Search

The Pr-based cobaltate Pr0.5Ca0.5CoO3 (PCCO) presents in bulk form a singular simultaneous valence and spin-state transition that turns the metallic state into insulator, and displays a large and ultrafast photoresponse in the insulating phase. Epitaxial thin films of PCCO have been grown by deposition on LaAlO3(001) (LAO) substrate, chosen to minimize the mismatch with the film. The films grow epitaxially with two times the substrate periodicity (2a0×2a0×2a0) and the long perovskite axis perpendicular to the surface. We report a reduction of the structural symmetry from Pnma (bulk) to P212121 (film). The P212121 symmetry revealed by synchrotron x ray remains at low temperatures. These PCCO films are metallic, and ferromagnetic below TC=170K, confirming the stabilization of excited Co3+ spin states and the suppression of the concurrent Co spin-state, valence, and metal-insulator transitions. Z-contrast imaging and electron-energy-loss spectroscopy elemental maps reveal long-range ordered oxygen vacancy planes unexpectedly stacking parallel to the interface, in spite of the tensile character of the PCCO/LAO heterostructure. In contrast to the general tendency reported for strained La0.5Sr0.5CoO3−δ (LSCO) films, we show that a nominal tensile strain can be also compatible with the presence of alternating O vacancy planes parallel to the interface, instead of perpendicular to it. That occurs thanks to the double cell of the film and the formation of the (1/2, 1/2, 1/2) superstructure studied in this work. These results expand the possibilities of controlling interfacial physical properties via engineering of ordered oxygen-defect structures in strongly correlated oxides., We acknowledge financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN), through Projects No. MAT2015-686760-02-2-P, No. RTI2018-098537-B-C21, and No. RTI2018-098537-BC22, and “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496). The former are cofunded by ERDF of European Union, as Project No. EFA194/16 TNSI (POCTEFA/UE-FEDER). We thank also the Spanish MICINN and the Consejo Superior de Investigaciones Científicas for financial support and for provision of synchrotron radiation facilities (Grant No. CRG-BM25B, ESRF). J.G. acknowledges the Ramon y Cajal program (Grant No. RYC-2012-11709). Research at ORNL (STEM-EELS) was supported by the US Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SJP and MV).

Trabajo presentado al Nanoselect Network Of Excellence (NOE) Annual Meeting, celebrado en Sant Feliu de Guíxols, Girona (España) del 10 al 12 de julio de 2017.

Resumen del trabajo presentado al European Congress and Exhibition on Advanced Materials and Processes (EUROMAT), celebrado en Tesalónica (Grecia9 del 17 al 22 de septiembre de 2017., The spin-state of trivalent cobalt is being examined in cobaltites with CoO6 octahedral units because of its proved ability to condition their transport, magnetic and electronic properties. We have investigated metal-insulator and conductivity transitions in a variety of cobalt compounds with perovskite and related structures such as LnCoO3, Ln1-xAxCoO3, LnBaCo2O5.50 or Ba2Co9O14 (BCO) presenting spin-state instabilities. Neutron and synchrotron diffraction, x-ray absorption and x-ray emission spectroscopies, and x-ray magnetic circular dichroism techniques have all been employed to probe the electronic and spin state of Co ions in different regimes and across the magnetostructural transitions of these compounds. In particular cases films could also be grown by pulsed laser deposition. In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr close to ~100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 (PCCO) ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating. Instead of spin-state changes, the magnetostructural transition and the magnetic anomalies in the PSCO perovskite can be ascribed to a magnetic symmetry transition from Im'm'a to Fm'm'm, which encompasses a sudden reorientation of the magnetization axis by 45º within the a-b plane and the appearance of conjugated magnetic domains. Finally, Ba2Co9O14 presents five different Co sites and its low temperature magnetic symmetry is PS-1. Two out of the three low-spin CoO6 octahedral sites adopt a higher spin state at the conductivity transition around 567 K. The origin of the strong moment reduction found at the nominally “divalent high spin” Co5 octahedral positions was carefully investigated.

