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SrMnO3 has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO3 films grown on several substrates imposing varying degrees of strain from slightly compressive (−0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (tc) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above tc. Yet, even for films thicker than tc, the atomic monolayers below tc are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn3+/Mn4+ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn3+ cations reduces the effective epitaxial strain in the SrMnO3 films and, thus, the accessibility to the strain-induced multiferroic phase.

Chemical doping and epitaxy can be used to tailor the magnetoelectric properties of multiferroic thin films, such as SrMnO3. Here, we study the dependence of the magnetic order temperatures of Sr1−xBaxMnO3 thin films on epitaxial strain and Ba content. Combining low-energy muon spin spectroscopy and scanning transmission electron microscopy, the broadness of the magnetic transition is attributed to the presence of a Mn-O-Mn angle gradient along the out-of-plane direction. We also demonstrate that the unit cell volume is the key parameter to determine the Néel temperature in Sr1−xBaxMnO3 thin films showing G-type antiferromagnetic order. The occurrence of a simultaneously ferroelectric and ferromagnetic ground state at high strain levels is suggested for the Sr0.8Ba0.2MnO3 thin film deposited on TbScO3.

The perovskite (Sr,Ba)MnO3 system is an ideal candidate for tailoring electrical and magnetoelectric properties through the accurate control of Ba content and epitaxial strain due to the strong coupling between polar instability, spin order, and lattice. Here, first, the polar order is proved to be induced in Sr1−xBaxMnO3 thin films through lattice expansion either by epitaxial strain or chemical pressure, which correlates with the evolution of the dielectric properties. Second, due to strong spin–phonon coupling, a large magnetoelectric response is found in the (Sr,Ba)MnO3 system, in which the dielectric constant drops up to 50% when the antiferromagnetic order emerges, larger than most magnetoelectric oxides. More important, this coupling between magnetism and dielectric properties can be tuned from ≈18% to ≈50% by appropriately selecting Ba content and epitaxial strain. Third, a clear trend of increasing the band gap energy on increasing the unit cell volume either by epitaxial strain or chemical pressure is found, which opens the way for engineering the semiconducting properties of (Sr,Ba)MnO3 system at will. Thus, this work proves the possibility to design the electrical response and the magnetoelectric coupling in (Sr,Ba)MnO3 system.

Spin-phonon coupling is investigated in epitaxially strained Sr1-xBaxMnO3 thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the Sr0.6Ba0.4MnO3 composition grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a 17% hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with Néel temperature (TN) around 150 K. Thus, our results confirm that epitaxial Sr0.6Ba0.4MnO3 thin films display strong spin-phonon coupling below TN, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the Sr1-xBaxMnO3 system.

