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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.

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.