M.A.G. Aranda

Selective spin-state switch and metal-insulator transition in GdBaCo2O5.5

Ultrahigh resolution synchrotron diffraction data for GdBaCo 2 O 5 . 5 throw new light on the metal-insulator transition of Co 3 + Ba-cobaltites. An anomalous expansion of CoO 6 octahedra is observed at the phase transitionon heating, while CoO 5 pyramids show the normal shrinking at the closing of the gap. The insulator-to-metal transition is attributed to a sudden excitation of some electrons in the octahedra (t 6 2 g state) into the Co e g band (final t 4 2 g e 2 g state). The t 5 2 g e 1 g state in the pyramids does not change and the structural study also rules out a d 3 x 2 r 2 /d 3 y 2 - r 2 orbital ordering at T M I .

Selective spin-state and metal-insulator transitions in GdBaCo 2 O 5:5

Abstract By means of ultra-high resolution synchrotron diffraction and calorimetry measurements we have studied the metal–insulator transition in GdBaCo 2 O 5.5 . The appearance of the metallic state is attributed to a sudden excitation of some electrons in the octahedra ( t 2 g 6 state) into the Co e g band (final t 2 g 4 e g 2 state). In contrast, the t 2 g 5 e g 1 state in the pyramids does not change at the transition. Calorimetry measurements show that the insulator-to-metal transition is first order and the entropy change estimated from the latent heat corroborates the pictured scheme.

Charge and orbital order in rare-earth and Bi manganites: a comparison

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

Room temperature charge and orbital ordering and phase coexistence in Bi0.5Sr0.5MnO3

We present high-resolution neutron and synchrotron powder diffraction, magnetotransport, and magnetization measurements on Bi0.5Sr0.5MnO3. We report that, for the whole range of temperature 1.5 K≤T≤300 K, the sample studied exhibits coexistence of two phases. The majority phase presents charge and orbital order at least below 350 K. The existence and stability of charge order up to this high temperature agrees with the fact that magnetic fields higher than 45 T are not sufficient to melt the charge-ordered state at 4 K. This is in great contrast with the behaviour of non-Bi-based Ln1/2Sr1/2MnO3 isostructural oxides.

Spin state transition: the origin of structural, magnetic and metal–insulator transitions in GdBaCo2O5+δ (δ≈0.5)

The metal–insulator transition in GdBaCo2O5:5 at TMIE360 K has been investigated by synchrotron X-ray powder diffraction and magnetic measurements. We have found that,at TMI remarkable structural distortions and magnetic changes take place. These changes evidence a spin state transition of Co ions at this temperature that is the origin of the metal–insulator transition. r 2002 Elsevier Science B.V. All rights reserved.

Electronic and magnetic transitions in Bi–Sr–Mn–O oxides: high temperature charge-ordering

Abstract Neutron/synchrotron diffraction data and magnetic measurements provide direct evidence of charge/orbital ordering at anomalously high temperatures in (Bi,Sr)MnO 3 manganites. We report on the electronic and magnetic transitions of several oxides of this family. A mechanism based on the 6s 2 lone pair of Bi is proposed to strongly decrease the mobility of e g electrons and markedly favor charge-ordering.

Structural, spin state, and magnetic transitions in GdBaCo2O5+δ (δ≈0.5)

Abstract Structural, spin state, magnetic and field-induced transitions have been studied in GdBaCo 2 O 5.5 over a wide range of temperatures. We have found that, concomitant with a metal–insulator transition at 370 K , remarkable structural changes take place. These changes evidence the onset of orbital order at this temperature. Magnetization measurements, and the observation of a field-induced transition, below 250 K , by applying high magnetic fields, confirm that, below this temperature, an antiferromagnetic arrangement of the magnetic moments is present.

Phase segregation and orbital ordering in Bi1-xCaxMnO3 (x ≥ 0.75) : a neutron and synchrotron X-ray diffraction study

Abstract High-resolution synchrotron X-ray and neutron powder diffraction of Bi 1− x Ca x MnO 3 ( x ≥0.75) has shown that these systems present a phase segregation coinciding with the electronic localization of the e g electrons. The very precise results obtained provide evidence of a mechanism for phase segregation based on subtle compositional heterogeneity effects.