Hamburg 2001 – wissenschaftliches Programm

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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

Dienstag, 27. März 2001, 14:45–19:00, Foyer S 3

Magnetic ordering and spin-state transition in R_1−xA_xCoO₃ — •M. Paraskevopoulos, A. Krimmel, J. Hemberger, and A. Loidl — Experimentalphysik V, Institut fuer Physik, Universitaet Augsburg.

The subtle balance between crystal-field energy Δ_cf and intraatomic exchange energy J_ex 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 = ( ⟨ r_A ⟩ − ⟨ r_O ⟩)/(√2(⟨ r_B ⟩−⟨ r_O ⟩)) . 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 Δ_cf and J_ex can easily be disturbed yielding transitions among different electronic configurations. This possibility motivated us to investigate the R_1−xA_xCoO₃ series and detailed magnetization, resistivity, heat capacity and neutron scattering studies were performed. All samples reveal ferromagnetic order with increasing T_C for increasing average ionic radius ⟨ r_A ⟩ and we observed a spin-state transition as a function of ⟨ r_A ⟩. 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 ⟨ r_A ⟩. 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.