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España)., Cobalt-based perovskites are attracting the interest as strongly correlated electron systems due to the relevance of the cobalt spin state for their functional properties. The spin state of trivalent cobalt is a key feature in half-doped Pr0.50A0.50CoO3 (A: Ca or Sr) perovskites. In these compounds the spin-lattice coupling gives rise to non-conventional magnetostructural and electronic transitions, the interpretation of which has been controversial. The properties of Pr0.50Sr0.50CoO3 (PSCO) differ from those in Pr0.50Ca0.50CoO3 (PCCO), where a spin-state change has been proved to induce a partial Pr3+ to Pr4+ valence shift and a exotic metal-insulator transition around 80 K. Moreover, recent works have confirmed the importance of the spin-lattice coupling in Pr0.50Sr0.50CoO3 reporting magnetostructural changes at 120K. We have investigated the properties of PSCO (always metallic, and ferromagnetic below TC ~ 230 K) across the unexpected magnetostructural transition at TS ~120 K [3-6]. X-ray absorption experiments (XAS) at the Pr M4,5 and L3 edges rule out any trace of Pr4+ below TS. By means of x-ray circular dichroism (XMCD) at Co L edges, the unusual and coupled evolution of the orbital and spin moments demonstrate the relevance of the spin-orbit coupling for the stabilization of two distinct ferromagnetic phases in this metallic perovskite. The Co spin state model is discussed on the basis of charge transfer multiplet calculations of the dichroic spectral shape. In addition, neutron diffraction sheds light on the link between the Imma→I4/mcm symmetry change and the original magnetic behavior of PSCO at TS.

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to ∼100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This paper investigates and describes the relationship between the Imma→I4/mcm symmetry change [Padilla-Pantoja, García-Muñoz, Bozzo, Jirák, and Herrero-Martín, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1∼120K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im′m′a (mx≠0, mz=0). The magnetic space group of the FM1 phase is Fm′m′m (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45 within the ab plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm′m′m symmetry but having perpendicular spin orientations along the diagonals in the xy plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field μ0H[z]30mT is able to reorient the magnetization within the ab plane, whereas a higher field (μ0H] 1.2T at 2 K) is necessary to align the Co moments perpendicular to the ab plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45, was not observed previously in analogous cobaltites without praseodymium., We thank the spanish Ministerio de Economía y Competitividad (MINECO) for financial support under Projects No. MAT2012-38213-C02-02 and -01 and No. MAT2015-68760-2-2-P and -1-P (cofunded by the European Regional Development Fund [ERDF] from the European Union) and No. CSD2007-00041 (NANOSELECT). J.P.-P. and J.A.R.V. thank CSIC for JAE-Pre and JAE-Doc contracts, respectively.

Trabajo presentado a la 61st Annual Conference on Magnetism and Magnetic Materials (MMM) celebrada en New Orleans (USA) del 31 de octubre al 4 de noviembre de 2016., We thank financial support from MINECO under projects MAT2012-38213-C02 and MAT2015-68760-C2, cofunded by ERDF from EU.

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to ~100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This paper investigates and describes the relationship between the Imma¿I4/mcm symmetry change [Padilla-Pantoja, García-Muñoz, Bozzo, Jirák, and Herrero-Martín, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1~120K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im'm'a (mx¿0, mz=0). The magnetic space group of the FM1 phase is Fm'm'm (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45° within the ab plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm'm'm symmetry but having perpendicular spin orientations along the diagonals in the xy plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field µ0H[¿z]¿30mT is able to reorient the magnetization within the ab plane, whereas a higher field (µ0H[¿z]¿1.2T at 2 K) is necessary to align the Co moments perpendicular to the ab plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45°, was not observed previously in analogous cobaltites without praseodymium.

