Raquel Cortés-Gil

Magnetic Structure and Electronic Study of Complex Oxygen-Deficient Manganites

Neutron diffraction and X-ray absorption near-edge structure (XANES) studies have been performed in La0.5Ca0.5MnO2.5, La0.5Sr0.5MnO2.5 and Nd0.5Sr0.5MnO2.5 oxygen-deficient perovskite compounds obtained by topotactic reduction. They all exhibit a brownmillerite structure with G-type antiferromagnetic ordering. Mn2+, Mn3+ and Mn4+ coexist at the octahedral sites, whereas only Mn2+ is placed in the tetrahedral positions. A magnetic moment of 1.6 microB has been detected at the tetrahedral layers, which can be explained by assuming Mn2+ is in a low-spin configuration.

Unexpected ferromagnetic ordering enhancement with crystallite size growth observed in La0.5Ca0.5MnO3 nanoparticles

In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as crystallite size is reduced to 15 nm. However, contrary to previously reported works, an enhancement of saturation magnetization is observed as crystallite size increases from 15 to 22 nm. This unexpected behavior is accompanied by an unusual cell volume variation that seems to induce ferromagnetic-like behavior at expense of antiferromagnetic one. Besides, field cooled hysteresis loops show exchange bias field and coercivity enhancement for increasing cooling fields, which suggest a kind of core-shell structure with AFM-FM coupling for crystallite sizes as small as 15 nm. It is expected that inner core orders antiferromagnetically, whereas uncompensated surface spins behave as spin glass with ferromagnetic-like ordering.

Magnetoresistance in La0.5Sr0.5MnO2.5

Resistance measurements indicate the presence of magnetoresistance in the La(0.5)Sr(0.5)MnO(2.5) brownmillerite related compound. An 80 % of magnetoresistance is found at 75 K. In spite of the partial break-up occurring at the 3D network of octahedra sharing corners, characteristic of the full oxygen content perovskite phase, the oxygen deficient compound exhibits complex magnetic and electric properties. Such behavior can be explained on the basis of ferromagnetic and metallic clusters randomly distributed at the octahedral layers separated from each other by an insulating antiferromagnetic matrix. AC susceptibility measurements suggest spin glass behavior at low temperature as a consequence of the competition between different magnetic interactions.

Revisiting the Role of Vacancies in Manganese Related Perovskites

Oxygen engineering is an important tool for on-demand tailoring of manganese related perovskites into opti- mized performances. Anionic vacancies can be induced in both doped and undoped manganites by means of topotactic re- duction processes under oxygen controlled atmosphere. This has given rise to the stabilization of new phases as a conse- quence of ordering of oxygen vacancies in which Mn 2+ appears due to the reducing process. Different reduction pathways are proposed for LaMnO3 and doped systems. FM interactions remain at the octahedral layers when vacancies are long- range ordered. The peculiar magnetic behaviour of new layered perovskite superstructures is discussed.

Temperature dependence of the magnetic properties in LaMnO3+δ

Data are presented on the thermal dependence of the hysteretic properties of cationic vacancies including manganite samples of composition LaMnO3+δ (δ=0.05 and 0.12). Our results evidence the presence in both samples of two magnetic phases having ferro- and antiferromagnetic orders, respectively. The temperature dependence of the coercivity and relaxational properties of the samples is closely linked to the connectivity of the magnetic moment bearing Mn3+–Mn4+ ferromagnetic clusters that demagnetize independently in the case of the δ=0.05 sample and collectively in that of the δ=0.12 one, as evidenced from the activation volume results (δ=0.05) which yielded a size of the same order magnitude as that obtained in previous works for the Mn3+–Mn4+ ferromagnetic cluster size.

Surprising resistivity decrease in manganites with constant electronic density

A decrease of eight orders of magnitude in the resistance of (La0.5Ca0.5)zMnO3 has been detected when the electronic density is kept constant while the calcium content is modified by introducing cationic vacancies. This effect is related to the disappearance of the charge ordering state and the emergence of an antiferromagnetic–ferromagnetic transition. Moreover, high values of the colossal magnetoresistance above room temperature are attained.