El desarrollo de materiales multiferroicos con acoplamiento magnetoeléctrico ha adquirido un gran interés en los últimos años. Estos materiales permitirían la creación de dispositivos de bajo consumo energético en el campo de la espintrónica. Como resultado su implementación en memorias de almacenamiento de información permitiría la escritura por medio de campos eléctricos con su posterior lectura a partir de la respuesta magnética. Esta tecnología mejoraría las memorias ferroeléctricas (FeRAMs) y magnéticas (MRAMs) de acceso aleatorio conocidas hasta la fecha. El >Santo Grial> de esta tecnología sería la creación de materiales multiferroicos que presenten en una misma fase ferroelectricidad y ferromagnetismo por encima de temperatura ambiente con un acoplamiento entre ambas propiedades eficiente., En esta búsqueda se han seguido diferentes enfoques tanto teóricos como experimentales. La ingeniería de deformación (strain engineering) de óxidos complejos se ha convertido en una de las estrategias más novedosas y con mayor éxito hasta la fecha. Uno de los puntos clave se centra en la gran variedad de composiciones, propiedades y aplicaciones de estos materiales. Además, el crecimiento epitaxial de láminas delgadas es, entre los métodos habituales para producir materiales, el que induce mayor calidad cristalina. Esto permite el estudio de sus propiedades intrínsecas minimizando la influencia de defectos locales, como las fronteras de grano. Asimismo, el crecimiento epitaxial permite tanto el control de las propiedades de las películas bajo la selección adecuada del sustrato que ejerce una tensión de tipo biaxial como la estabilización de fases complejas que no son estables en el material masivo. Por ejemplo, el material BiFeO3 se ha convertido en el óxido multiferroico más estudiado hasta la fecha, ya que por medio de la tensión epitaxial la polarización espontánea aumenta con respecto a la del material masivo, conservando su ordenamiento antiferromagnético. Recientemente, ha surgido, el interés en un nuevo material con propiedades multiferroicas inducidas por tensión epitaxial, EuTiO3, si bien presenta bajas temperaturas de ordenamiento. Estudios teóricos muestran que la familia de las manganitas de alcalinotérreos con estructura perovskita, AMnO3, podría presentar una fase ferroeléctrica si se expande de forma artificial el volumen de la celda unidad. Este aumento del volumen se puede lograr al incrementar el radio iónico del elemento en la posición A, cambiando el catión de Ca a Ba. Lo más importante en estos materiales es el alto acoplamiento magnetoeléctrico que se espera debido a que es el desplazamiento del catión magnético, Mn4+, de su posición centrosimétrica en la celda unidad lo que induce la distorsión ferroeléctrica. La dificultad se presenta en la estabilización de la estructura de tipo perovskita necesaria para alcanzar el ordenamiento ferroeléctrico, ya que la fase hexagonal es mucho más estable a temperatura ambiente. Las predicciones teóricas mencionadas se demostraron a través del dopaje de SrMnO3 masivo con Ba por encima de 45%. La reducción, de la tetragonalidad (relacionada con la polarización espontánea) a la temperatura de Néel en este material demuestra el fuerte acoplamiento magnetoeléctrico. Estudios teóricos posteriores en películas delgadas de SrMnO3 predijeron la creación de un estado ferroeléctrico al aumentar la tensión epitaxial ejercida en el óxido, así como un cambio en el tipo de ordenamiento antiferromagnético pudiendo convertirse incluso en ferromagnético. En esta tesis, el primer objetivo fue la estabilización de películas delgadas de SrMnO3 con estructura de perovskita pseudocúbica gracias al crecimiento epitaxial. Para este propósito, se eligió la técnica de deposición por láser pulsado como herramienta óptima gracias a su versatilidad y éxito en la preparación de óxidos complejos. Se estudió la estructura y microestructura de las películas delgadas por medio de difracción de rayos X y microscopías de transmisión de electrones. Se determinaron el espesor crítico de las películas para la relajación y su módulo de Poisson a través del estudio de muestras depositadas en sustratos con diferentes, parámetros de red. Se determinó la estequiometría de las películas, más específicamente el contenido en oxígeno, por medio de espectroscopias de fotoelectrones de rayos X y de pérdida de energía por electrones en microscopía electrónica de transmisión. Finalmente, se han caracterizado películas delgadas epitaxiales de composición Sr1-xBaxMnO3 preparadas en nuestro laboratorio. Se estudiaron las propiedades polares de las películas a partir de diversas técnicas. El estudio se centró inicialmente en la demostración de la emergencia de un orden polar en películas tensionadas de SrMnO3. Estos experimentos se desarrollaron en el marco de una colaboración con el grupo Multifunctional Ferroic Group en el ETH (Zürich), bajo la supervisión del Prof. M. Fiebig. Para ello se utilizó la técnica de generación de segundo armónico (SHG). A continuación se estudió la influencia del contenido de oxígeno en las propiedades polares de películas de SrMnO3. Estas propiedades fueron medidas por medio de diversas técnicas de microscopía electrónica de transmisión obteniendo la posición de los átomos con resolución sub-angstrom. Con estas técnicas se midieron variaciones locales, en la posición de los iones con respecto a su posición centrosimétrica. Finalmente, se comenzó el estudio de las propiedades polares de películas de SrMnO3 con diferentes dopajes de Ba. La selección de un sustrato particular para los experimentos permite fijar el parámetro de red en el plano, y bajo diferentes dopajes en Ba se modifica el parámetro de red fuera del plano, y por lo tanto la tetragonalidad. Por medio de la técnica de SHG se inició un estudio sobre la dependencia de las propiedades polares de las películas con su contenido de Ba. Estas películas presentan un ordenamiento de tipo antiferromagnético. Se obtuvo su temperatura de Néel (TN) por medio de medidas de espectroscopia de muones de baja energía desarrolladas en el Paul Scherrer Institute. El principal objetivo fue la demostración del diagrama de fases magnético asociado con la tensión epitaxial propuesto por Lee y Rabe. Mediante el dopaje con Ba, se estudiaron los cambios en TN asociados a la tensión epitaxial y volumen de la celda unidad. A continuación se acopló las películas antiferromagnéticas de SrMnO3 con películas ferromagnéticas de La0.33Sr0.67MnO3 a través del efecto exchange bias con objeto de dar funcionalidad al estado polar antiferromagnético de la película delgada. Por último, se caracterizaron las propiedades dieléctricas de las películas mediante espectroscopia de impedancias. Para ello, se depositaron electrodos interdigitados por la técnica de pulverización catódica combinada con litografía óptica. Las medidas de espectroscopia se desarrollaron a distintas temperaturas y frecuencias del campo eléctrico aplicado, y se obtuvieron la resistencia y la capacidad equivalentes. La observación de un descenso brusco de la capacidad al enfriar por debajo de la temperatura de ordenamiento magnético nos permitió confirmar la existencia de acoplamiento magnetoeléctrico. Como conclusión se estudió por medio de diferentes técnicas la multiferroicidad en películas delgadas de Sr1-xBaxMnO3 sometidas a tensión epitaxial. Este trabajo presenta un pequeño avance en la investigación de materiales multiferroicos con un acoplamiento magnetoeléctrico eficiente y abre nuevas rutas a posteriores estudios que ayuden a su implementación en la electrónica de spin.

