnetic Thin Films: The Role of Inverse Faraday Effect and Magnetic Circular Dichroism

[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

Magneto-thermal effects induced by electric currents in ferromagnetic nanostrips

Spintronics is a field of physics that has been booming since the discovery of Giant Magnetoresistance (GMR), a finding awarded with the Nobel Prize in 2007. Modern Spintronics is in constant search for applications of nanomagnetism such as future non-volatile memories, logic gates even functional devices for neuromorphic computing. At the core of all these applications is the transfer of angular momentum from the electric current to the local magnetization. This thesis studies three key aspects in modern spintronics: Joule heating, magnetic domain wall motion, and Skyrmions. Our study on Joule heating focuses on analysing the relevant parameters that control the heating process on a ferromagnetic nanostrip when an electric current flows through it. To do this, simulations were performed using COMSOL Multiphysics software, studying in detail the influence of parameters such as interface thermal resistance, the thermal conductivity of the substrate and the strip, and the influence of the strip dimensions. As a result, we proposed a simple model to estimate the temperature of a nanostrip, using only parameters that are easily accessible to experimentalists. When the electric current density is very large, typically larger than 1011 Am−2, the nanodevices can get damaged due to different mechanisms such as electro-migration or simply by evaporation. Most of the spintronic experiments operate with a current density typically larger than 1011 Am−2, so it is always important to generate mechanisms to improve heat dissipations. In this thesis, we propose a method to dissipate thermal energy and reduce the temperature of the strips. It involves coating the strips with materials with good thermal conductivity, so the top surface of the nanostrip can act also as a conductive channel. The top layers provide also structural protection to the devices. We have analysed this fabrication route both theoretically through simulations and experimentally, demonstrating that the proposed fabrication route is very effective enhancing the heat dissipation. Once the heating process and the build-up of heat gradients is properly characterized, the goal was to study the magnetization process in a strip made out of a magnetic material with perpendicular magnetic anisotropy (PMA), in the presence of thermal gradients. After different attempts, we centred the study on the motion of magnetic domain walls on Pt/Co/Pt ferromagnetic strips. These samples showed a very defined large PMA and we studied how the propagation field is altered by a DC electric current, combining the influence of heat and Spin Orbit torques, such as Rashba and/or spin-Hall. Finally, this thesis includes the results of the research conducted during an internship in Spintec, France. Following some theoretical predictions related to the possible existence of planar Skyrmions in samples with in-plane magnetic anisotropy, we conducted an experimental work dedicated to find them. The measurements were performed using both Magnetic Force Microscope and Photoemission Electron Microscope via the Magnetic Circular Dichroism X-ray technique to test the existence of planar Skyrmions. We also conducted MuMax3 simulations to contrast the experimental results. RESUMEN La espintrónica es un campo de la física en imparable auge que desde el descubrimiento de la Magnetorresistencia Gigante (GMR), a la que se le concedió el premio Nobel en 2017. La espintrónica moderna busca aplicaciones del nanomagnetismo como las futuras memorias no volátiles, puertas lógicas o incluso dispositivos funcionales para computación neuromórfica. En el núcleo de estas aplicaciones está la transferencia de momento angular desde la corriente eléctrica a la imanación local. Esta tesis estudia tres aspectos clave de la espintrónica actual: Calentamiento por efecto Joule, movimiento de paredes de dominio magnéticas y estudio de Skyrmions. Nuestro estudio sobre el calentamiento por efecto Joule se centra en analizar los parámetros que influyen en el calentamiento de nanocintas ferromagnéticas cuando una corriente eléctrica las atraviesa. Para ello se han realizado simulaciones con el software COMSOL Multiphysics, estudiando en detalle la influencia de parámetros como la resistencia térmica de interfaz, la conductividad térmica del sustrato y de la cinta y las dimensiones del ferromagnético. Como resultado, hemos propuesto un modelo simple para estimar la temperatura que alcanza la nanocinta, usando exclusivamente parámetros a los que un científico experimental puede acceder fácilmente. Cuando se utilizan densidades de corriente eléctrica elevadas, típicamente mayores de 1011 Am−2, los dispositivos se pueden dañar por diferentes causas, por ejemplo, electro-migración o simplemente la evaporación. La mayoría de los experimentos espintrónicos se realizan con densidades de corriente superiores a 1011 Am−2 y es por tanto siempre conveniente encontrar mecanismos que ayuden a la disipación térmica. En esta tesis se propone un método para disipar energía térmica y reducir la temperatura de las cintas. Consiste en recubrir las cintas con materiales con buena conductividad térmica, de manera que se habilite la superficie superior de la cinta como canal de conducción de calor. Estas capas aportan además una protección estructural a los dispositivos. Hemos analizado este método de fabricación teóricamente a través de simulaciones y experimentalmente, demostrando que este método de fabricación es realmente efectivo ayudando con la disipación del calor. Una vez que el proceso de calentamiento y el desarrollo de gradientes térmicos está bien caracterizado, el objetivo era estudiar el proceso de imanación en una cinta hecha de un material magnético con anisotropía perpendicular al plano (PMA), en presencia de gradientes térmicos. Tras diferentes intentos, centramos el estudio en el movimiento de paredes de dominio en cintas de Pt/Co/Pt. Estas muestras muestran una anisotropía perpendicular al plano bien definida y estudiamos como el campo de propagación se veía alterado en presencia de una corriente eléctrica DC, combinando la influencia del calor con los pares nacidos del acoplo espín-órbita, como el Rashba y/o el efecto Hall de espín. Finalmente, esta tesis incluye los resultados de la investigación llevada a cabo durante una estancia en Spintec, Francia. Partiendo de las predicciones teóricas sobre la posible existencia de Skyrmions planares en muestras con anisotropía magnética en el plano, se planteó una investigación para tratar de observar este tipo de Skyrmions. Las medidas se realizaron tanto con Microscopio de Fuerza Magnética como en Microscopio de Foto-emisión de Electrones mediante la técnica de Rayos X de Dicroísmo Circular Magnético probando la existencia de Skyrmions planares. Las medidas experimentales se han apoyado en simulaciones MuMax3 también realizadas en esta tesis

Micromagnetic Modeling of All Optical Switching of Ferromagnetic Thin Films: The Role of Inverse Faraday Effect and Magnetic Circular Dichroism

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

Fast current-induced skyrmion motion in synthetic antiferromagnets

This is the author’s version of the work. It is posted here bypermission of the AAAS for personal use, not for redistribution. The definitive version was published in Science 384, 307–312 (2024) , DOI: 10.1126/science.add5751International audienceMagnetic skyrmions are topological magnetic textures that hold great promise as nanoscale bits of information in memory and logic devices. Although room-temperature ferromagnetic skyrmions and their current-induced manipulation have been demonstrated, their velocity has been limited to about 100 meters per second. In addition, their dynamics are perturbed by the skyrmion Hall effect, a motion transverse to the current direction caused by the skyrmion topological charge. Here, we show that skyrmions in compensated synthetic antiferromagnets can be moved by current along the current direction at velocities of up to 900 meters per second. This can be explained by the cancellation of the net topological charge leading to a vanishing skyrmion Hall effect. Our results open an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks