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Hamburg 2001 – scientific programme

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AM: Magnetismus

AM 9: Poster: Magnetowid. (1-17), Dü. Schichten (18-34), Oberfl
ächenmag. (35,36), Mikr. Methoden (37-45), Mikromag. (46-58), Phasenüberg. (59-77), Spektroskop. (78-91), Nanokr.Mat.(92-96), Anisotrop. (97-101), Schmelzen(102-104),Sonst/postdeadl.(105-109)

AM 9.68: Poster

Tuesday, March 27, 2001, 14:45–19:00, Foyer S 3

Magnetic ordering and spin-state transition in R1−xAxCoO3 — •M. Paraskevopoulos, A. Krimmel, J. Hemberger, and A. Loidl — Experimentalphysik V, Institut fuer Physik, Universitaet Augsburg.

The subtle balance between crystal-field energy Δcf and intraatomic exchange energy Jex for Co3+ leads to a peculiar spin-state transition observed in LaCoO3. In general the magnetic and electronic properties of ABO3 perovskites can be tuned by modifying the lattice parameters. The different lattice distortions are determined by the tolerance factor t = ( ⟨ rA ⟩ − ⟨ rO ⟩)/(√2(⟨ rB ⟩−⟨ rO ⟩)) . When t is closed to 1, the cubic perovskite structure is obtained. As t decreases the lattice symmetry transforms to rhombohedral and subsequently to orthorhombic. Concomitantly the crystal field changes at the B site. In most cases this effect can be neglected. However in cobalt based perovskites the balance between Δcf and Jex can easily be disturbed yielding transitions among different electronic configurations. This possibility motivated us to investigate the R1−xAxCoO3 series and detailed magnetization, resistivity, heat capacity and neutron scattering studies were performed. All samples reveal ferromagnetic order with increasing TC for increasing average ionic radius ⟨ rA ⟩ and we observed a spin-state transition as a function of ⟨ rA ⟩. For R = Nd and Sm, a further magnetic transition at low temperatures has been found. Our measurements show that this second transition is not connected to structural changes, i.e. is not related to the variation of ⟨ rA ⟩. We propose that this transition is due to an intimate interplay between the d and f electrons subsystems of the cobalt and rare earth ions respectively.

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