Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

We investigate the influence of a well-defined reversible biaxial strain $\ensuremath{\leqslant}0.12%$ on the magnetization $(M)$ of epitaxial ferromagnetic manganite films. $M$ has been recorded depending on temperature, strain and magnetic field in $20--50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick films. This is accomplished by reversibly compressing the isotropic in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT (001) substrates by application of an electric field $E\ensuremath{\leqslant}12\phantom{\rule{0.3em}{0ex}}\mathrm{kV}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The magnitude of the total variable in-plane strain has been derived. Strain-induced shifts of the ferromagnetic Curie temperature $({T}_{C})$ of up to $19\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were found in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ films and are quantitatively analyzed for LSMO within a cubic model. The effective magnetoelectric coupling coefficient $\ensuremath{\alpha}={\ensuremath{\mu}}_{0}dM∕dE\ensuremath{\leqslant}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}\mathrm{s}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ at ambient temperature in the magnetic-film-ferroelectric-substrate system results from the strain-induced $M$ change. It corresponds to an enhancement of ${\ensuremath{\mu}}_{0}\ensuremath{\Delta}M\ensuremath{\leqslant}19\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ upon biaxial compression of 0.1%. The extraordinary large $\ensuremath{\alpha}$ originates from the combination of three crucial properties: (i) the strong strain dependence of $M$ in the ferromagnetic manganites, (ii) large piezo-strain of the PMN-PT substrates, and (iii) effective elastic coupling at the film-substrate interface.

Influence of strain on the magnetization and magnetoelectric effect in La 0.7 A 0.3 Mn O 3 ∕ PMN − PT ( 001 ) ( A = Sr , Ca )

Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites

The rapid development of information technology has led to urgent requirements for high efficiency and ultralow power consumption. In the past few decades, neuromorphic computing has drawn extensive attention due to its promising capability in processing massive data with extremely low power consumption. Here, we offer a comprehensive review on emerging artificial neuromorphic devices and their applications. In light of the inner physical processes, we classify the devices into nine major categories and discuss their respective strengths and weaknesses. We will show that anion/cation migration-based memristive devices, phase change, and spintronic synapses have been quite mature and possess excellent stability as a memory device, yet they still suffer from challenges in weight updating linearity and symmetry. Meanwhile, the recently developed electrolyte-gated synaptic transistors have demonstrated outstanding energy efficiency, linearity, and symmetry, but their stability and scalability still need to be optimized. Other emerging synaptic structures, such as ferroelectric, metal–insulator transition based, photonic, and purely electronic devices also have limitations in some aspects, therefore leading to the need for further developing high-performance synaptic devices. Additional efforts are also demanded to enhance the functionality of artificial neurons while maintaining a relatively low cost in area and power, and it will be of significance to explore the intrinsic neuronal stochasticity in computing and optimize their driving capability, etc. Finally, by looking into the correlations between the operation mechanisms, material systems, device structures, and performance, we provide clues to future material selections, device designs, and integrations for artificial synapses and neurons.

A comprehensive review on emerging artificial neuromorphic devices

The question whether $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ accepts doping with tetravalent cations such as cerium and thus allows the preparation of electron-doped mixed-valent lanthanum manganites has been discussed controversially so far. Against the background of this problem, we present a comparative x-ray photoemission (XPS) study of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCeMO) and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films. We focus on the exchange splitting of the Mn $3s$ core level peak, which is a direct indicator of the Mn valence and allows us to quantify the Mn valence in the outermost $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of the films. We demonstrate that, depending on the oxygen content, the Mn valence can be tuned between a mixed ${\mathrm{Mn}}^{3+∕4+}$ state and a mixed ${\mathrm{Mn}}^{2+∕3+}$ state in both the LCeMO and the LCMO film. The oxygen content was varied by heating in ultrahigh vacuum for deoxygenation and in an oxygen atmosphere for reoxidation. In the LCeMO film, the deoxygenation not only changes the Mn valence, but also the Ce valence is driven from the $4+$ towards the $3+$ state.

XPS investigation of Mn valence in lanthanum manganite thin films under variation of oxygen content

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

A.-M. Haghiri-Gosnet

Papers

CMR manganites: physics, thin films and devices