Search

We present a comparative study of the magnetic transitions in metallic Pr0.50Sr0.50CoO3 (PSCO) perovskites prepared in polycrystalline and thin film forms. As the bulk system, the strained epitaxial PSCO (010) film grown on LAO (100) is metallic in all the temperature range, with a ferromagnetic transition at 225K, close to Tc ~ 235K in the ceramic PSCO specimen. Unlike the bulk system, the PSCO film does not show the second magnetic transition on cooling. In the ceramic sample, the second magnetic transition is coupled to an orthorhombic-to-monoclinic symmetry change. There is a contraction of the average bond distance in the monoclinic phase below Ta, but the bond length is not modified across the transition. The orthorhombic to monoclinic structural transition stabilizes four short Pr-O2 bonds to basal oxygens in CoO6 octahedra. A strong hybridization of Pr 4f and O 2p orbitals in these bonds can be activated at Ta and probably assist the magnetostructural transition. [ABSTRACT FROM AUTHOR]

We have performed temperature-dependent x-ray magnetic circular dichroism on ferromagnetic Pr0.5Sr0.5CoO3 at the Co L2,3 edges to analyze the correlation of the spin and orbital angular components of the magnetization with the singular magnetostructural anomalies at ~ 120 K. We found a sizable orbital angular momentum in Co atoms that follows a temperature dependence very similar to that of the spin momentum. Their sum agrees with the bulk magnetization. Taken together with recent x-ray absorption spectra showing the absence of changes in the electronic filling of Co 3d states across the magnetostructural transition, our results 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. [ABSTRACT FROM AUTHOR]

Synchrotron x-ray diffraction and absorption, together with magnetic and thermoelectric measurements, have been used to investigate the mechanism driving the metal to insulator transition in YBaCo2O5.50. Results show a structural and magnetic transition concomitant with the metal to insulator transition. Absorption spectroscopy at the Co L3 edge as a function of temperature reveals minor changes across the transition. The set of experimental results are hardly conciliative with a high to low spin state change of Co ions at octahedral sites in this cobaltite. [ABSTRACT FROM AUTHOR]

The mixed Ba2Co9O14Co2+/Co3+ system undergoes an insulator-insulator transition at TSS~567K that arises from a spin-state transition at trivalent cobalt sites. Below this temperature, Co1, Co2, and Co4 are nonmagnetic (S=0, low spin). Ferromagnetically aligned Co5 spins are sandwiched between antiparallel planes of Co3 spins below TN≈41K. The successive antiferromagnetic trilayers are inverted along the c axis (compatible with Cc2/c,Cc2/m, or PS-1 magnetic space groups, depending on the moment orientation in the ab plane). The origin of the resistivity drop on warming was investigated by means of neutron and x-ray diffraction, x-ray absorption and emission spectroscopies, and x-ray magnetic circular dichroism. Charge-transfer multiplet calculations confirm that the divalent Co sites are both in an S=3/2 high spin state. Independently, the analysis of measured Co Kβ x-ray emission spectroscopy spectra agrees with this model. The magnetic moment from divalent Co5 ions is not fully ordered, likely due to the competition between magnetic anisotropy and weak supersuperexchange interactions, but not to covalency effects. Results agree with the spin blockade of electronic transport being partially removed at the octahedral trimers and also at the Co4O6 units within the CdI2-type layer. [ABSTRACT FROM AUTHOR]