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Voltage control of magnetism through electric field-induced oxygen motion (magneto-ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in a large variety of magnetic devices, such as magnetic micro-electro-mechanical systems (MEMS), magnetic logics, spin electronics, or neuromorphic computing, i.e., envisaging ultra-low power emulation of the biological synapse. Boosting the induced changes in magnetization, magneto-ionic motion and cyclability (endurance) continue to be key challenges to turn magneto-ionic phenomena into real applications. Here, we demonstrate that, without degrading cyclability, room temperature magneto-ionic motion in electrolyte-gated paramagnetic and fairly thick (> 100 nm) Co3O4 films largely depends on the configuration used to apply the electric field. In particular, magneto-ionic effects are significantly increased both in terms of generated magnetization (6 times larger: from 118.5 to 699.2 emu cm-3) and speed (35 times faster: from 33.1 to 1170.8 emu cm-3 h-1) if the electric field is applied across a conducting buffer layer (grown underneath the Co3O4 films), instead of directly contacting Co3O4. This is attributed to a greater uniformity and strength of the applied electric field when using the conducting layer. These results may trigger the use of oxygen magneto-ionics into promising new technologies, such as magnetic MEMS or brain-inspired computing, which require endurance and moderate speeds of operation.

So far, magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ion/vacancy and, to a lesser extent, lithium and hydrogen. Here, we demonstrate efficient, room-temperature, voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a single CoN film (without an ion-reservoir layer). Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4, both layers showing a nanocrystalline face-centered-cubic structure and reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results are appealing for the use of magneto-ionics in nitride semiconductor devices, in applications requiring endurance and moderate speeds of operation, such as brain-inspired computing., Comment: 35 pages, 5 main figures, 1 Table, supplementary information included

La nanocalorimetria abre la posibilidad de realizar medidas calorimétricas en capas finas o ultra finas debido al substancial aumento de sensibilidad que presenta respecto a los sistemas comerciales. Basándonos en esa premisa, este trabajo de investigación se ha dedicado al desarrollo de nanocalorimetros basados en membranas que incorporan calentadores y termómetros en capa fina, que permiten reducir la capacidad calorífica de la celda calorimétrica y por tanto al aumento den sensibilidad.En la primera parte de la tesis describimos las técnicas de procesado de semiconductores que se usan para fabricar los microdispositivos con una especial atención a su estabilidad térmica a alta temperatura. Se muestra que usando como actuador una la combinación metálica de Pt/Ti cubierta por Al2O3, se puede utilizar con gran reproducibilidad para calentar/sensar hasta temperaturas de alrededor de 1200K.La nanocalorimetria adiabática se presenta en el capitulo 4. La técnica trabaja a ritmos de calentamiento alrededor de 104 K/s. A estos ritmos de calentamiento se pueden estudiar transformaciones de fase en capas ultrafinas con una sensitividad en energía inferior al nJ. El ruido pico a pico asociado a las medidas de capacidad calorífica es de alrededor a 20 pJ/K, para transformaciones reversibles. La dependencia con el tamaño del punto de fusión y de la entalpía de transformación para capas finas de In se han analizado como estudio preliminar. También presentamos una nueva metodología para poder evaluar la potencia de pérdidas a altas temperaturas. Empleando esta metodología se ha determinado la capacidad calorífica de capas muy finas de Ni alrededor de la transición de curie. Se presencia un estudio en el que se evidencia como los efectos de tamaño tienen un rol fundamental en la transición. La última parte de este capítulo presenta el análisis de capas ultrafinas de Ge encapsuladas entre capas de SiO2 cuando son sometidas a calentamientos ultrarápidos hasta 1200K. Se describen la transformación de amorfo a líquido así como la dependencia con el tamaño de la fusión y el sobreenfriamiento de nanocristales de Ge.En el capítulo 5 presentamos un nuevo sistema de control digital que hemos desarrollado para trabajar con los nanocalorímetros en modo compensación de potencia a ritmos de calentamiento que se expanden desde 0.1 hasta 103 K/s. Este sistema ha permitido analizar muestras de microgramos con sensitividades energéticas de µJ. Este nuevo desarrollo abre la posibilidad al estudio de transformaciones cinéticamente limitadas que típicamente necesitan de ritmos de calentamiento bajos, como por ejemplo para analizar los procesos de RTA (rápidos recocidos térmicos) usados en la industria microelectrónica.Finalmente, en los apéndices tratamos la teoría de control calorimetría y la cristalización de capas finas de Ge de diferentes espesores., Thin film calorimetry opens the possibility to perform calorimetric measurements on ultra-thin or thin films due to the substantial increase in sensitivity compared to commercial systems. Based on this premise, the present research work deals with the development of membrane-based nanocalorimeters incorporating thin film heaters and thermometers which can work with high sensitivity because of their very low thermal mass. In the first part we describe semiconductor processing techniques that are used to fabricate the microdevices with a special care devoted to their high temperature thermomechanical stability. It is shown that alumina coated Pt/Ti resistive elements can be reproducibly used for heating/sensing up to temperatures around 1200 K. Quasi-adiabatic nanocalorimetry is presented in Chapter 4. The technique works at heating rates above 104 K/s. At these rates phase transitions in ultra-thin films can be measured with energy sensitivity in the nJ range. The associated noise in heat capacity is around 20 pJ/K, for reversible transitions. The size-dependent melting point and enthalpy of ultra-thin films of In is analyzed as a case study. A new methodology to account for power losses at high temperatures is presented in this chapter. By employing this methodology the heat capacity of very thin films of Ni at the Curie transition is determined. It is shown that size effects also play a key role in this transition. The last part of this chapter is devoted to the analysis of ultra-thin films of Ge embedded within SiO2 layers during ultrafast heating up to 1200K. The melting of the amorphous phase along with the size-dependent melting and supercooling of Ge nanocrystals is also described. In Chapter 5 we present a new digital-based control system which has been developed to work in power compensation mode at heating rates spanning from 0.1 to 103 K/s. It permits to analyze samples in the microgram range with a energy sensitivity around the µJ. This new development opens the possibility to study kinetically limited transformations that typically need for lower rates or to mimic real conditions similar to those achieved in rapid thermal processing in the microelectronic industry. Finally, several appendix dealing with control theory, calorimetry and the crystallization of Ge films of different thickness are also presented.