Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 16: Posters Correlated Electrons, Measuring Devices, Cryotechnique

TT 16.45: Poster

Samstag, 5. März 2005, 11:00–16:30, Poster TU C

Interplay between structure, magnetism and ordering phenomena in La_2−xSr_xCoO₄ — •M. Cwik¹, M. Benomar¹, M. Haider¹, T. Lorenz¹, Y. Sidis², and M. Braden¹ — ¹II. Physikalisches Institut, Universität zu Köln — ²Laboratoire Léon Brillouin, C.E.A./C.N.R.S., France

La₂CoO₄ is an antiferromagnetic insulator with T_N ∼ 275 K and Co²⁺ in the S=3/2 high-spin state [1]. Doping with Sr induces a Co²⁺/Co³⁺ mixed valency, destroys long-range antiferromagnetic order and removes the concomitant orthorhombic distortion of the K₂NiF₄ structure. In the half-doped compound La_1.5Sr_0.5CoO₄ spin and charge order have been found [2,3]. However, the effective magnetic coupling and the role of the three possible Co³⁺ spin states are not yet clarified. So far, it is also unknown how variations of doping affect both types of order in the La_2−xSr_xCoO₄ series. We present a study of structure, spin order, charge order, magnetic fluctuations, resistivity, and susceptibility on high-quality single crystals for 0.2≤x≤1.0. We find a drastic change in the tetragonal CoO₆-octahedra distortion which is not consistent with a simple ionic picture and incommensurable spin order next to the half-doped compound. An analysis of the magnon dispersion suggests an effective three-dimensional magnetic coupling scheme away from a simple frustrated antiferromagnetic square lattice with competing nearest- and next-nearest-neighbor interaction.

[1] K. Yamada et al., Phys. Rev. B 39, 2236 (1989)

[2] I. A. Zaliznyak et al., Phys. Rev. Lett. 85, 4353 (2000)

[3] I. A. Zaliznyak et al., Phys. Rev. B 64, 195117 (2001)