María Elena Gómez

The escalating energy demands of modern artificial intelligence have exposed fundamental limitations in conventional computing architectures. Neuromorphic computing, which seeks to emulate the brain’s massively parallel and energy-efficient information processing, offers a compelling alternative, but its realization requires materials that naturally exhibit nonlinear, multi-state, and adaptive behavior characteristic of biological neurons and synapses. This review argues that strongly correlated electron materials, particularly Mott insulators, provide exactly this physical substrate. We present the essential physics of metal-insulator transitions in vanadium oxides (VO2, V2O3) and perovskite manganites in an accessible manner and describe how their switching behaviors map onto neuromorphic primitives: the neuristor (artificial neuron) and the synaptor (artificial synapse). The review draws extensively on the authors’ experimental work: direct imaging of nanotextured phase coexistence in V2O3; an MIT Fingerprinting approach to engineer multi-state resistive switching in VO2; the demonstration of subthreshold firing in Mott nanodevices; and 1 electrically controlled multi-state analog memory in phase-separated manganites. The review concludes by identifying the main challenges and the most promising directions to translate these laboratory demonstrations into practical brain-inspired computing systems.

For the past eight decades, understanding the physical mechanisms that occur at the interface between two different materials has been an open question driven by the breaking of crystalline symmetry and the reduction of atomic coordination, which modify the type and orbital occupation of atoms in the interface. Surface science began studying interface phenomena and has advanced significantly thanks to new and sophisticated tools for controlled fabrication and monolayer characterization. This focus has stimulated scientific and technological advancements, especially in the context of electronic devices and a wide range of interdisciplinary applications, such as biointerfaces, next-generation ultrasensitive detectors and actuators, and improved tribological properties. Here we provide a historical context of emerging interfacial phenomena and their influence on properties exhibited in junctions, multilayers, and heterostructures. We discuss advances in magnetic superlattice systems that enhance the information density of hard drives. We also explore electronic and magnetic interface states, as well as new functionalities in interfaces within heterojunctions based on complex oxides. Finally, we highlight advances and perspectives in surface science. En las últimas ocho décadas la comprensión de los mecanismos físicos que ocurren en la superficie límite entre dos materiales diferentes ha estado en constante evolución. Su interés radica en la ruptura de la simetría cristalina y la reducción de la coordinación atómica, las cuales producen modificaciones en el tipo y la ocupación orbital de los átomos en la interfaz. El estudio de los fenómenos interfaciales dio inicio a la ciencia de superficies, cuyo avance ha sido significativo en la medida en que aparecen nuevas y sofisticadas herramientas, tanto para la fabricación controlada de interfaces, como para la caracterización en el rango de la monocapa atómica. El interés se ha centrado no solo en el estímulo científico si no en el tecnológico, sobre todo en el contexto de los dispositivos electrónicos y, más recientemente, en una amplia gama de aplicaciones interdisciplinarias, como las biointerfaces, los detectores y actuadores ultrasensibles de última generación y el mejoramiento de las propiedades tribológicas. En este trabajo presentamos, primero, un contexto histórico de los diferentes fenómenos interfaciales que han ido surgiendo y su influencia en las propiedades que exhiben donde quiera que hay una interfaz, como en el caso de junturas, multicapas y heteroestructuras. Presentamos, asimismo, los avances en los sistemas de superredes magnéticas de importancia en el mejoramiento de la densidad de información de los discos duros, y discutimos sobre los estados interfaciales electrónicos y magnéticos y las nuevas funcionalidades en las interfaces de heterojunturas basadas en óxidos complejos. Por último, resaltamos algunos avances y perspectivas en el campo de la ciencia de superficies.

We studied in detail the in-plane magnetic properties of heterostructures based on a ferroelectric BaTiO₃ overlayer deposited on a ferromagnetic La_2/3Sr_1/3MnO₃ film grown epitaxially on pseudocubic (001)-oriented SrTiO₃, (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 and LaAlO₃ substrates. In this configuration, the combination of both functional perovskites constitutes an artificial multiferroic system with potential applications in spintronic devices based on the magnetoelectric effect. La_2/3Sr_1/3MnO₃ single layers and BaTiO₃/La_2/3Sr_1/3MnO₃ bilayers using the pulsed-laser deposition technique. We analyzed the films structurally through X-ray reciprocal space maps and high-angle annular dark field microscopy, and magnetically via thermal demagnetization curves and in-plane magnetization versus applied magnetic field loops at room temperature. Our results indicate that the BaTiO₃ layer induces an additional strain in the La_2/3Sr_1/3MnO₃ layers close to their common interface. The presence of BaTiO₃ on the surface of tensile-strained La_2/3Sr_1/3MnO₃ films transforms the in-plane biaxial magnetic anisotropy present in the single layer into an in-plane uniaxial magnetic anisotropy. Our experimental evidence suggests that this change in the magnetic anisotropy only occurs in tensile-strained La_2/3Sr_1/3MnO₃ film and is favored by an additional strain on the La_2/3Sr_1/3MnO₃ layer promoted by the BaTiO₃ film. These findings reveal an additional mechanism that alters the magnetic behavior of the ferromagnetic layer, and consequently, deserves further in-depth research to determine how it can modify the magnetoelectric coupling of this hybrid multiferroic system.

We have deposited ferroelectric BaTiO₃ (BTO), and ferromagnetic La_2/3Ca_1/3MnO₃ bilayers on SrTiO₃ substrates by pulsed-laser deposition at pure oxygen atmosphere and substrate temperature of 820 °C, keeping fixed the magnetic layer thickness (t_LCMO) at 50 nm and varying the thickness of the ferroelectric layer (t_BTO) from 20 to 100 nm. X-ray diffraction analysis indicates a textured growth of both layers. The lattice parameter of the La_2/3Ca_1/3MnO₃ layer is slightly modified by the presence of the BaTiO₃ layer. Resistivity versus temperature Arrhenius plots show a small polaron thermally activated conductivity in bilayers with activation energies from 120 to 135 meV. Polarization loops at the BaTiO₃ measured at room temperature, using the La_2/3Ca_1/3MnO₃ layer as bottom electrode, show a maximum remnant polarization of 20 μC/cm₂ and minimal coercive field of 70 kV/cm. Thus, the bilayers maintain marked ferroelectric and ferromagnetic properties on individual layers.

Ferrofluids based on magnetic Co_0.25Zn_0.75Fe₂O₄ ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO₄, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co_0.25Zn_0.75Fe₂O₄ samples showed superparamagnetic behavior, according to hysteresis loop results. Taking in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions.