Un gran numero de dispositivos magneticos en la electronica moderna se basan en el exchange bias, una interaccion magnetica que acopla un material ferromagnetico y uno antiferromagnetico, dando origen a un desplazamiento unidireccional del ciclo de histeresis ferromagnetico en una cantidad llamada exchange bias field. Establecer y optimizar el exchange bias implica un tratamiento termico en presencia de un campo magnetico externo, que permite controlar el grado de orden magnetico en el material antiferromagnetico. En esta tesis, demostramos un proceso alternativo para la generacion del exchange bias. Nuestro estudio se centra en uno de los sistemas mas comunes para aplicaciones tecnologicas basado en el exchange bias, el FeCo ferromagnetico acoplado al IrMn antiferromagnetico en forma de peliculas delgadas depositada por magnetron sputtering. Hemos encontrado las condiciones de deposicion que dejan la capa IrMn en una fase amorfa/nanocristalina metaestable. El estado desordenado del IrMn previene el establecimiento del exchange bias en la capa de FeCo adyacente, la cual muestra un ciclo de histeresis centrado y con baja coercitividad tipico de las peliculas delgadas ferromagneticas blandas. Durante los dias sucesivos a la deposicion, una transicion de fase estructural se desarrolla espontaneamente a temperatura ambiente en la capa de IrMn y se expande siguiendo un proceso de nucleacion y crecimiento bidimensional. A medida que la transicion de fase se propaga, el exchange bias se forma progresivamente en el FeCo segun la direccion de su imanacion remanente. La caracterizacion estructural de las bicapas de IrMn/FeCo revela que el IrMn, inicialmente amorfo/nanocristalino, experimenta una cristalizacion espontanea que conduce a una fase quimicamente desordenada de tipo γ, altamente cristalina. Una vez completada la transicion de fase, la capa de IrMn contiene granos cristalinos muy grandes, del orden de unos cientos de nanometros, con una gran densidad de defectos estructurales que se extienden a traves de su espesor. Las imagenes tomadas por microscopia electronica de transmision indican que el origen de estos defectos puede residir en la naturaleza policristalina de la capa de FeCo adyacente. La caracterizacion magnetica de las muestras cristalizadas revela un exchange bias moderadamente fuerte con buena estabilidad a temperatura ambiente. Las medidas muestran tambien unas respuestas magneticas y termicas inesperadas que no se pueden explicar con el modelo granular del exchange bias, generalmente utilizado para materiales policristalinos. Con el fin de justificar como cambian las propiedades de las muestras cristalizadas durante el recocido a varias temperaturas, analizamos los resultados a la luz de un modelo de fuerte fijacion de paredes antiferromagnetica, que indica que los defectos estructurales en la capa de IrMn juegan un papel importante tanto en la estabilidad como en la resistencia del exchange bias. Ademas, proponemos un posible mecanismo para el establecimiento inicial del exchange bias, lo que explicaria el aumento progresivo del exchange bias field durante su formacion espontanea, a medida que la fase cristalina se propaga. Finalmente, abordamos la interesante posibilidad de controlar la direccion del exchange bias, simplemente cambiando la imanacion de la capa de FeCo a medida que se propaga la transicion de fase. Esto permite el modelado microscopico de la direccion del exchange bias, lo que generalmente no es posible a menos que la muestra se trate localmente con laser o radiacion ionica. El estudio de este mecanismo para establecer y modelar el exchange bias en los sistemas de IrMn/FeCo puede contribuir a aclarar aspectos fundamentales de este fenomeno, asi como a nuevos desarrollos en el campo de la espintronica. ----------ABSTRACT---------- Most magnetic devices in advanced electronics rely on the exchange bias effect, a magnetic interaction that couples a ferromagnetic and an antiferromagnetic material, resulting in a unidirectional displacement of the ferromagnetic hysteresis loop by an amount called the “exchange bias field”. Setting and optimizing the exchange bias involves a thermal treatment in presence of an external magnetic field, which allows to control the degree of magnetic order in the antiferromagnetic material. In this thesis we demonstrate an alternative process for the generation of exchange bias. Our study focuses on one of the most common systems for technological applications based on exchange bias, which is the ferromagnetic FeCo coupled to the antiferromagnetic IrMn in the form of a multilayered thin film deposited by magnetron sputtering. We have found the deposition conditions which leave the IrMn layer in a metastable amorphous/nanocrystalline phase. The disordered state of the IrMn prevents the setting of the exchange bias in the adjacent FeCo layer, which shows a centered hysteresis loop with low coercivity, typical of soft ferromagnetic thin films. During the days after deposition, a structural phase transition in the IrMn layer develops spontaneously at room temperature and spreads according to a two-dimensional nucleation and growth process. As the phase transition propagates, the FeCo gets progressively exchange-biased according to the direction of its remanent magnetization. The structural characterization of the IrMn/FeCo bilayers reveals that the as-deposited amorphous/nanocrystalline IrMn undergoes a spontaneous crystallization which leads to a highly crystalline, chemically-disordered γ-phase. Once the phase transition is completed, the IrMn layer consists of very large single-crystal grains, of the order of few hundred nanometers, with a large density of structural defects which extend through its thickness. The images taken by transmission electron microscopy indicate that the origin of these defects may lie in the polycrystalline nature of the adjacent FeCo layer. The magnetic characterization of the crystallized samples shows a moderately strong exchange bias with good stability at room temperature. It also reveals some unexpected magnetic and thermal responses which cannot be explained with the granular model for the exchange bias, generally used for polycrystalline materials. In order to justify the properties of the crystallized samples upon field annealing, we analyze the results in light of a strong antiferromagnetic domain walls pinning model, which indicates that the structural defects in the IrMn layer may play an important role in both the stability and the strength of the exchange bias. Furthermore, we propose a possible mechanism for the initial setting of the exchange bias, which would explain the progressive increase of the exchange bias field during its spontaneous formation, as the crystalline phase propagates. Finally, we report the interesting possibility of controlling the direction of the exchange bias, simply by switching the magnetization of the FeCo layer as the phase transition propagates. This allows microscopic patterning of the direction of the exchange bias, which is generally not possible unless the sample is locally treated with laser or ion radiation. The study of this mechanism for establishing and tailoring the exchange bias in IrMn/FeCo systems can contribute toward the clarification of fundamental aspects of this phenomenon, as well as toward further developments in the field of spintronics.

