Raquel Cortés-Gil

The inclusion of La-Mn vacancies in LaMnO₃ nanoparticles leads to a noticeable change in conductivity behavior. The sample retains its overall insulator characteristic, with a typical thermal activation mechanism at high temperatures, but it presents high magnetoconductivity below 200 K. The activation energy decreases linearly with the square of the reduced magnetization and vanishes when the sample is magnetized at saturation. Therefore, it turns out that electron hopping between Mn³⁺ and Mn⁴⁺ largely contributes to the conductivity below the Curie temperature. The influence of the applied magnetic field on conductivity also supports the hypothesis of hopping contribution, and the electric behavior can be explained as being due to an increase in the hopping probability via spin alignment.

Magnetoimpedance spectroscopy was carried out on phase-separated La _0.5 Ca _0.5 MnO ₃ polycrystalline manganites.

Ruddlesden–Popper oxides, (AO)(ABO₃)_n, occupy a prominent place in the landscape of materials research because of their intriguing potential applications. Compositional modifications to the cation sublattices,AorB, have been explored in order to achieve enhanced functionalities. However, changes to the anionic sublattice have been much less explored. In this work, new oxygen-deficient manganese Ruddlesden–Popper-related phases, La_0.5Ca_2.5Mn₂O_6.5and La_0.5Ca_2.5Mn₂O_6.25, have been synthesized by controlled reduction of the fully oxidizedn= 2 term La_0.5Ca_2.5Mn₂O₇. A complete structural and compositional characterization, by means of neutron diffraction, electron diffraction and atomically resolved scanning transmission electron microscopy and electron energy-loss spectroscopy techniques, allows the proposition of a topotactic reduction pathway through preferential oxygen removal in the [MnO₂] layers along [031] and [0{\bar 1}3] directions. The gradual decrease of the Mn oxidation state, accommodated by short-range ordering of anionic vacancies, reasonably explains the breaking of ferromagnetic interactions reinforcing the emergence of antiferromagnetic ones. Additional short-range order–disorder phenomena of La and Ca cations have been detected in the reduced La_0.5Ca_2.5Mn₂O_7−δ, as previously reported in the parent compound.

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 Mn4+ holes localization around Ca2+ 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 Mn3+ and Mn4+ 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 Mn4+ holes are bound to substitutional divalent Ca2+ ions.

In this paper, the physical properties of half-doped manganite La_0.5Ca_0.5MnO₃ 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.

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_0.56Sr_0.44MnO_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.

A decrease of eight orders of magnitude in the resistance of.(La0.5Ca0.5)(z)MnO3 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.