High-resolution mapping of electronic transport phenomena plays an increasingly important role for the characterization of ferroic domains and their functionality. At present, spatially resolved electronic transport data are commonly gained from local two-point measurements, collected in line-by-line scans with a conducting nanosized probe. Here, we introduce an innovative experimental approach based on low-energy electron microscopy. As a model case, we study polar domains of varying conductance in strained SrMnO3. By a direct comparison with conductive atomic force and electrostatic force microscopy, we reveal that the applied low-energy electron-microscopy experiment can be considered as an inverse I(V) measurement, providing access to the local electronic conductance with nanoscale resolution and short data-acquisition times in the order of 10-102 ms. Low-energy electrons thus hold yet unexplored application opportunities as a minimal-invasive probe for local electronic transport phenomena, opening a promising route towards spatially resolved, high-throughput sampling at the nanoscale.

Engineering strain in oxides is a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties, but presently remains challenging. Promising intrinsic multiferroics are some manganites with the cubic ABO3 perovskite structure. For instance, previous works have shown that the antiferromagnetic paraelectric SrMnO3 (SMO) could become ferroelectric upon artificial expansion of the unit cell by epitaxial strain. In this case, the induced lattice distortion is expected to induce ferroelectricity by the off-centering of the magnetic cation Mn4+. In this work we explore the existence of polar gradients induced by epitaxial strain in 10-nm thick SMO thin films grown by pulsed laser deposition on LSAT (100) substrates, whose polar nature was predicted theoretically, and recently observed experimentally. Aberration-corrected scanning transmission electron microscopy (STEM) experiments have been carried out to determine the strain fields and the local atomic displacements within the unit cell from Annular Bright Field (ABF) images with atomic resolution. As a result, we map the polar rotation of the ferroelectric polarization at atomic resolution, both far from and near the domain walls and find flexoelectricity resulting from vertical strain gradients (see Fig.1). The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Herein we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.

The urge to bend conventional rules in developing multiferroic materials with strong magnetoelectric coupling, has been partially solved by strain engineering. Among these novel strain-induced multiferroic materials, SrMnO3 was proposed as an optimal candidate as strain promotes the polar distortion and associated off-centring of the magnetic Mn atoms. The prediction of the polar instability under strain was successfully verified and shown to be concomitant with the emergence of conduction domains. However, the properties of these thin films not only depend on the imposed strain but also on the mechanism to accommodate the in-plane lattice parameter, where off-stoichiometry is as additional mechanism besides the widely-known modification of the bond lengths or the octahedral tilt patterns in stoichiometric perovskites. Here, we show experimental evidence for the previously proposed increase of the oxygen vacancy content on increasing tensile epitaxial strain. Furthermore, a novel dependence of the oxygen stoichiometry on the thickness of the films has been found, allowing to control the amount of these defects by both the applied strain and the film thickness. In these films, the Poisson’s ratio has been experimentally obtained lower than in other manganites. Density functional theory calculations confirm the low Poisson's ratio for both stoichiometric SrMnO3 and films containing oxygen vacancies. The control of the content of oxygen vacancies in magnetoelectric materials opens new routes to induce unexpected properties in strained thin films.