Experimental Evidence of the Origin of Nanophase Separation in Low Hole-Doped Colossal Magnetoresistant Manganites

While being key to understanding their intriguing physical properties, the origin of nanophase separation in manganites and other strongly correlated materials is still unclear. Here, experimental evidence is offered for the origin of the controverted phase separation mechanism in the representative La1-xCaxMnO3 system. For low hole densities, direct evidence of Mn(4+) holes localization around Ca(2+) ions is experimentally provided by means of aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy. These localized holes give rise to the segregated nanoclusters, within which double exchange hopping between Mn(3+) and Mn(4+) remains restricted, accounting for the insulating character of perovskites with low hole density. This localization is explained in terms of a simple model in which Mn(4+) holes are bound to substitutional divalent Ca(2+) ions.

Analytical Atomic-Resolution Microscopy of Oxygen Deficient Ca2Mn3O8-d

Manganese related perovskites (AMnO3, A=alkaline earth) present a wide range of fascinating functional properties due to the ability of Mn to adopt several oxidation states and different coordination polyhedra. Regarding their catalytic behaviour, CaMnO3-δ selectively oxidizes, at least on a laboratory scale, propene to benzene and 2-methyl propene [1]. Moreover, the Ca-Mn-O system presents a great variety of oxides with different Ca/Mn ratio and crystalline structure and a particular behaviour: their reduction process leads to a rock-salt type structure which, in most cases, can be again oxidized to the starting material [2]. Here we show the combination of oxygen engineering performed under adequate controlled atmosphere with local characterization techniques like atomic resolution electron microscopy associated to Energy Electron Loss Spectroscopy (EELS) to provide a complete characterization of other member of the Ca-Mn-O system. In particular, Ca2Mn3O8 crystallizes in a monoclinic layered structure [3] with sheets of MnIV in octahedral coordination held together by Ca2+ cations alternately stacked along a axis (Fig. 1). The total reduction process of this material leads to Ca2Mn3IIO5 with rock-salt type structure. By means of partial reduction, different samples have been stabilized in the Ca2Mn3O8-d system. Among them, Ca2Mn3O6.5, a new layered calcium manganese oxide with only Mn3+, has been stabilized. The different samples obtained in the Ca2Mn3O8-δ system have been characterized by using High Angle Annular Dark Field (HAADF) and Annular Bright Field (ABF) imaging associated to EELS and Energy Dispersive Spectroscopic (EDS) in an aberration-corrected JEOL JEMARM200cF electron microscope. The structural evolution and the local variation of the Mn oxidation state in different phases with different anionic composition will be discussed.

Chemical Analysis at Atomic Resolution of Isolated Extended Defects in an Oxygen‐Deficient, Complex Manganese Perovskite

A general approach to the structural and analytical characterization of complex bulk oxides that exploits the advantage of the atomic spatial resolution and the analytical capability of aberration-corrected microscopy is described. The combined use of imaging and spectroscopic techniques becomes necessary to the complete characterization of the oxygen-deficient colossal magnetoresistant La(0.56)Sr(0.44)MnO(2.5)-related perovskite. In this compound, the formation of isolated (La/Sr)O and MnO rock-salt-type planar defects are identified from atomically resolved High Angle Annular Dark Field (HAADF) images. The location of the oxygen atomic columns from Annular Bright Field (ABF) images indicates edge-sharing MnO6 octahedra in the MnO planes and the study performed by Electron Energy Loss Spectroscopy (EELS) reveals different Mn oxidation states derived from the corner- or edge-sharing MnO6 octahedra environment.

Ab initio study of the influence of nanoscale doping inhomogeneities in the phase separated state of La1-xCaxMnO3.

The chemical influence in the phase separation phenomenon that occurs in perovskite manganites is discussed by means of ab initio calculations. Supercells have been used to simulate a phase separated state, that occurs at Ca concentrations close to the localized itinerant crossover. We have first considered a model with two types of magnetic ordering coexisting within the same compound. This is not stable. However, a non-isotropic distribution of chemical dopants is found to be the ground state. This leads to regions in the system with different effective concentrations, that would always accompany the magnetic phase separation at the same nanometric scale, with hole-rich regions being more ferromagnetic in character and hole-poor regions being in the antiferromagnetic region of the phase diagram, as long as the system is close to a phase crossover.