/pdf/spontaneous-exchange-bias-formation-driven-by-a-structural-2v0iu7s1ju.pdf

Spontaneous exchange bias formation driven by a structural phase transition in the antiferromagnetic material

Thin film phase transformation kinetics: From theory to experiment

The Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is widely used to describe phase transformation kinetics. This description, however, is not valid in finite size domains, in particular, thin films. A new computational model incorporating the level-set method is employed to study phase evolution in thin film systems. For both homogeneous (bulk) and heterogeneous (surface) nucleation, nucleation density and film thickness were systematically adjusted to study finite-thickness effects on the Avrami exponent during the transformation process. Only site-saturated nucleation with isotropic interface-kinetics controlled growth is considered in this paper. We show that the observed Avrami exponent is not constant throughout the phase transformation process in thin films with a value that is not consistent with the dimensionality of the transformation. Finite-thickness effects are shown to result in reduced time-dependent Avrami exponents when bulk nucleation is present, but not necessarily when surface nucleation is present.

/pdf/modeling-interface-controlled-phase-transformation-kinetics-1soaqt42oj.pdf

Modeling interface-controlled phase transformation kinetics in thin films

Abstract Ultrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass. 

/pdf/ultrastable-glasses-new-perspectives-for-an-old-problem-1twdlcpt.pdf

Ultrastable glasses: new perspectives for an old problem

Aluminum is an attractive anode material for lithium-ion batteries (LIBs) owing to its low cost, light weight, and high specific capacity. However, utilization of Al-based anodes is significantly limited by drastic capacity fading during cycling. Herein, a systematic study is performed to investigate the kinetics of electrochemical lithiation of Al thin films to understand the mechanisms governing the phase transformation, by using an operando light microscopy platform. Operando videos reveal that nuclei appear at random positions and expand to form quasi-circular patches that grow and merge until the phase transformation is complete. Based on this direct evidence, models of the lithiation processes in Al anodes are discussed and reaction-controlled kinetics are suggested. The growth rate of LiAl depends on the potential and increases considerably as higher overpotentials are approached. Lastly, improved cycling performance of Al-based anodes can be realized by two approaches: 1) by controlling the lithiation extent, the cycling life of Al thin film is extended from 5 cycles to 25 cycles; 2) the performance can be optimized by adjusting the kinetics. Together, this work offers a renewed promise for the commercialization of Al-based anodes in LIBs where the performance requirements are compatible with the proposed cycle life-extending strategies.

Improvement of the Cycling Performance of Aluminum Anodes through Operando Light Microscopy and Kinetic Analysis.

The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model is widely used to quantify the isothermal crystallization kinetics. The present work reports an analytical solution for the crystallization kinetics in the special case of plate-shaped samples with a finite thickness. As a result, we obtained an adapted JMAK model revealing the thickness range which influences the crystallization kinetics mode significantly. The analytical solution also provides theoretical bounds for the film thickness, where the assumption of 2D or 3D kinetics is accurate. Finally, the conclusions related to amorphous silicon and amorphous nickel-titanium thin films are reported.

Solomon Agbo

Papers

Finite-thickness effect on crystallization kinetics in thin films and its adaptation in the Johnson-Mehl-Avrami-Kolmogorov model