Poster presented at the VI European Conference on Neutron Scattering, held in Zaragoza (Spain) from August 30th to September 4th, 2015. Magnetism, Superconductivity and other Electronic Phenomena., The spin-state nature across the two electronic transitions in LaCoO3 has been matter of controversy for many years. Two competing pictures in LaCoO3 involve the spin transitions either LS→(HS+LS)→IS or LS→IS→(HS+IS). The ground and thermally excited states of LnCoO3 perovskites (Ln:lanthanide or rare earth) with heavy rare-earths and more distorted structures are also attracting great interest. We present neutron powder diffraction (NPD) studies aimed to characterize the electronic phenomena associated with the spin-state of Co3+ ions in the highly distorted ErCoO3 cobaltite. NPD measurements carried out on the high-resolution D2B diffractometer up to 1000 K were combined with low temperature measurements using several ILL diffractometers (which confirmed the ordering of Er moments below 2 K). To our knowledge, ErCoO3 (prepared under moderate pressure conditions) is the smallest Co3+ simple perovskite investigated so far in a temperature range up to 1000 K. The orthorhombic Pbnm symmetry was confirmed in this range, but the lattice evolution proves a remarkable occupation of excited magnetic states on heating above TSS≈575 K. The spin-state crossover extends over a broad temperature interval and can be monitored by an atypical augmentation of the rthorhombic strain (s). The anomalous contributions to the lattice above that temperature are clearly visible in the evolution of the anisotropic or global linear thermal expansion coefficients. The occupation of excited magnetic states (S=1 or 2) is activated at TSS≈575 K but it develops progressively along the interval 575-780 K. Above ~800 K the progression decreases. An estimation of the anomalous volume expansion accumulated at 1000 K due to spin state excitations gives ~2 %. Several correlated structural modifications above TSS are driven by the anomalous expansion of the CoO6 octahedra, which is partially anisotropic. The activation of IS or HS spin states brings about the augmentation of the Co-O distances in the a-b plane respect to the distance along the c-axis. This fact produces a non monotonous evolution of the tilting between adjacent octahedra and the intensification of the orthorhombic distortion. In spite of this, some evidences suggest the stabilization of a metallic state in the thermally excited phase.

Resumen del trabajo presentado a la 19th International Conference on Magnetism celebrada en Korea del 8 al 13 de julio de 2012.-- et al., The electronic and magnetic properties of cobalt-based perovskites are largely determined by the electronic filling of the Co 3d valence band. The characteristic spin state degree of freedom in many cobaltites is correlated to the electron mobility and thus it can play an important role in metal-insulator transitions (MIT). MIT are found in a wide range of compositions of the (Pr1-yYy)1-xCaxCoO3 series, coupled to magnetic and structural changes. At low temperatures, x-ray diffraction confirms a crystal cell volume contraction occurs due to the appearance of a certain quantity of Pr4+ ions. The electrons lost by Pr ions move to Co sites. The expected increase of the average Co-O distance is not observed, may be due to a compensation effect by a likely Co ions spin state change. We present here a comprehensive study of these compounds by means of x-ray absorption and emission spectroscopies, which leads us to confirm these hypothesis also quantifying the intersite charge donation across the MIT. The observed photoinduced phase transition in Pr0.5Ca0.5CoO3 under illumination by laser radiation, where metallic domains can be stimulated, could thus be related to induced spin state changes.

Resumen del trabajo presentado al: "European Magnetic Research Society-Spring Meeting" (Symposium D), celebrado en Strasbourg (Francia) del 14 al 17 de mayo de 2012.-- et al., The spin degree of freedom in cobalt oxides with perovskite structure is a fundamental parameter modulating the occurrence and characteristics of metal-insulator transitions due to the implications it imposes on the electron mobility. Nearly half-doped Pr-Ca cobaltites show a metal-insulator transition associated to magnetic and structural changes. The crystal cell volume contraction observed is explained as due to a net intersite charge transfer from a fraction of Pr to Co at the transition, thus leading to a further oxidation of Pr ions (formally from +3 to +3+d) while Co gets reduced. At the same time, the average spin state of Co ions (~ Co3.5+ in the metallic phase) is observed to decrease. This can be attributed to the appearance of low spin (LS) state Co3+ ions. The magnitude of these changes and the transition temperature can be optimized by introducing a partial substitution of Pr by Y in small quantities. The results obtained by x-ray absorption and x-ray emission spectroscopies we will present here for this family can help us to understand the observed photoinduced phase transition in Pr0.5Ca0.5CoO3 under illumination by laser radiation, where metallic domains can be stimulated, and which seems to be related to induced spin state changes.