The possibility of controlling the magnetization (polarization) by an electric (magnetic) field in materials displaying ferroelectric and magnetic order in the same phase, so-called multiferroic materials, has triggered great amount of research in the last few years. However, very few of them have been proved to show strong magnetoelectric coupling, mainly due to the different mechanisms of ferroelectricity and magnetism in these compounds. Exception can be found, though, in perovskite AMnO3 system (A = Ca, Sr, Ba), in which Mn4+ is expected to be able to drive both the magnetic order and the required non-centrosymmetric distortion for ferroelectric order. Yet ferroelectricity is allowed in this system, solely, when the unit cell volume is large enough to promote Mn off-centring, which may be achieved by using increasingly larger A cation, as partially replacing Sr with Ba, or artificially expanding the lattice parameters by epitaxial strain engineering. However, increasing the size of the A-cation destabilizes the perovskite structure, becoming different non-ferroelectric hexagonal polymorphs the ground state phase. In this work, we report on the synthesis of the perovskite phase of (Sr1-xBax)MnO3 solid solution for Ba content ranging 0.2 ≤ x ≤ 0.5 in thin films [5]. Moreover, we have managed to grow epitaxial tensile-strained (Sr1xBax)MnO3 films, ranging from 0% to 4% strain. In bulk, high hydrostatic pressures are used for synthesizing the metastable pseudocubic phase. Here, instead, we use an alternative powerful mechanism for films, the so-called epitaxial stabilization. Still, we demonstrate that the stability of the perovskite phase over the non-ferroelectric hexagonal phase is only found in a very delicate balance of growth conditions, which severely worsens on increasing Ba content. Finally, using Scanning Transmission Electron Microscopy and electrical measurements based on impedance spectroscopy we have assessed the polar character of (Sr1-xBax)MnO3 films

The development of multiferroic materials with strong magnetoelectric effect has been the object of intensive study in recent years due to their potential applications in data storage technology. The goal is to increase the coupling between magnetic and polar properties, and AMnO3 perovskites (being A = Ca, Sr or Ba) have been proposed as novel candidates with the condition that the unit cell volume is artificially expanded. Two different approaches have been carried out: epitaxial strain engineering in SrMnO3 thin films and Ba doping in bulk SrMnO3. Here we report on the in-plane strain induced polar properties of SrMnO3 thin films probed by optical Second Harmonic Generation (SHG) [4]. Furthermore, the effect of Ba doping in Sr1-xBaxMnO3 films has been studied. Three different Ba-contents have been chosen (x=0.2, 0.3, 0.4). The evolution of the unit cell tetragonality has been analysed using high resolution XRD. With increasing Ba content, the c/a evolution suggests a possible rotation of the polar axis towards the out of plane direction. The correlation between the tetragonality of Sr1-xBaxMnO3 and the polar state of the film will also be shown.

Local perturbations in complex oxides, such as domain walls, strain and defects, are of interest because they can modify the conduction or the dielectric and magnetic response, and can even promote phase transitions. Here, we show that the interaction between different types of local perturbations in oxide thin films is an additional source of functionality. Taking SrMnO 3 as a model system, we use nonlinear optics to verify the theoretical prediction that strain induces a polar phase, and apply density functional theory to show that strain simultaneously increases the concentration of oxygen vacancies. These vacancies couple to the polar domain walls, where they establish an electrostatic barrier to electron migration. The result is a state with locally structured room-temperature conductivity consisting of conducting nanosized polar domains encased by insulating domain boundaries, which we resolve using scanning probe microscopy. Our 'nanocapacitor' domains can be individually charged, suggesting stable capacitance nanobits with a potential for information storage technology.

Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer.